RFC 2264
User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3)
RFC 2264: User-based Security Model (USM) for version 3 of the Simple Network Management Protocol (SNMPv3) 🎀
okay bestie, RFC 2264 is giving foundational SNMP security 🔐✨. It's a Standards Track doc dated January 1998, written by U. Blumenthal and B. Wijnen, both from IBM T. J. Watson Research 💅. Nothing in the title block says it obsoletes or updates anything, so it's not replacing or patching an older spec.
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. 📜
Copyright Notice
Copyright (C) The Internet Society (1997). All Rights Reserved. ©️🌸
Abstract
okay so this doc describes the User-based Security Model (USM) for SNMP version 3, which is meant for use in the SNMP architecture [RFC2261] 🦋. It defines the Elements of Procedure for providing SNMP message level security, and it also includes a MIB for remotely monitoring and managing the configuration parameters for this Security Model. main character energy, honestly 👑.
Table of Contents ✨
1. Introduction 3
1.1. Threats 4
1.2. Goals and Constraints 5
1.3. Security Services 6
1.4. Module Organization 7
1.4.1. Timeliness Module 7
1.4.2. Authentication Protocol 8
1.4.3. Privacy Protocol 8
1.5. Protection against Message Replay, Delay and Redirection 8
1.5.1. Authoritative SNMP engine 8
1.5.2. Mechanisms 8
1.6. Abstract Service Interfaces. 10
1.6.1. User-based Security Model Primitives for Authentication 11
1.6.2. User-based Security Model Primitives for Privacy 11
2. Elements of the Model 12
2.1. User-based Security Model Users 12
2.2. Replay Protection 13
2.2.1. msgAuthoritativeEngineID 13
2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime 14
2.2.3. Time Window 15
2.3. Time Synchronization 15
2.4. SNMP Messages Using this Security Model 16
2.5. Services provided by the User-based Security Model 17
2.5.1. Services for Generating an Outgoing SNMP Message 17
2.5.2. Services for Processing an Incoming SNMP Message 19
2.6. Key Localization Algorithm. 21
3. Elements of Procedure 21
3.1. Generating an Outgoing SNMP Message 22
3.2. Processing an Incoming SNMP Message 25
4. Discovery 30
5. Definitions 31
6. HMAC-MD5-96 Authentication Protocol 45
6.1. Mechanisms 45
6.1.1. Digest Authentication Mechanism 46
6.2. Elements of the Digest Authentication Protocol 46
6.2.1. Users 46
6.2.2. msgAuthoritativeEngineID 47
6.2.3. SNMP Messages Using this Authentication Protocol 47
6.2.4. Services provided by the HMAC-MD5-96 Authentication Module 47
6.2.4.1. Services for Generating an Outgoing SNMP Message 47
6.2.4.2. Services for Processing an Incoming SNMP Message 48
6.3. Elements of Procedure 49
6.3.1. Processing an Outgoing Message 49
6.3.2. Processing an Incoming Message 50
7. HMAC-SHA-96 Authentication Protocol 51
7.1. Mechanisms 51
7.1.1. Digest Authentication Mechanism 51
7.2. Elements of the HMAC-SHA-96 Authentication Protocol 52
7.2.1. Users 52
7.2.2. msgAuthoritativeEngineID 52
7.2.3. SNMP Messages Using this Authentication Protocol 53
7.2.4. Services provided by the HMAC-SHA-96 Authentication Module 53
7.2.4.1. Services for Generating an Outgoing SNMP Message 53
7.2.4.2. Services for Processing an Incoming SNMP Message 54
7.3. Elements of Procedure 54
7.3.1. Processing an Outgoing Message 55
7.3.2. Processing an Incoming Message 55
8. CBC-DES Symmetric Encryption Protocol 56
8.1. Mechanisms 56
8.1.1. Symmetric Encryption Protocol 57
8.1.1.1. DES key and Initialization Vector. 57
8.1.1.2. Data Encryption. 58
8.1.1.3. Data Decryption 59
8.2. Elements of the DES Privacy Protocol 59
8.2.1. Users 59
8.2.2. msgAuthoritativeEngineID 59
8.2.3. SNMP Messages Using this Privacy Protocol 60
8.2.4. Services provided by the DES Privacy Module 60
8.2.4.1. Services for Encrypting Outgoing Data 60
8.2.4.2. Services for Decrypting Incoming Data 61
8.3. Elements of Procedure. 61
8.3.1. Processing an Outgoing Message 61
8.3.2. Processing an Incoming Message 62
9. Intellectual Property 62
10. Acknowledgements 63
11. Security Considerations 64
11.1. Recommended Practices 64
11.2. Defining Users 66
11.3. Conformance 67
12. References 67
13. Editors' Addresses 69
A.1. SNMP engine Installation Parameters 70
A.2. Password to Key Algorithm 71
A.2.1. Password to Key Sample Code for MD5 71
A.2.2. Password to Key Sample Code for SHA 72
A.3. Password to Key Sample Results 73
A.3.1. Password to Key Sample Results using MD5 73
A.3.2. Password to Key Sample Results using SHA 74
A.4. Sample encoding of msgSecurityParameters 74
B. Full Copyright Statement 761. Introduction ✨
okay so the Architecture for describing Internet Management Frameworks [RFC2261] says that an SNMP engine is made up of four pieces 🧚:
- a Dispatcher
- a Message Processing Subsystem,
- a Security Subsystem, and
- an Access Control Subsystem.
Applications use the services of these subsystems 💖.
It's really important to get the SNMP architecture and its terminology down, so you can see where the Security Model in this doc fits in and how it talks to the other subsystems 🫶. The reader is expected to have already read and understood the SNMP architecture as defined in [RFC2261], so no skipping that one bestie 🙈.
This memo [RFC2264] describes the User-based Security Model as it's used inside the SNMP Architecture. The big idea is that we use the traditional concept of a user (identified by a userName) and attach security information to that user 🎀.
This memo describes using HMAC-MD5-96 and HMAC-SHA-96 as the authentication protocols, and CBC-DES as the privacy protocol 🔥. The User-based Security Model does allow other protocols to be used instead of these, or alongside them 🌈. So HMAC-MD5-96, HMAC-SHA-96 and CBC-DES each get their own self-contained sections, which means they can be swapped out or supplemented later on 🦋.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] 💜.
1.1. Threats 🕵️♀️
okay so bestie, a bunch of the classic network-protocol threats totally apply to network management, which means they'd apply to any SNMP Security Model too. Other threats just don't fit the network management problem, so we can skip those. This section covers the principal threats, the secondary threats, and the ones that matter less. 💅
The principal threats this SNMP Security Model SHOULD protect against are:
- Modification of Information The modification threat is the danger that some unauthorized entity may alter in-transit SNMP messages generated on behalf of an authorized user in such a way as to effect unauthorized management operations, including falsifying the value of an object. 😤
- Masquerade The masquerade threat is the danger that management operations not authorized for some user may be attempted by assuming the identity of another user that has the appropriate authorizations. 🎭
Two secondary threats are also identified. The Security Model defined in this memo provides limited protection against:
- Disclosure The disclosure threat is the danger of eavesdropping on the exchanges between managed agents and a management station. Protecting against this threat may be required as a matter of local policy. 👀
- Message Stream Modification The SNMP protocol is typically based upon a connection-less transport service which may operate over any sub-network service. The re-ordering, delay or replay of messages can and does occur through the natural operation of many such sub-network services. The message stream modification threat is the danger that messages may be maliciously re-ordered, delayed or replayed to an extent which is greater than can occur through the natural operation of a sub-network service, in order to effect unauthorized management operations. 🌀
There are at least two threats that an SNMP Security Model need not protect against. The security protocols defined in this memo do not provide protection against:
- Denial of Service This SNMP Security Model does not attempt to address the broad range of attacks by which service on behalf of authorized users is denied. Indeed, such denial-of-service attacks are in many cases indistinguishable from the type of network failures with which any viable network management protocol must cope as a matter of course. 🙃
- Traffic Analysis This SNMP Security Model does not attempt to address traffic analysis attacks. Indeed, many traffic patterns are predictable (devices may be managed on a regular basis by a relatively small number of management applications), and therefore there is no significant advantage afforded by protecting against traffic analysis. 📊
1.2. Goals and Constraints 🎯
Based on everything we just covered about threats in the SNMP network management environment, the goals of this SNMP Security Model are as follows.
- Provide for verification that each received SNMP message has not been modified during its transmission through the network. ✅
- Provide for verification of the identity of the user on whose behalf a received SNMP message claims to have been generated. 🪪
- Provide for detection of received SNMP messages, which request or contain management information, whose time of generation was not recent. ⏰
- Provide, when necessary, that the contents of each received SNMP message are protected from disclosure. 🔒
Besides the main goal of supporting secure network management, the design of this SNMP Security Model is also shaped by the following constraints.
- When the requirements of effective management in times of network stress are inconsistent with those of security, the design should prefer the former. 🌪️
- Neither the security protocol nor its underlying security mechanisms should depend upon the ready availability of other network services (e.g., Network Time Protocol (NTP) or key management protocols). 🧘♀️
- A security mechanism should entail no changes to the basic SNMP network management philosophy. 💖
1.3. Security Services 🛡️
The security services needed to support the goals of this SNMP Security Model are as follows.
- Data Integrity is the provision of the property that data has not been altered or destroyed in an unauthorized manner, nor have data sequences been altered to an extent greater than can occur non-maliciously. 🧩
- Data Origin Authentication is the provision of the property that the claimed identity of the user on whose behalf received data was originated is corroborated. 🫡
- Data Confidentiality is the provision of the property that information is not made available or disclosed to unauthorized individuals, entities, or processes. 🤫
- Message timeliness and limited replay protection is the provision of the property that a message whose generation time is outside of a specified time window is not accepted. Note that message reordering is not dealt with and can occur in normal conditions too. ⌛
For the protocols specified in this memo, it is not possible to assure the specific originator of a received SNMP message. Instead, it is the user on whose behalf the message was originated that is authenticated. 🎀
For these protocols, it is not possible to obtain data integrity without data origin authentication, nor is it possible to obtain data origin authentication without data integrity. Further, there is no provision for data confidentiality without both data integrity and data origin authentication. 🔗
The security protocols used in this memo are considered acceptably secure at the time of writing. However, the procedures allow for new authentication and privacy methods to be specified at a future time if the need arises. 🔮
1.4. Module Organization 🧩
The security protocols defined in this memo are split into three different modules. Each one has its own specific responsibilities, and together they realize the goals and security services described above. 🍓
- The authentication module MUST provide for:
- Data Integrity,
- Data Origin Authentication
- The timeliness module MUST provide for:
- Protection against message delay or replay (to an extent greater than can occur through normal operation)
The privacy module MUST provide for
- Protection against disclosure of the message payload.
The timeliness module is fixed for the User-based Security Model, while there is provision for multiple authentication and/or privacy modules. Each of those implements a specific authentication or privacy protocol, respectively. 🧚
1.4.1. Timeliness Module ⏳
Section 3 (Elements of Procedure) uses the timeliness values in an SNMP message to do timeliness checking. The timeliness check is only performed if authentication is applied to the message. Since the complete message is checked for integrity, we can assume that the timeliness values in a message that passes the authentication module are trustworthy. 💯
1.4.2. Authentication Protocol 🔑
Section 6 describes the HMAC-MD5-96 authentication protocol, which is the first authentication protocol that MUST be supported with the User-based Security Model. Section 7 describes the HMAC-SHA-96 authentication protocol, which is another authentication protocol that SHOULD be supported with the User-based Security Model. In the future, additional or replacement authentication protocols may be defined as new needs arise. 🌟
The User-based Security Model prescribes that, if authentication is used, then the complete message is checked for integrity in the authentication module. 🔍
For a message to be authenticated, it needs to pass the authentication check by the authentication module and the timeliness check, which is a fixed part of this User-based Security Model. ✅
1.4.3. Privacy Protocol 🙈
Section 8 describes the CBC-DES Symmetric Encryption Protocol, which is the first privacy protocol to be used with the User-based Security Model. In the future, additional or replacement privacy protocols may be defined as new needs arise. 🦋
The User-based Security Model prescribes that the scopedPDU is protected from disclosure when a message is sent with privacy. 🫣
The User-based Security Model also prescribes that a message needs to be authenticated if privacy is in use. 💜
1.5. Protection against Message Replay, Delay and Redirection 🔁
1.5.1. Authoritative SNMP engine 👑
In order to protect against message replay, delay and redirection, one of the SNMP engines involved in each communication is designated to be the authoritative SNMP engine. When an SNMP message contains a payload which expects a response (for example a Get, GetNext, GetBulk, Set or Inform PDU), then the receiver of such messages is authoritative. When an SNMP message contains a payload which does not expect a response (for example an SNMPv2-Trap, Response or Report PDU), then the sender of such a message is authoritative. 🌸
1.5.2. Mechanisms ✨
okay so here's how they actually pull it off 🦋
- Okay bestie, first up 🕰️ we want to stop delayed or replayed messages from sneaking in, and I mean delays and replays way beyond what normal network lag could explain. So every message carries a set of timeliness indicators for the authoritative SNMP engine. An SNMP engine checks them to see if a received message is recent. It may also check that a message is at least as recent as the last message it got from that same source. A non-authoritative SNMP engine uses received authentic messages to advance its notion of the timeliness indicators at the remote authoritative source.
An SNMP engine MUST also use a mechanism to match incoming Responses to outstanding Requests, and it MUST drop any Responses that do not match an outstanding request. For example, a msgID can be inserted in every message to cater for this functionality. 🧚
These mechanisms let you detect authenticated messages whose time of generation was not recent. Main character energy 💅
This protection against delay or replay does not imply nor provide any protection against unauthorized deletion or suppression of messages. Also, an SNMP engine may not be able to detect message reordering if all the messages involved are sent within the Time Window interval. Other mechanisms defined independently of the security protocol can also be used to detect the re-ordering, replay, deletion, or suppression of messages containing Set operations (e.g., the MIB variable snmpSetSerialNo [RFC1907]). 🙈
- Verification that a message sent to/from one authoritative SNMP engine cannot be replayed to/as-if-from another authoritative SNMP engine 🔐
Included in each message is an identifier unique to the authoritative SNMP engine associated with the sender or intended recipient of the message.
A Report, Response or Trap message sent by an authoritative SNMP engine to one non-authoritative SNMP engine can potentially be replayed to another non-authoritative SNMP engine. The latter non-authoritative SNMP engine might (if it knows about the same userName with the same secrets at the authoritative SNMP engine) as a result update its notion of timeliness indicators of the authoritative SNMP engine, but that is not considered a threat. In this case, A Report or Response message will be discarded by the Message Processing Model, because there should not be an outstanding Request message. A Trap will possibly be accepted. Again, that is not considered a threat, because the communication was authenticated and timely. It is as if the authoritative SNMP engine was configured to start sending Traps to the second SNMP engine, which theoretically can happen without the knowledge of the second SNMP engine anyway. Anyway, the second SNMP engine may not expect to receive this Trap, but is allowed to see the management information contained in it. Honestly, this one's giving "it's not a threat, it's just a vibe" 🫠
- Detection of messages which were not recently generated ⏳
A set of time indicators are included in the message, indicating the time of generation. Messages without recent time indicators are not considered authentic. In addition, an SNMP engine MUST drop any Responses that do not match an outstanding request. This however is the responsibility of the Message Processing Model.
This memo allows the same user to be defined on multiple SNMP engines 👯♀️ Each SNMP engine keeps a value called snmpEngineID, which uniquely identifies that SNMP engine. This value is included in each message sent to/from the SNMP engine that is authoritative (see section 1.5.1). When a message arrives, an authoritative SNMP engine checks the value to make sure it's the intended recipient, and a non-authoritative SNMP engine uses the value to make sure the message gets processed with the correct state information.
Each SNMP engine maintains two values, snmpEngineBoots and snmpEngineTime, which taken together give an indication of time at that SNMP engine ⏰ Both of these values are included in an authenticated message sent to/received from that SNMP engine. On receipt, the values are checked to make sure the indicated timeliness value is within a Time Window of the current time. The Time Window is an administrative upper bound on acceptable delivery delay for protocol messages. Think of it as the cutoff for "this is too late, bestie" 🚫
For an SNMP engine to generate a message which an authoritative SNMP engine will accept as authentic, and to verify that a message received from that authoritative SNMP engine is authentic, such an SNMP engine must first achieve timeliness synchronization with the authoritative SNMP engine. See section 2.3. 🫶
1.6. Abstract Service Interfaces ✨
Abstract service interfaces have been defined to describe the conceptual interfaces between the various subsystems within an SNMP entity. Similarly, a set of abstract service interfaces have been defined within the User-based Security Model (USM) to describe the conceptual interfaces between the generic USM services and the self-contained authentication and privacy services. It's giving layers, layers, and more layers 🧁
These abstract service interfaces are defined by a set of primitives that define the services provided and the abstract data elements that must be passed when the services are invoked. This section lists the primitives that have been defined for the User-based Security Model. 📋
1.6.1. User-based Security Model Primitives for Authentication 🔑
The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the authentication service:
statusInformation =
authenticateOutgoingMsg(
IN authKey -- secret key for authentication
IN wholeMsg -- unauthenticated complete message
OUT authenticatedWholeMsg -- complete authenticated message
)
statusInformation =
authenticateIncomingMsg(
IN authKey -- secret key for authentication
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- complete authenticated message
)1.6.2. User-based Security Model Primitives for Privacy 🔒
The User-based Security Model provides the following internal primitives to pass data back and forth between the Security Model itself and the privacy service:
statusInformation =
encryptData(
IN encryptKey -- secret key for encryption
IN dataToEncrypt -- data to encrypt (scopedPDU)
OUT encryptedData -- encrypted data (encryptedPDU)
OUT privParameters -- filled in by service provider
)
statusInformation =
decryptData(
IN decryptKey -- secret key for decrypting
IN privParameters -- as received on the wire
IN encryptedData -- encrypted data (encryptedPDU)
OUT decryptedData -- decrypted data (scopedPDU)
)2. Elements of the Model 🧩
This section contains definitions required to realize the security model defined by this memo. Let's get into the nitty gritty 💜
2.1. User-based Security Model Users 👤
Management operations using this Security Model make use of a defined set of user identities. For any user on whose behalf management operations are authorized at a particular SNMP engine, that SNMP engine must have knowledge of that user. An SNMP engine that wishes to communicate with another SNMP engine must also have knowledge of a user known to that engine, including knowledge of the applicable attributes of that user.
A user and its attributes are defined as follows:
userName: A string representing the name of the user.securityName: A human-readable string representing the user in a format that is Security Model independent.authProtocol: An indication of whether messages sent on behalf of this user can be authenticated, and if so, the type of authentication protocol which is used. Two such protocols are defined in this memo:- the HMAC-MD5-96 authentication protocol.
- the HMAC-SHA-96 authentication protocol.
authKey: If messages sent on behalf of this user can be authenticated, the (private) authentication key for use with the authentication protocol. Note that a user's authentication key will normally be different at different authoritative SNMP engines. TheauthKeyis not accessible via SNMP. The length requirements of theauthKeyare defined by theauthProtocolin use. 🤫authKeyChangeandauthOwnKeyChange: The only way to remotely update the authentication key. Does that in a secure manner, so that the update can be completed without the need to employ privacy protection. 🔄privProtocol: An indication of whether messages sent on behalf of this user can be protected from disclosure, and if so, the type of privacy protocol which is used. One such protocol is defined in this memo: the CBC-DES Symmetric Encryption Protocol.privKey: If messages sent on behalf of this user can be en/decrypted, the (private) privacy key for use with the privacy protocol. Note that a user's privacy key will normally be different at different authoritative SNMP engines. TheprivKeyis not accessible via SNMP. The length requirements of theprivKeyare defined by theprivProtocolin use. 🗝️privKeyChangeandprivOwnKeyChange: The only way to remotely update the encryption key. Does that in a secure manner, so that the update can be completed without the need to employ privacy protection. 💌
2.2. Replay Protection 🔁
Each SNMP engine maintains three objects:
snmpEngineID, which (at least within an administrative domain) uniquely and unambiguously identifies an SNMP engine.snmpEngineBoots, which is a count of the number of times the SNMP engine has re-booted/re-initialized sincesnmpEngineIDwas last configured; and,snmpEngineTime, which is the number of seconds since thesnmpEngineBootscounter was last incremented.
Each SNMP engine is always authoritative with respect to these objects in its own SNMP entity. It is the responsibility of a non-authoritative SNMP engine to synchronize with the authoritative SNMP engine, as appropriate. 🤝
An authoritative SNMP engine is required to maintain the values of its snmpEngineID and snmpEngineBoots in non-volatile storage. Those two need to survive a reboot, so no cheating with RAM only 💾
2.2.1. msgAuthoritativeEngineID 🪪
The msgAuthoritativeEngineID value contained in an authenticated message is used to defeat attacks in which messages from one SNMP engine to another SNMP engine are replayed to a different SNMP engine. It represents the snmpEngineID at the authoritative SNMP engine involved in the exchange of the message. Basically it's the ID card that stops messages from getting passed off to the wrong engine 🚫🪄
When an authoritative SNMP engine is first installed, it sets its local value of snmpEngineID according to a enterprise-specific algorithm (see the definition of the Textual Convention for SnmpEngineID in the SNMP Architecture document [RFC2261]).
2.2.2. msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime ✨
okay so msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime are the fields that stop replay attacks, where somebody grabs an old authenticated message and tries to send it again after it's already expired 🙅. They carry the snmpEngineBoots and snmpEngineTime values from the authoritative SNMP engine that's part of the exchange 🕰️
Here's the lowkey genius part 💅 An SNMP engine does NOT need a clock that keeps ticking while it's powered off 🔌❌. Instead, every time an engine reboots, it grabs snmpEngineBoots, adds one to it, saves that to non-volatile storage, and resets snmpEngineTime to zero. No fancy hardware needed, bestie 🧚
When an SNMP engine is first installed, both snmpEngineBoots and snmpEngineTime get set to zero 🌱 If snmpEngineTime ever hits its max value of 2147483647, then snmpEngineBoots goes up as if the engine rebooted, snmpEngineTime resets to zero, and it starts counting up again 🔄
Every time an authoritative SNMP engine reboots, any SNMP engines that are holding its snmpEngineBoots and snmpEngineTime values need to re-synchronize before they send correctly authenticated messages to it. Check Section 2.3 for the (re-)synchronization steps 🤝 Also, the procedures do allow a notification to be accepted as authentic by a receiving SNMP engine when it was sent by an authoritative SNMP engine that has rebooted since the receiver last (re-)synchronized. Not me being impressed 😭
If an authoritative SNMP engine is ever unable to figure out its latest snmpEngineBoots value, then it MUST set snmpEngineBoots to 2147483647. 🚨
Once the local value of snmpEngineBoots sits at 2147483647, it latches there forever, and any authenticated message will always cause a notInTimeWindow authentication failure 💀
Warning, bestie: an engine stuck at2147483647can't just be fixed remotely. Manual intervention is REQUIRED. Someone has to physically visit the engine and reconfigure it, either with a newsnmpEngineIDvalue or with new secret values for the authentication and privacy protocols of every user known to that SNMP engine. 🛠️
Also, even if an SNMP engine rebooted once a second, it would still take about 68 years to reach the max value of 2147483647. That's giving long-term stability 🦋
2.2.3. Time Window 🌸
The Time Window is the window of time in which a message generated on behalf of any user is valid ⏰ This memo says the exact same Time Window, 150 seconds, is used for ALL users, so nobody gets special treatment 👑
2.3. Time Synchronization 🫶
Time synchronization is REQUIRED for a non-authoritative SNMP engine before it can proceed with authentic communications. It's considered done once the non-authoritative engine has gotten its own local copy of the authoritative SNMP engine's snmpEngineBoots and snmpEngineTime values from that authoritative engine 📥 Those values MUST be (and REMAIN) within the authoritative SNMP engine's Time Window, so the local copy has to stay loosely synced with what's actually stored at the authoritative engine ⏳
On top of keeping those copies, a non-authoritative SNMP engine MUST also keep one more local variable, latestReceivedEngineTime 🧠 This records the highest snmpEngineTime value it has ever received from the authoritative engine. It's what stops someone from replaying messages to freeze the non-authoritative engine's idea of snmpEngineTime so it never moves forward 🚫
A non-authoritative SNMP engine MUST keep local copies of these values for EACH authoritative SNMP engine it wants to talk to 💜 Since every authoritative SNMP engine is uniquely and unambiguously identified by its snmpEngineID, the non-authoritative engine MAY use that value as a key to cache its local copies 🗝️
Time synchronization happens as part of the procedure for receiving an SNMP message (Section 3.2, step 7b). So no explicit time synchronization procedure is REQUIRED for a non-authoritative SNMP engine. Just know that whenever the local value of snmpEngineID changes (for example, through discovery), or whenever secure communications are first set up with an authoritative SNMP engine, the local values of snmpEngineBoots and latestReceivedEngineTime SHOULD be set to zero. That makes time synchronization happen when the next authentic message arrives 🎀
2.4. SNMP Messages Using this Security Model 💖
An SNMP message using this Security Model follows the message format defined in the version-specific Message Processing Model document (for example, [RFC2262]) 📨
The msgSecurityParameters field in SNMPv3 messages is an OCTET STRING. Its value is the BER serialization of the ASN.1 sequence below 🧁
USMSecurityParametersSyntax DEFINITIONS IMPLICIT TAGS ::= BEGIN
UsmSecurityParameters ::=
SEQUENCE {
-- global User-based security parameters
msgAuthoritativeEngineID OCTET STRING,
msgAuthoritativeEngineBoots INTEGER (0..2147483647),
msgAuthoritativeEngineTime INTEGER (0..2147483647),
msgUserName OCTET STRING (SIZE(1..32)),
-- authentication protocol specific parameters
msgAuthenticationParameters OCTET STRING,
-- privacy protocol specific parameters
msgPrivacyParameters OCTET STRING
}
ENDHere's what each field in that sequence does 👇
msgAuthoritativeEngineIDtells you thesnmpEngineIDof the authoritative SNMP engine in the exchange 🪪msgAuthoritativeEngineBootstells you thesnmpEngineBootsvalue at that authoritative engine 🔁msgAuthoritativeEngineTimetells you thesnmpEngineTimevalue at that authoritative engine ⌚msgUserNamesays which user (principal) the message is being exchanged on behalf of 👤msgAuthenticationParametersare defined by whatever authentication protocol the message uses, as set in theusmUserAuthProtocolcolumn of that user's entry in theusmUserTable🔐msgPrivacyParametersare defined by whatever privacy protocol the message uses, as set in theusmUserPrivProtocolcolumn of that user's entry in theusmUserTable) 🙈
See appendix A.4 for an example of the BER encoding of the msgSecurityParameters field ✨
2.5. Services provided by the User-based Security Model 🦋
This section covers the services the User-based Security Model provides, along with their inputs and outputs 💅
The services are written up as primitives of an abstract service interface, and their inputs and outputs are written up as abstract data elements that get passed around in those abstract service primitives 🧚
2.5.1. Services for Generating an Outgoing SNMP Message ✨
okay so when the Message Processing (MP) Subsystem asks the User-based Security module to secure an outgoing SNMP message, it must use the right service that the Security module provides 💅. there are two of these, bestie:
- A service to generate a Request message 🎀. the abstract service primitive looks like this:
statusInformation = -- success or errorIndication
generateRequestMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
)- A service to generate a Response message 🦋. the abstract service primitive looks like this:
statusInformation = -- success or errorIndication
generateResponseMsg(
IN messageProcessingModel -- typically, SNMP version
IN globalData -- message header, admin data
IN maxMessageSize -- of the sending SNMP entity
IN securityModel -- for the outgoing message
IN securityEngineID -- authoritative SNMP entity
IN securityName -- on behalf of this principal
IN securityLevel -- Level of Security requested
IN scopedPDU -- message (plaintext) payload
IN securityStateReference -- reference to security state
-- information from original
-- request
OUT securityParameters -- filled in by Security Module
OUT wholeMsg -- complete generated message
OUT wholeMsgLength -- length of generated message
)the abstract data elements passed as parameters in these primitives work like this 🌸:
statusInformation: tells you whether encoding and securing the message was successful. if it wasn't, it tells you what the problem was.essageProcessingModel: the SNMP version number for the message to be generated. the User-based Security module doesn't use this one 🙈.globalData: the message header, aka its administrative info. the User-based Security module doesn't use this either.maxMessageSize: the maximum message size, as included in the message, of the sending SNMP entity. not used by the User-based Security module.securityParameters: the security parameters. these get filled in by the User-based Security module.securityModel: the securityModel in use. it should be the User-based Security Model. the User-based Security module doesn't use this data.securityName: together with the snmpEngineID, it identifies a row in the usmUserTable that gets used for securing the message. the securityName has a format that doesn't depend on the Security Model. for a response this parameter is ignored and the value from the cache is used instead 🧚.securityLevel: the Level of Security. the User-based Security module uses it to figure out whether the message needs protecting from disclosure and whether it needs authenticating. for a response this is ignored and the cached value is used.securityEngineID: the snmpEngineID of the authoritative SNMP engine a Request message is going to. for a response it's implied to be the processing SNMP engine's snmpEngineID, so if you do specify it, it gets ignored 💅.scopedPDU: the message payload. the data is opaque as far as the User-based Security Model is concerned 🫶.securityStateReference: a handle/reference to cachedSecurityData that gets used when securing an outgoing Response message. it's the exact same handle/reference the User-based Security module generated when it processed the incoming Request this Response answers. iconic, right? 👑wholeMsg: the fully encoded and secured message, ready to go out on the wire 🚀.wholeMsgLength: the length of the encoded and secured message (wholeMsg).
when the process is done, the User-based Security module returns statusInformation. if it worked, you get the completed message with privacy and authentication applied, as long as the specified securityLevel asked for them ✨. if it didn't work, you get an errorIndication 😭.
2.5.2. Services for Processing an Incoming SNMP Message 🔍
so, when the Message Processing (MP) Subsystem asks the User-based Security module to check that an incoming message is secure, it must use the service that's provided for incoming messages. the abstract service primitive is:
statusInformation = -- errorIndication or success
-- error counter OID/value if error
processIncomingMsg(
IN messageProcessingModel -- typically, SNMP version
IN maxMessageSize -- of the sending SNMP entity
IN securityParameters -- for the received message
IN securityModel -- for the received message
IN securityLevel -- Level of Security
IN wholeMsg -- as received on the wire
IN wholeMsgLength -- length as received on the wire
OUT securityEngineID -- authoritative SNMP entity
OUT securityName -- identification of the principal
OUT scopedPDU, -- message (plaintext) payload
OUT maxSizeResponseScopedPDU -- maximum size of the Response PDU
OUT securityStateReference -- reference to security state
) -- information, needed for responsethe abstract data elements passed as parameters in these primitives work like this 🌷:
statusInformation: tells you whether the process was successful. if it wasn't, the statusInformation includes the OID and the value of the error counter that got incremented 🙃.messageProcessingModel: the SNMP version number as received in the message. the User-based Security module doesn't use this.maxMessageSize: the maximum message size, as included in the message. the User-based Security module uses this value to calculate the maxSizeResponseScopedPDU 🧮.securityParameters: the security parameters, exactly as they were received in the message.securityModel: the securityModel in use. it should be the User-based Security Model. the User-based Security module doesn't use this data.securityLevel: the Level of Security. the User-based Security module uses it to figure out whether the message needs protecting from disclosure and whether it needs authenticating 🔐.wholeMsg: the whole message, exactly as it was received.wholeMsgLength: the length of the message as it was received (wholeMsg).securityEngineID: the snmpEngineID that was pulled out of the field msgAuthoritativeEngineID and that got used to look up the secrets in the usmUserTable 🗝️.securityName: the security name representing the user on whose behalf the message was received. the securityName has a format that's independent of the Security Model.scopedPDU: the message payload. the data is opaque as far as the User-based Security Model is concerned.maxSizeResponseScopedPDU: the maximum size of a scopedPDU that can be included in a possible Response message. the User-based Security module works this out from the mms (as received in the message) and the space needed for the message header, including the securityParameters, for that Response message 🎀.securityStateReference: a handle/reference to cachedSecurityData that gets used when securing an outgoing Response message. when the Message Processing Subsystem asks the User-based Security module to generate a response to this incoming message, it has to pass this handle/reference back in 💖.
when the process finishes, the User-based Security module returns statusInformation and, if it was successful, the extra data elements the message needs for further processing 🦋. if it wasn't successful, you get an errorIndication, possibly with an OID and value pair for whichever error counter got incremented.
2.6. Key Localization Algorithm. 🗝️
okay this one is so clever 🧠. a localized key is a secret key shared between a user U and one authoritative SNMP engine E. even though a user might only have one password, and therefore one key for the whole network, the actual secrets shared between the user and each authoritative SNMP engine are different. the trick that makes this happen is called key localization [Localized-key] 💜.
first, if a user uses a password, that password gets converted into a key Ku using one of the two algorithms described in Appendices A.2.1 and A.2.2 🌈.
to turn the key Ku into a localized key Kul of user U at the authoritative SNMP engine E, you append the snmpEngineID of the authoritative SNMP engine to the key Ku, then append the key Ku to the result. that way the snmpEngineID gets wrapped up inside two copies of the user's key Ku 🎁. then you run a secure hash function on it. which hash function depends on the authentication protocol defined for this user U at authoritative SNMP engine E. this document defines two authentication protocols with their associated algorithms, based on MD5 and SHA 🔥. the output of the hash function is the localized key Kul for user U at the authoritative SNMP engine E ✨.
3. Elements of Procedure ✨
okay, this section is where the good stuff is 🌸. it describes the security related procedures that an SNMP engine follows when it processes SNMP messages according to the User-based Security Model 💅.
3.1. Generating an Outgoing SNMP Message ✨
okay so this section is the recipe 🍳 an SNMP engine follows whenever it builds a message with a management operation (a request, a response, a notification, or a report) on behalf of a user at a particular securityLevel. let's get into it 💅
- a) If any securityStateReference is passed (Response message), then information concerning the user is extracted from the cachedSecurityData. The securityEngineID and the securityLevel are extracted from the cachedSecurityData. The cachedSecurityData can now be discarded.
Otherwise,
b) based on the securityName, information concerning the user at the destination snmpEngineID, specified by the securityEngineID, is extracted from the Local Configuration Datastore (LCD, usmUserTable). If information about the user is absent from the LCD, then an error indication (unknownSecurityName) is returned to the calling module. 🙈
- If the securityLevel specifies that the message is to be protected from disclosure, but the user does not support both an authentication and a privacy protocol then the message cannot be sent. An error indication (unsupportedSecurityLevel) is returned to the calling module. no privacy AND auth support? the message just doesn't go out 💀
- If the securityLevel specifies that the message is to be authenticated, but the user does not support an authentication protocol, then the message cannot be sent. An error indication (unsupportedSecurityLevel) is returned to the calling module.
- a) If the securityLevel specifies that the message is to be protected from disclosure, then the octet sequence representing the serialized scopedPDU is encrypted according to the user's privacy protocol. To do so, a call is made to the privacy module that implements the user's privacy protocol according to the abstract primitive:
statusInformation = -- success or failure
encryptData(
IN encryptKey -- user's localized privKey
IN dataToEncrypt -- serialized scopedPDU
OUT encryptedData -- serialized encryptedPDU
OUT privParameters -- serialized privacy parameters
)statusInformation indicates if the encryption process was successful or not.
encryptKey is the user's localized private privKey, the secret key that can be used by the encryption algorithm.
dataToEncrypt is the serialized scopedPDU, the data that is to be encrypted.
encryptedData is the encryptedPDU, which represents the encrypted scopedPDU, encoded as an OCTET STRING.
privParameters are the privacy parameters, encoded as an OCTET STRING.
If the privacy module returns failure, then the message cannot be sent and an error indication (encryptionError) is returned to the calling module.
If the privacy module returns success, then the returned privParameters are put into the msgPrivacyParameters field of the securityParameters and the encryptedPDU serves as the payload of the message being prepared.
Otherwise,
b) If the securityLevel specifies that the message is not to be protected from disclosure, then the NULL string is encoded as an OCTET STRING and put into the msgPrivacyParameters field of the securityParameters, and the plaintext scopedPDU serves as the payload of the message being prepared.
- The snmpEngineID is encoded as an OCTET STRING into the msgAuthoritativeEngineID field of the securityParameters. Note that an empty (zero length) snmpEngineID is OK for a Request message, because that will cause the remote (authoritative) SNMP engine to return a Report PDU with the proper snmpEngineID included in the msgAuthoritativeEngineID in the securityParameters of that returned Report PDU. 🦋
- a) If the securityLevel specifies that the message is to be authenticated, then the current values of snmpEngineBoots and snmpEngineTime corresponding to the snmpEngineID from the LCD are used.
Otherwise,
b) If this is a Response message, then the current value of snmpEngineBoots and snmpEngineTime corresponding to the local snmpEngineID from the LCD are used.
Otherwise,
c) If this is a Request message, then a zero value is used for both snmpEngineBoots and snmpEngineTime. This zero value gets used if snmpEngineID is empty.
The values are encoded as INTEGER respectively into the msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields of the securityParameters. 🧚
- The userName is encoded as an OCTET STRING into the msgUserName field of the securityParameters.
- a) If the securityLevel specifies that the message is to be authenticated, the message is authenticated according to the user's authentication protocol. To do so, a call is made to the authentication module that implements the user's authentication protocol according to the abstract service primitive:
statusInformation =
authenticateOutgoingMsg(
IN authKey -- the user's localized authKey
IN wholeMsg -- unauthenticated message
OUT authenticatedWholeMsg -- authenticated complete message
)statusInformation indicates if authentication was successful or not.
authKey is the user's localized private authKey, the secret key that can be used by the authentication algorithm.
wholeMsg is the complete serialized message to be authenticated.
authenticatedWholeMsg is the same as the input given to the authenticateOutgoingMsg service, but with msgAuthenticationParameters properly filled in.
If the authentication module returns failure, then the message cannot be sent and an error indication (authenticationFailure) is returned to the calling module.
If the authentication module returns success, then the msgAuthenticationParameters field is put into the securityParameters and the authenticatedWholeMsg represents the serialization of the authenticated message being prepared.
Otherwise,
b) If the securityLevel specifies that the message is not to be authenticated, then the NULL string is encoded as an OCTET STRING into the msgAuthenticationParameters field of the securityParameters. The wholeMsg is now serialized and then represents the unauthenticated message being prepared.
- The completed message with its length is returned to the calling module with the statusInformation set to success. 🎀
3.2. Processing an Incoming SNMP Message ✨
this section is the flip side 🔄 the procedure an SNMP engine follows whenever it receives a message with a management operation on behalf of a user at a particular securityLevel.
the release of state information isn't always spelled out step by step, to keep things simple 🧹 as a general rule, if state information is available when a message gets discarded, the state information should also be released. also, when an error indication with an OID and value for an incremented counter is returned, the available information (like securityStateReference) must be passed back to the caller so it can generate a Report PDU. this part is lowkey important, don't skip it bestie 👑
- If the received securityParameters is not the serialization (according to the conventions of [RFC1906]) of an OCTET STRING formatted according to the UsmSecurityParameters defined in section 2.4, then the snmpInASNParseErrs counter [RFC1907] is incremented, and an error indication (parseError) is returned to the calling module. Note that we return without the OID and value of the incremented counter, because in this case there is not enough information to generate a Report PDU. 💀
- The values of the security parameter fields are extracted from the securityParameters. The securityEngineID to be returned to the caller is the value of the msgAuthoritativeEngineID field. The cachedSecurityData is prepared and a securityStateReference is prepared to reference this data. Values to be cached are:
- msgUserName
- securityEngineID
- securityLevel
- If the value of the msgAuthoritativeEngineID field in the securityParameters is unknown, then:
a) a non-authoritative SNMP engine that performs discovery may optionally create a new entry in its Local Configuration Datastore (LCD) and continue processing;
or
b) the usmStatsUnknownEngineIDs counter is incremented, and an error indication (unknownEngineID) together with the OID and value of the incremented counter is returned to the calling module.
- Information about the value of the msgUserName and msgAuthoritativeEngineID fields is extracted from the Local Configuration Datastore (LCD, usmUserTable). If no information is available for the user, then the usmStatsUnknownUserNames counter is incremented and an error indication (unknownSecurityName) together with the OID and value of the incremented counter is returned to the calling module. 🙈
- If the information about the user indicates that it does not support the securityLevel requested by the caller, then the usmStatsUnsupportedSecLevels counter is incremented and an error indication (unsupportedSecurityLevel) together with the OID and value of the incremented counter is returned to the calling module.
- If the securityLevel specifies that the message is to be authenticated, then the message is authenticated according to the user's authentication protocol. To do so, a call is made to the authentication module that implements the user's authentication protocol according to the abstract service primitive:
statusInformation = -- success or failure
authenticateIncomingMsg(
IN authKey -- the user's localized authKey
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- checked for authentication
)statusInformation indicates if authentication was successful or not.
authKey is the user's localized private authKey, the secret key that can be used by the authentication algorithm.
wholeMsg is the complete serialized message to be authenticated.
authenticatedWholeMsg is the same as the input given to the authenticateIncomingMsg service, but after authentication has been checked.
If the authentication module returns failure, then the message cannot be trusted, so the usmStatsWrongDigests counter is incremented and an error indication (authenticationFailure) together with the OID and value of the incremented counter is returned to the calling module. not trusted = counter goes up and the error goes back, no exceptions 🚫
If the authentication module returns success, then the message is authentic and can be trusted, so processing continues. ✨
- okay bestie, step 7 is where the time checks get SERIOUS 🕰️✨ If the securityLevel says the message is authenticated, the local snmpEngineBoots and snmpEngineTime that match the msgAuthoritativeEngineID value get pulled out of the Local Configuration Datastore 💅
a) If the extracted msgAuthoritativeEngineID is the same as your own snmpEngineID (so you're the authoritative engine 👑), the message counts as outside of the Time Window if ANY of these is true:
- the local snmpEngineBoots is 2147483647;
- the msgAuthoritativeEngineBoots field differs from the local snmpEngineBoots; or
- the msgAuthoritativeEngineTime field differs from your local snmpEngineTime by more than +/- 150 seconds.
If it's outside the Time Window, the usmStatsNotInTimeWindows counter goes up 📈 and you return an error indication (notInTimeWindow) along with the OID and value of that counter 🙈
b) If the extracted msgAuthoritativeEngineID is NOT the same as your snmpEngineID (so you're not the authoritative one, bestie 🫶), then:
- if at least one of these is true:
- the extracted msgAuthoritativeEngineBoots is greater than your local snmpEngineBoots; or
- the extracted msgAuthoritativeEngineBoots equals your local snmpEngineBoots AND the extracted msgAuthoritativeEngineTime is greater than latestReceivedEngineTime,
then the LCD entry for that msgAuthoritativeEngineID gets updated 🔄. You set the local snmpEngineBoots to the msgAuthoritativeEngineBoots field, the local snmpEngineTime to the msgAuthoritativeEngineTime field, and latestReceivedEngineTime to the value of the value of the msgAuthoritativeEngineTime field.
- if any of these is true, the message counts as outside of the Time Window:
- the local snmpEngineBoots is 2147483647;
- the msgAuthoritativeEngineBoots field is less than your local snmpEngineBoots; or
- the msgAuthoritativeEngineBoots field equals your local snmpEngineBoots AND the msgAuthoritativeEngineTime field is more than 150 seconds less than your local snmpEngineTime.
If it's outside the Time Window, you return an error indication (notInTimeWindow) 🚫
Note that this means a too old (possibly replayed) message has been detected, and it's deemed unauthentic. Not it 💀
Note that this procedure lets msgAuthoritativeEngineBoots in the message be greater than your local snmpEngineBoots. That way, messages from an authoritative engine that rebooted since the receiving engine last (re-)synchronized can still be accepted as authentic 🦋
Note that this procedure does not allow automatic time synchronization when the non-authoritative engine is really out of sync, meaning the authoritative engine is more than 150 seconds behind the non-authoritative engine 🥺
- a) If the securityLevel says the message was protected from disclosure, the OCTET STRING representing the encryptedPDU gets decrypted using the user's privacy protocol, so you end up with an unencrypted serialized scopedPDU value 🔐 To do that, you call the privacy module that implements the user's privacy protocol, using this abstract primitive:
statusInformation = -- success or failure
decryptData(
IN decryptKey -- the user's localized privKey
IN privParameters -- as received on the wire
IN encryptedData -- encryptedPDU as received
OUT decryptedData -- serialized decrypted scopedPDU
)Here's what each piece means:
- statusInformation tells you whether the decryption worked or not.
- decryptKey is the user's localized private privKey, the secret key the decryption algorithm can use.
- privParameters is the msgPrivacyParameters, encoded as an OCTET STRING.
- encryptedData is the encryptedPDU, which represents the encrypted scopedPDU, encoded as an OCTET STRING.
- decryptedData is the serialized scopedPDU, if decryption is successful.
If the privacy module returns failure, the message can't be processed 😭 So the usmStatsDecryptionErrors counter goes up and you return an error indication (decryptionError) together with the OID and value of that counter.
If the privacy module returns success, the decrypted scopedPDU is the message payload that goes back to the calling module 🎉
Otherwise,
b) The scopedPDU component is assumed to be in plain text, and it's the message payload that goes back to the calling module 📝
- The maxSizeResponseScopedPDU gets calculated. This is the maximum size allowed for a scopedPDU in a possible Response message. It leaves room for a message header that allows the same securityLevel as the received Request 📏
- The securityName for the user is retrieved from the usmUserTable 🧾
- The security data gets cached as cachedSecurityData, so a possible response to this message can use the same authentication and privacy secrets, the same securityLevel, and the same msgAuthoritativeEngineID value. Here's what gets saved:
- msgUserName,
- usmUserAuthProtocol, usmUserAuthKey
- usmUserPrivProtocol, usmUserPrivKey
- securityEngineID, securityLevel
- The statusInformation is set to success, and you return to the calling module, passing back the OUT parameters as specified in the processIncomingMsg primitive ✨
4. Discovery 🔍
The User-based Security Model needs a discovery process to get enough info about other SNMP engines so they can talk to each other 🤝 Discovery means a non-authoritative SNMP engine has to learn the authoritative SNMP engine's snmpEngineID value before communication can go ahead. This can happen by generating a Request message with a securityLevel of noAuthNoPriv, a msgUserName of "initial", a msgAuthoritativeEngineID value of zero length, and the varBindList left empty. The response will be a Report message with the snmpEngineID of the authoritative SNMP engine as the value of the msgAuthoritativeEngineID field inside the msgSecurityParameters field. It also contains a Report PDU with the usmStatsUnknownEngineIDs counter in the varBindList 💖
If authenticated communication is required, the discovery process should also set up time synchronization with the authoritative SNMP engine ⏰ You can do this by sending an authenticated Request message where msgAuthoritativeEngineID is set to the newly learned snmpEngineID, and msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime are both set to zero. The response will be a Report message with the up to date values of the authoritative SNMP engine's snmpEngineBoots and snmpEngineTime, as the value of the msgAuthoritativeEngineBoots and msgAuthoritativeEngineTime fields respectively. It also has the usmStatsNotInTimeWindows counter in the varBindList of the Report PDU. After that, time synchronization happens automatically through the procedures in section 3.2 step 7b. See also section 2.3 🧚
5. Definitions 🎀
Here's the MIB module, and it's giving very official vibes 💅
SNMP-USER-BASED-SM-MIB DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
OBJECT-IDENTITY,
snmpModules, Counter32 FROM SNMPv2-SMI
TEXTUAL-CONVENTION, TestAndIncr,
RowStatus, RowPointer,
StorageType, AutonomousType FROM SNMPv2-TC
MODULE-COMPLIANCE, OBJECT-GROUP FROM SNMPv2-CONF
SnmpAdminString, SnmpEngineID,
snmpAuthProtocols, snmpPrivProtocols FROM SNMP-FRAMEWORK-MIB;
snmpUsmMIB MODULE-IDENTITY
LAST-UPDATED "9711200000Z" -- 20 Nov 1997, midnight
ORGANIZATION "SNMPv3 Working Group"
CONTACT-INFO "WG-email: snmpv3@tis.com
Subscribe: majordomo@tis.com
In msg body: subscribe snmpv3
Chair: Russ Mundy
Trusted Information Systems
postal: 3060 Washington Rd
Glenwood MD 21738
USA
email: mundy@tis.com
phone: +1-301-854-6889
Co-editor Uri Blumenthal
IBM T. J. Watson Research
postal: 30 Saw Mill River Pkwy,
Hawthorne, NY 10532
USA
email: uri@watson.ibm.com
phone: +1-914-784-7964
Co-editor: Bert Wijnen
IBM T. J. Watson Research
postal: Schagen 33
3461 GL Linschoten
Netherlands
email: wijnen@vnet.ibm.com
phone: +31-348-432-794
"
DESCRIPTION "The management information definitions for the
SNMP User-based Security Model.
"
::= { snmpModules 4 }
-- Administrative assignments ****************************************
usmMIBObjects OBJECT IDENTIFIER ::= { snmpUsmMIB 1 }
usmMIBConformance OBJECT IDENTIFIER ::= { snmpUsmMIB 2 }
-- Identification of Authentication and Privacy Protocols ************
usmNoAuthProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "No Authentication Protocol."
::= { snmpAuthProtocols 1 }
usmHMACMD5AuthProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The HMAC-MD5-96 Digest Authentication Protocol."
REFERENCE "- H. Krawczyk, M. Bellare, R. Canetti HMAC:
Keyed-Hashing for Message Authentication,
RFC2104, Feb 1997.
- Rivest, R., Message Digest Algorithm MD5, RFC1321.
"
::= { snmpAuthProtocols 2 }okay so we're still in the MIB, bestie 🎀 the next protocol up is HMAC-SHA, and it's giving main character energy 💅
usmHMACSHAAuthProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The HMAC-SHA-96 Digest Authentication Protocol."
REFERENCE "- H. Krawczyk, M. Bellare, R. Canetti, HMAC:
Keyed-Hashing for Message Authentication,
RFC2104, Feb 1997.
- Secure Hash Algorithm. NIST FIPS 180-1.
"
::= { snmpAuthProtocols 3 }
usmNoPrivProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "No Privacy Protocol."
::= { snmpPrivProtocols 1 }
usmDESPrivProtocol OBJECT-IDENTITY
STATUS current
DESCRIPTION "The CBC-DES Symmetric Encryption Protocol."
REFERENCE "- Data Encryption Standard, National Institute of
Standards and Technology. Federal Information
Processing Standard (FIPS) Publication 46-1.
Supersedes FIPS Publication 46,
(January, 1977; reaffirmed January, 1988).
- Data Encryption Algorithm, American National
Standards Institute. ANSI X3.92-1981,
(December, 1980).
- DES Modes of Operation, National Institute of
Standards and Technology. Federal Information
Processing Standard (FIPS) Publication 81,
(December, 1980).
- Data Encryption Algorithm - Modes of Operation,
American National Standards Institute.
ANSI X3.106-1983, (May 1983).
"
::= { snmpPrivProtocols 2 }Next up is the textual conventions bit, and this is where it gets spicy 🌶️ The KeyChange convention is basically how you rotate a secret key without ever sending the actual key over the wire. Iconic, honestly 😭✨
-- Textual Conventions ***********************************************
KeyChange ::= TEXTUAL-CONVENTION
STATUS current
DESCRIPTION
"Every definition of an object with this syntax must identify
a protocol P, a secret key K, and a hash algorithm H
that produces output of L octets.
The object's value is a manager-generated, partially-random
value which, when modified, causes the value of the secret
key K, to be modified via a one-way function.
The value of an instance of this object is the concatenation
of two components: first a 'random' component and then a
'delta' component.
The lengths of the random and delta components
are given by the corresponding value of the protocol P;
if P requires K to be a fixed length, the length of both the
random and delta components is that fixed length; if P
allows the length of K to be variable up to a particular
maximum length, the length of the random component is that
maximum length and the length of the delta component is any
length less than or equal to that maximum length.
For example, usmHMACMD5AuthProtocol requires K to be a fixed
length of 16 octets and L - of 16 octets.
usmHMACSHAAuthProtocol requires K to be a fixed length of
20 octets and L - of 20 octets. Other protocols may define
other sizes, as deemed appropriate.
When a requestor wants to change the old key K to a new
key keyNew on a remote entity, the 'random' component is
obtained from either a true random generator, or from a
pseudorandom generator, and the 'delta' component is
computed as follows:
- a temporary variable is initialized to the existing value
of K;
- if the length of the keyNew is greater than L octets,
then:
- the random component is appended to the value of the
temporary variable, and the result is input to the
the hash algorithm H to produce a digest value, and
the temporary variable is set to this digest value;
- the value of the temporary variable is XOR-ed with
the first (next) L-octets (16 octets in case of MD5)
of the keyNew to produce the first (next) L-octets
(16 octets in case of MD5) of the 'delta' component.
- the above two steps are repeated until the unused
portion of the delta component is L octets or less,
- the random component is appended to the value of the
temporary variable, and the result is input to the
hash algorithm H to produce a digest value;
- this digest value, truncated if necessary to be the same
length as the unused portion of the keyNew, is XOR-ed
with the unused portion of the keyNew to produce the
(final portion of the) 'delta' component.
For example, using MD5 as the hash algorithm H:
iterations = (lenOfDelta - 1)/16; /* integer division */
temp = keyOld;
for (i = 0; i < iterations; i++) {
temp = MD5 (temp || random);
delta[i*16 .. (i*16)+15] =
temp XOR keyNew[i*16 .. (i*16)+15];
}
temp = MD5 (temp || random);
delta[i*16 .. lenOfDelta-1] =
temp XOR keyNew[i*16 .. lenOfDelta-1];
The 'random' and 'delta' components are then concatenated as
described above, and the resulting octet string is sent to
the receipient as the new value of an instance of this
object.
At the receiver side, when an instance of this object is set
to a new value, then a new value of K is computed as follows:
- a temporary variable is initialized to the existing value
of K;
- if the length of the delta component is greater than L
octets, then:
- the random component is appended to the value of the
temporary variable, and the result is input to the
the hash algorithm H to produce a digest value, and
the temporary variable is set to this digest value;
- the value of the temporary variable is XOR-ed with
the first (next) L-octets (16 octets in case of MD5)
of the delta component to produce the first (next)
L-octets (16 octets in case of MD5) of the new value
of K.
- the above two steps are repeated until the unused
portion of the delta component is L octets or less,
- the random component is appended to the value of the
temporary variable, and the result is input to the
hash algorithm H to produce a digest value;
- this digest value, truncated if necessary to be the same
length as the unused portion of the delta component, is
XOR-ed with the unused portion of the delta component to
produce the (final portion of the) new value of K.
For example, using MD5 as the hash algorithm H:
iterations = (lenOfDelta - 1)/16; /* integer division */
temp = keyOld;
for (i = 0; i < iterations; i++) {
temp = MD5 (temp || random);
keyNew[i*16 .. (i*16)+15] =
temp XOR delta[i*16 .. (i*16)+15];
}
temp = MD5 (temp || random);
keyNew[i*16 .. lenOfDelta-1] =
temp XOR delta[i*16 .. lenOfDelta-1];
The value of an object with this syntax, whenever it is
retrieved by the management protocol, is always the zero
length string.
"
SYNTAX OCTET STRINGNow the stats 📊 these counters are basically the tea on every dropped packet 🍵 each one tells you exactly why a packet got yeeted
-- Statistics for the User-based Security Model **********************
usmStats OBJECT IDENTIFIER ::= { usmMIBObjects 1 }
usmStatsUnsupportedSecLevels OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they requested a
securityLevel that was unknown to the SNMP engine
or otherwise unavailable.
"
::= { usmStats 1 }
usmStatsNotInTimeWindows OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they appeared
outside of the authoritative SNMP engine's window.
"
::= { usmStats 2 }
usmStatsUnknownUserNames OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they referenced a
user that was not known to the SNMP engine.
"
::= { usmStats 3 }
usmStatsUnknownEngineIDs OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they referenced an
snmpEngineID that was not known to the SNMP engine.
"
::= { usmStats 4 }
usmStatsWrongDigests OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they didn't
contain the expected digest value.
"
::= { usmStats 5 }
usmStatsDecryptionErrors OBJECT-TYPE
SYNTAX Counter32
MAX-ACCESS read-only
STATUS current
DESCRIPTION "The total number of packets received by the SNMP
engine which were dropped because they could not be
decrypted.
"
::= { usmStats 6 }Last bit of this chunk is the usmUser group 🦋 it's the table of users, plus a spin lock so multiple apps don't step on each other's toes when they're changing secrets 🫶
-- The usmUser Group ************************************************
usmUser OBJECT IDENTIFIER ::= { usmMIBObjects 2 }
usmUserSpinLock OBJECT-TYPE
SYNTAX TestAndIncr
MAX-ACCESS read-write
STATUS current
DESCRIPTION "An advisory lock used to allow several cooperating
Command Generator Applications to coordinate their
use of facilities to alter secrets in the
usmUserTable.
"
::= { usmUser 1 }
-- The table of valid users for the User-based Security Model ********
usmUserTable OBJECT-TYPE
SYNTAX SEQUENCE OF UsmUserEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "The table of users configured in the SNMP engine's
Local Configuration Datastore (LCD)."
::= { usmUser 2 }
usmUserEntry OBJECT-TYPE
SYNTAX UsmUserEntry
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A user configured in the SNMP engine's Local
Configuration Datastore (LCD) for the User-based
Security Model.
"
INDEX { usmUserEngineID,
usmUserName
}
::= { usmUserTable 1 }
UsmUserEntry ::= SEQUENCE
{
usmUserEngineID SnmpEngineID,
usmUserName SnmpAdminString,
usmUserSecurityName SnmpAdminString,
usmUserCloneFrom RowPointer,
usmUserAuthProtocol AutonomousType,
usmUserAuthKeyChange KeyChange,
usmUserOwnAuthKeyChange KeyChange,
usmUserPrivProtocol AutonomousType,
usmUserPrivKeyChange KeyChange,
usmUserOwnPrivKeyChange KeyChange,
usmUserPublic OCTET STRING,
usmUserStorageType StorageType,
usmUserStatus RowStatus
}okay so we're deep in the usmUser table now and every single column is giving main character energy 💅🎀
usmUserEngineID OBJECT-TYPE
SYNTAX SnmpEngineID
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "An SNMP engine's administratively-unique identifier.
In a simple agent, this value is always that agent's
own snmpEngineID value.
The value can also take the value of the snmpEngineID
of a remote SNMP engine with which this user can
communicate.
"
::= { usmUserEntry 1 }
usmUserName OBJECT-TYPE
SYNTAX SnmpAdminString (SIZE(1..32))
MAX-ACCESS not-accessible
STATUS current
DESCRIPTION "A human readable string representing the name of
the user.
This is the (User-based Security) Model dependent
security ID.
"
::= { usmUserEntry 2 }
usmUserSecurityName OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-only
STATUS current
DESCRIPTION "A human readable string representing the user in
Security Model independent format.
The default transformation of the User-based Security
Model dependent security ID to the securityName and
vice versa is the identity function so that the
securityName is the same as the userName.
"
::= { usmUserEntry 3 }okay the usmUserName and engine ID combo is the key to the whole row, so these two are kinda the VIPs 🔑👑
usmUserCloneFrom OBJECT-TYPE
SYNTAX RowPointer
MAX-ACCESS read-create
STATUS current
DESCRIPTION "A pointer to another conceptual row in this
usmUserTable. The user in this other conceptual
row is called the clone-from user.
When a new user is created (i.e., a new conceptual
row is instantiated in this table), the privacy and
authentication parameters of the new user are cloned
from its clone-from user.
The first time an instance of this object is set by
a management operation (either at or after its
instantiation), the cloning process is invoked.
Subsequent writes are successful but invoke no
action to be taken by the receiver.
The cloning process fails with an 'inconsistentName'
error if the conceptual row representing the
clone-from user is not in an active state when the
cloning process is invoked.
Cloning also causes the initial values of the secret
authentication key and the secret encryption key of
the new user to be set to the same value as the
corresponding secret of the clone-from user.
When this object is read, the ZeroDotZero OID
is returned.
"
::= { usmUserEntry 4 }
usmUserAuthProtocol OBJECT-TYPE
SYNTAX AutonomousType
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An indication of whether messages sent on behalf of
this user to/from the SNMP engine identified by
usmUserEngineID, can be authenticated, and if so,
the type of authentication protocol which is used.
An instance of this object is created concurrently
with the creation of any other object instance for
the same user (i.e., as part of the processing of
the set operation which creates the first object
instance in the same conceptual row). Once created,
the value of an instance of this object can not be
changed.
If a set operation tries to set a value for an unknown
or unsupported protocol, then a wrongValue error must
be returned.
"
DEFVAL { usmHMACMD5AuthProtocol }
::= { usmUserEntry 5 }
usmUserAuthKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An object, which when modified, causes the secret
authentication key used for messages sent on behalf
of this user to/from the SNMP engine identified by
usmUserEngineID, to be modified via a one-way
function.
The associated protocol is the usmUserAuthProtocol.
The associated secret key is the user's secret
authentication key (authKey). The associated hash
algorithm is the algorithm used by the user's
usmUserAuthProtocol.
When creating a new user, it is an 'inconsistentName'
error for a Set operation to refer to this object
unless it is previously or concurrently initialized
through a set operation on the corresponding value
of usmUserCloneFrom.
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 6 }
usmUserOwnAuthKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "Behaves exactly as usmUserAuthKeyChange, with one
notable difference: in order for the Set operation
to succeed, the usmUserName of the operation
requester must match the usmUserName that
indexes the row which is targeted by this
operation.
The idea here is that access to this column can be
public, since it will only allow a user to change
his own secret authentication key (authKey).
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 7 }
usmUserPrivProtocol OBJECT-TYPE
SYNTAX AutonomousType
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An indication of whether messages sent on behalf of
this user to/from the SNMP engine identified by
usmUserEngineID, can be protected from disclosure,
and if so, the type of privacy protocol which is used.
An instance of this object is created concurrently
with the creation of any other object instance for
the same user (i.e., as part of the processing of
the set operation which creates the first object
instance in the same conceptual row). Once created,
the value of an instance of this object can not be
changed.
If a set operation tries to set a value for an unknown
or unsupported protocol, then a wrongValue error must
be returned.
"
DEFVAL { usmNoPrivProtocol }
::= { usmUserEntry 8 }
usmUserPrivKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "An object, which when modified, causes the secret
encryption key used for messages sent on behalf
of this user to/from the SNMP engine identified by
usmUserEngineID, to be modified via a one-way
function.
The associated protocol is the usmUserPrivProtocol.
The associated secret key is the user's secret
privacy key (privKey). The associated hash
algorithm is the algorithm used by the user's
usmUserAuthProtocol.
When creating a new user, it is an 'inconsistentName'
error for a set operation to refer to this object
unless it is previously or concurrently initialized
through a set operation on the corresponding value
of usmUserCloneFrom.
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 9 }
usmUserOwnPrivKeyChange OBJECT-TYPE
SYNTAX KeyChange -- typically (SIZE (0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "Behaves exactly as usmUserPrivKeyChange, with one
notable difference: in order for the Set operation
to succeed, the usmUserName of the operation
requester must match the usmUserName that indexes
the row which is targeted by this operation.
The idea here is that access to this column can be
public, since it will only allow a user to change
his own secret privacy key (privKey).
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 10 }
usmUserPublic OBJECT-TYPE
SYNTAX OCTET STRING (SIZE(0..32))
MAX-ACCESS read-create
STATUS current
DESCRIPTION "A publicly-readable value which is written as part
of the procedure for changing a user's secret
authentication and/or privacy key, and later read to
determine whether the change of the secret was
effected.
"
DEFVAL { ''H } -- the empty string
::= { usmUserEntry 11 }
usmUserStorageType OBJECT-TYPE
SYNTAX StorageType
MAX-ACCESS read-create
STATUS current
DESCRIPTION "The storage type for this conceptual row.
Conceptual rows having the value 'permanent'
must allow write-access at a minimum to:
- usmUserAuthKeyChange, usmUserOwnAuthKeyChange
and usmUserPublic for a user who employs
authentication, and
- usmUserPrivKeyChange, usmUserOwnPrivKeyChange
and usmUserPublic for a user who employs
privacy.
Note that any user who employs authentication or
privacy must allow its secret(s) to be updated and
thus cannot be 'readOnly'.
"
DEFVAL { nonVolatile }
::= { usmUserEntry 12 }
usmUserStatus OBJECT-TYPE
SYNTAX RowStatus
MAX-ACCESS read-create
STATUS current
DESCRIPTION "The status of this conceptual row.
Until instances of all corresponding columns are
appropriately configured, the value of the
corresponding instance of the usmUserStatus column
is 'notReady'.
In particular, a newly created row cannot be made
active until the corresponding usmUserCloneFrom,
usmUserAuthKeyChange, usmUserOwnAuthKeyChange,
usmUserPrivKeyChange and usmUserOwnPrivKeyChange
have all been set.
The RowStatus TC [RFC1903] requires that this
DESCRIPTION clause states under which circumstances
other objects in this row can be modified:
The value of this object has no effect on whether
other objects in this conceptual row can be modified.
"
::= { usmUserEntry 13 }
-- Conformance Information *******************************************
usmMIBCompliances OBJECT IDENTIFIER ::= { usmMIBConformance 1 }
usmMIBGroups OBJECT IDENTIFIER ::= { usmMIBConformance 2 }
-- Compliance statements6. HMAC-MD5-96 Authentication Protocol ✨
Okay bestie, this section is all about HMAC-MD5-96 🔐. It's the very first authentication protocol defined for the User-based Security Model 👑. It uses the MD5 hash function from [MD5], in the HMAC mode from [RFC2104], and then chops the output down to 96 bits ✂️.
This protocol is identified by usmHMACMD5AuthProtocol 🆔.
Over time, other authentication protocols may be defined, either to replace this one or to sit alongside it 🦋.
6.1. Mechanisms
- In support of data integrity, a message digest algorithm is required. A digest is calculated over an appropriate portion of an SNMP message and included as part of the message sent to the recipient 💌.
- In support of data origin authentication and data integrity, a secret value is prepended to the SNMP message before the digest is computed. The calculated digest is partially inserted into the SNMP message before transmission, and the prepended value is never transmitted 🤫. The secret value is shared by all SNMP engines authorized to originate messages on behalf of the appropriate user 🫶.
6.1.1. Digest Authentication Mechanism
The Digest Authentication Mechanism defined in this memo gives you two main things 🎀:
- verification of the integrity of a received message, i.e., the message received is the message sent 🔍.
The integrity of the message is protected by computing a digest over an appropriate portion of the message. The digest is computed by the originator of the message, transmitted with the message, and verified by the recipient of the message 📬.
- verification of the user on whose behalf the message was generated 🪪.
A secret value known only to SNMP engines authorized to generate messages on behalf of a user is used in HMAC mode (see [RFC2104]). It also recommends truncating the hash-function output that's used as the Message Authentication Code 💅.
This protocol uses the MD5 [MD5] message digest algorithm. A 128-bit MD5 digest is calculated in a special (HMAC) way over the designated portion of an SNMP message, and the first 96 bits of that digest get included as part of the message sent to the recipient. The size of the digest carried in a message is 12 octets. The size of the private authentication key (the secret) is 16 octets. For the details, see section 6.3 🧚.
6.2. Elements of the Digest Authentication Protocol
This section has the definitions you need to realize the authentication module defined in this section of this memo 📚.
6.2.1. Users
Authentication using this protocol makes use of a defined set of userNames. For any user on whose behalf a message must be authenticated at a particular SNMP engine, that SNMP engine must have knowledge of that user. An SNMP engine that wants to talk to another SNMP engine must also know a user known to that engine, including the applicable attributes of that user 🗣️.
A user and its attributes are defined as follows:
<userName>: A string representing the name of the user.<authKey>: A user's secret key to be used when calculating a digest. It MUST be 16 octets long for MD5 🔑.
6.2.2. msgAuthoritativeEngineID
The msgAuthoritativeEngineID value contained in an authenticated message specifies the authoritative SNMP engine for that particular message. (See the definition of SnmpEngineID in the SNMP Architecture document [RFC2261] 📖.)
A user's (private) authentication key is normally different at each authoritative SNMP engine, so the snmpEngineID is used to select the proper key for the authentication process 🗝️✨.
6.2.3. SNMP Messages Using this Authentication Protocol
Messages using this authentication protocol carry a msgAuthenticationParameters field as part of the msgSecurityParameters. For this protocol, the msgAuthenticationParameters field is the serialized OCTET STRING representing the first 12 octets of the HMAC-MD5-96 output done over the wholeMsg 🎁.
The digest is calculated over the wholeMsg, so if a message is authenticated, that also means all the fields in the message are intact and haven't been tampered with 🛡️. No cap, that's iconic.
6.2.4. Services provided by the HMAC-MD5-96 Authentication Module
This section describes the inputs and outputs that the HMAC-MD5-96 Authentication module expects and produces when the User-based Security module calls it for services 🧩.
6.2.4.1. Services for Generating an Outgoing SNMP Message
The HMAC-MD5-96 authentication protocol assumes that the caller does the selection of the authKey, and that the caller passes in the secret key to be used 🙌.
Upon completion, the authentication module returns statusInformation and, if the message digest was correctly calculated, the wholeMsg with the digest inserted at the proper place. The abstract service primitive is:
statusInformation = -- success or failure
authenticateOutgoingMsg(
IN authKey -- secret key for authentication
IN wholeMsg -- unauthenticated complete message
OUT authenticatedWholeMsg -- complete authenticated message
)The abstract data elements are:
statusInformation: An indication of whether the authentication process was successful. If not, it is an indication of the problem.authKey: The secret key to be used by the authentication algorithm. The length of this key MUST be 16 octets.wholeMsg: The message to be authenticated.authenticatedWholeMsg: The authenticated message (including the inserted digest) on output.
Note that the authParameters field is filled in by the authentication module, and this field should already be present in the wholeMsg before the Message Authentication Code (MAC) is generated ⚠️.
6.2.4.2. Services for Processing an Incoming SNMP Message
The HMAC-MD5-96 authentication protocol assumes that the caller does the selection of the authKey, and that the caller passes in the secret key to be used 🙌.
Upon completion, the authentication module returns statusInformation and, if the message digest was correctly calculated, the wholeMsg as it was processed. The abstract service primitive is:
statusInformation = -- success or failure
authenticateIncomingMsg(
IN authKey -- secret key for authentication
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- complete authenticated message
)The abstract data elements are:
statusInformation: An indication of whether the authentication process was successful. If not, it is an indication of the problem.authKey: The secret key to be used by the authentication algorithm. The length of this key MUST be 16 octets.authParameters: The authParameters from the incoming message.wholeMsg: The message to be authenticated on input, and the authenticated message on output.authenticatedWholeMsg: The whole message after the authentication check is complete.
6.3. Elements of Procedure
This section describes the procedures for the HMAC-MD5-96 authentication protocol 🛠️.
6.3.1. Processing an Outgoing Message
This section describes the procedure an SNMP engine follows whenever it needs to authenticate an outgoing message using the usmHMACMD5AuthProtocol 🚀.
- The msgAuthenticationParameters field is set to the serialization, according to the rules in [RFC1906], of an OCTET STRING containing 12 zero octets.
- From the secret authKey, two keys K1 and K2 are derived:
a) extend the authKey to 64 octets by appending 48 zero octets; save it as extendedAuthKey b) obtain IPAD by replicating the octet 0x36 64 times; c) obtain K1 by XORing extendedAuthKey with IPAD; d) obtain OPAD by replicating the octet 0x5C 64 times; e) obtain K2 by XORing extendedAuthKey with OPAD.
- Prepend K1 to the wholeMsg and calculate the MD5 digest over it according to [MD5].
- Prepend K2 to the result of step 4 and calculate the MD5 digest over it according to [MD5]. Take the first 12 octets of the final digest. This is the Message Authentication Code (MAC) 🎯.
- Replace the msgAuthenticationParameters field with the MAC obtained in step 5.
- The authenticatedWholeMsg is then returned to the caller, together with statusInformation indicating success 🎉.
6.3.2. Processing an Incoming Message 📥
okay so this is the receiving side, bestie 🫶 It's the procedure an SNMP engine follows whenever it has to authenticate an incoming message with the usmHMACMD5AuthProtocol 💅
- If the digest received in the msgAuthenticationParameters field is not 12 octets long, then a failure occurs and an errorIndication (authenticationError) is returned to the calling module. 🙈
- The MAC received in the msgAuthenticationParameters field is saved. 💾
- The digest in the msgAuthenticationParameters field is replaced by the 12 zero octets. 🧹
- From the secret authKey, two keys K1 and K2 are derived 🔑✨
- a) extend the authKey to 64 octets by appending 48 zero octets, and save it as extendedAuthKey
- b) obtain IPAD by replicating the octet 0x36 64 times
- c) obtain K1 by XORing extendedAuthKey with IPAD
- d) obtain OPAD by replicating the octet 0x5C 64 times
- e) obtain K2 by XORing extendedAuthKey with OPAD
- The MAC is calculated over the wholeMsg 🧮
- a) prepend K1 to the wholeMsg and calculate the MD5 digest over it
- b) prepend K2 to the result of step 5.a and calculate the MD5 digest over it
- c) the first 12 octets of the result of step 5.b is the MAC
Then the msgAuthenticationParameters field is replaced with the MAC value that was saved in step 2.
- Next, the newly calculated MAC is compared with the MAC saved in step 2. If they do not match, a failure occurs and an errorIndication (authenticationFailure) is returned to the calling module. 🚫
- The authenticatedWholeMsg and statusInformation indicating success are then returned to the caller. 🎉
7. HMAC-SHA-96 Authentication Protocol 🌈
this section is all about HMAC-SHA-96 🦋 It uses the SHA hash function described in [SHA-NIST], in the HMAC mode described in [RFC2104], and the output gets truncated to 96 bits ✂️
this protocol is identified by usmHMACSHAAuthProtocol 🏷️
over time, other authentication protocols might be defined, either to replace this one or to sit alongside it, so stay tuned 👀
7.1. Mechanisms 🛠️
- for data integrity, a message digest algorithm is required. A digest is calculated over an appropriate portion of an SNMP message and included as part of the message sent to the recipient. 🔍
- for data origin authentication and data integrity, a secret value is prepended to the SNMP message before the digest is computed. The calculated digest is then partially inserted into the message before it is transmitted. The prepended secret is never transmitted 🤫 The secret value is shared by all SNMP engines that are authorized to originate messages on behalf of the appropriate user. 🫢
7.1.1. Digest Authentication Mechanism 💖
the Digest Authentication Mechanism in this memo gives you two things:
- verification of the integrity of a received message, meaning that the message received is the message that was sent. 🧾
The integrity is protected by computing a digest over an appropriate portion of the message. The originator of the message computes the digest and transmits it with the message, and the recipient of the message verifies it.
- verification of the user on whose behalf the message was generated. 🕵️♀️
A secret value, known only to the SNMP engines that are authorized to generate messages on behalf of a user, is used in HMAC mode (see [RFC2104]). The hash-function output used as the Message Authentication Code is also recommended to be truncated.
this mechanism uses the SHA [SHA-NIST] message digest algorithm 🌸 A 160-bit SHA digest is calculated, in a special (HMAC) way, over the designated portion of an SNMP message, and the first 96 bits of that digest are included as part of the message sent to the recipient. The digest carried in a message is 12 octets long, and the private authentication key (the secret) is 20 octets long. For the details, see section 7.3. 📚
7.2. Elements of the HMAC-SHA-96 Authentication Protocol 🧩
this section has the definitions you need to build the authentication module described in this section of this memo 💅
7.2.1. Users 🙋♀️
authentication with this protocol uses a defined set of userNames. For any user on whose behalf a message must be authenticated at a particular SNMP engine, that engine has to know about that user. An SNMP engine that wants to talk to another SNMP engine must also know a user that is known to that engine, including the applicable attributes of that user. 🤝
a user and its attributes are defined like this:
<userName>: a string representing the name of the user.<authKey>: the user's secret key, used when calculating a digest. It MUST be 20 octets long for SHA. 🔐
7.2.2. msgAuthoritativeEngineID 👑
the msgAuthoritativeEngineID value in an authenticated message specifies the authoritative SNMP engine for that particular message (see the definition of SnmpEngineID in the SNMP Architecture document [RFC2261]). 📖
the user's (private) authentication key is normally different at each authoritative SNMP engine, so the snmpEngineID is used to pick the right key for the authentication process. 🗝️✨
7.2.3. SNMP Messages Using this Authentication Protocol 💌
messages using this authentication protocol carry a msgAuthenticationParameters field as part of the msgSecurityParameters. For this protocol, the msgAuthenticationParameters field is the serialized OCTET STRING representing the first 12 octets of the HMAC-SHA-96 output done over the wholeMsg. 🧁
the digest is calculated over the wholeMsg, so if a message is authenticated, that also means every field in the message is intact and hasn't been tampered with. 🛡️ no sneaky edits allowed 😤
7.2.4. Services provided by the HMAC-SHA-96 Authentication Module 🎀
this section describes the inputs and outputs that the HMAC-SHA-96 Authentication module expects and produces when the User-based Security module calls it for services 📞
7.2.4.1. Services for Generating an Outgoing SNMP Message 📤
HMAC-SHA-96 authentication assumes that the caller selects the authKey and passes in the secret key to be used 🔑
when it's done, the authentication module returns statusInformation and, if the message digest was calculated correctly, the wholeMsg with the digest inserted in the proper place. The abstract service primitive looks like this:
statusInformation = -- success or failure
authenticateOutgoingMsg(
IN authKey -- secret key for authentication
IN wholeMsg -- unauthenticated complete message
OUT authenticatedWholeMsg -- complete authenticated message
)the abstract data elements are:
- statusInformation: an indication of whether the authentication process was successful. If it wasn't, it indicates the problem.
- authKey: the secret key to be used by the authentication algorithm. The length of this key MUST be 20 octets. 🔐
- wholeMsg: the message to be authenticated.
- authenticatedWholeMsg: the authenticated message, including the inserted digest, on output.
note that the authParameters field is filled in by the authentication module, and this field should already be present in the wholeMsg before the Message Authentication Code (MAC) is generated. ⚠️
7.2.4.2. Services for Processing an Incoming SNMP Message 📥
HMAC-SHA-96 authentication assumes that the caller selects the authKey and passes in the secret key to be used 🗝️
when it's done, the authentication module returns statusInformation and, if the message digest was calculated correctly, the wholeMsg as it was processed. The abstract service primitive looks like this:
statusInformation = -- success or failure
authenticateIncomingMsg(
IN authKey -- secret key for authentication
IN authParameters -- as received on the wire
IN wholeMsg -- as received on the wire
OUT authenticatedWholeMsg -- complete authenticated message
)the abstract data elements are:
- statusInformation: an indication of whether the authentication process was successful. If it wasn't, it indicates the problem.
- authKey: the secret key to be used by the authentication algorithm. The length of this key MUST be 20 octets. 🔐
- authParameters: the authParameters from the incoming message.
- wholeMsg: the message to be authenticated on input, and the authenticated message on output.
- authenticatedWholeMsg: the whole message after the authentication check is complete.
7.3. Elements of Procedure 📋
this section describes the procedures for the HMAC-SHA-96 authentication protocol 🧠
7.3.1. Processing an Outgoing Message 📤
this section describes the procedure an SNMP engine follows whenever it must authenticate an outgoing message using the usmHMACSHAAuthProtocol 💅
- The msgAuthenticationParameters field is set to the serialization, according to the rules in [RFC1906], of an OCTET STRING containing 12 zero octets. 🧸
- From the secret authKey, two keys K1 and K2 are derived 🔑✨
- a) extend the authKey to 64 octets by appending 44 zero octets, and save it as extendedAuthKey
- b) obtain IPAD by replicating the octet 0x36 64 times
- c) obtain K1 by XORing extendedAuthKey with IPAD
- d) obtain OPAD by replicating the octet 0x5C 64 times
- e) obtain K2 by XORing extendedAuthKey with OPAD
- Prepend K1 to the wholeMsg and calculate the SHA digest over it according to [SHA-NIST]. 🧮
- Prepend K2 to the result of step 3 and calculate the SHA digest over it according to [SHA-NIST]. Take the first 12 octets of the final digest, which is the Message Authentication Code (MAC). ✂️
- Replace the msgAuthenticationParameters field with the MAC obtained in step 4. 🔁
- The authenticatedWholeMsg is then returned to the caller, together with statusInformation indicating success. 🎉
7.3.2. Processing an Incoming Message ✨
Okay bestie, this section is the step-by-step for what an SNMP engine does when it has to authenticate an incoming message with the usmHMACSHAAuthProtocol 🔐. Let's go!
- If the digest received in the
msgAuthenticationParametersfield is not 12 octets long, then a failure and anerrorIndication(authenticationError) are returned to the calling module. 💀 - The MAC received in the
msgAuthenticationParametersfield is saved. 📥 - The digest in the
msgAuthenticationParametersfield is replaced by the 12 zero octets. 🧸 - From the secret
authKey, two keys K1 and K2 are derived 🔑✨- a) extend the
authKeyto 64 octets by appending 44 zero octets; save it asextendedAuthKey - b) obtain
IPADby replicating the octet0x3664 times; - c) obtain K1 by XORing
extendedAuthKeywithIPAD; - d) obtain
OPADby replicating the octet0x5C64 times; - e) obtain K2 by XORing
extendedAuthKeywithOPAD.
- a) extend the
- The MAC is calculated over the
wholeMsg🍓- a) prepend K1 to the
wholeMsgand calculate the SHA digest over it; - b) prepend K2 to the result of step 5.a and calculate the SHA digest over it;
- c) first 12 octets of the result of step 5.b is the MAC.
- a) prepend K1 to the
The msgAuthenticationParameters field is replaced with the MAC value that was saved in step 2. 🫶
- The newly calculated MAC is compared with the MAC saved in step 2. If they do not match, then a failure and an
errorIndication(authenticationFailure) are returned to the calling module. 😭 - The
authenticatedWholeMsgandstatusInformationindicating success are then returned to the caller. 🎀
8. CBC-DES Symmetric Encryption Protocol 🌸
This section is all about the CBC-DES Symmetric Encryption Protocol, and it's giving main character energy 💅. It's the first privacy protocol defined for the User-based Security Model, so it's kind of a big deal 👑.
This protocol is identified by usmDESPrivProtocol.
Over time, other privacy protocols may be defined, either as a replacement for this protocol or in addition to it. Stay tuned 🦋.
8.1. Mechanisms 🔐
- In support of data confidentiality, an encryption algorithm is required. An appropriate portion of the message is encrypted before it gets sent. The User-based Security Model says the scopedPDU is the part that needs to be encrypted 🙈.
- A secret value combined with a timeliness value is used to create the en/decryption key and the initialization vector. The secret value is shared by all SNMP engines that are authorized to originate messages on behalf of the appropriate user 🧚.
8.1.1. Symmetric Encryption Protocol
The Symmetric Encryption Protocol defined in this memo provides support for data confidentiality 🔒. The designated portion of an SNMP message gets encrypted and then included as part of the message sent to the recipient.
Two organizations have published specifications defining the DES: the National Institute of Standards and Technology (NIST) [DES-NIST] and the American National Standards Institute [DES-ANSI]. Each definition has a companion Modes of Operation specification ([DESO-NIST] and [DESO-ANSI], respectively).
The NIST has also published three additional documents that implementors may find useful 📚:
- There is a document with guidelines for implementing and using the DES, including functional specifications for the DES and its modes of operation [DESG-NIST].
- There is a specification of a validation test suite for the DES [DEST-NIST]. The suite is designed to test all aspects of the DES and is useful for pinpointing specific problems 🔍.
- There is a specification of a maintenance test for the DES [DESM-NIST]. The test uses a minimal amount of data and processing to test all components of the DES. It gives a simple yes-or-no indication of correct operation and is useful to run as part of an initialization step, e.g., when a computer re-boots 🔄.
8.1.1.1. DES key and Initialization Vector. 🔑
The first 8 octets of the 16-octet secret (private privacy key) are used as a DES key. Since DES only uses 56 bits, the Least Significant Bit in each octet is disregarded 🙃.
The Initialization Vector for encryption is obtained with the following procedure.
The last 8 octets of the 16-octet secret (private privacy key) are used as pre-IV.
Okay so, to make sure the IV for two different packets encrypted by the same key is not the same (so the IV never repeats), we need to "salt" the pre-IV with something unique per packet 🧂. An 8-octet string is used as the "salt". The salt is made by concatenating the generating SNMP engine's 32-bit snmpEngineBoots with a local 32-bit integer that the encryption engine maintains. That integer is initialized to an arbitrary value at boot time ⏰.
The 32-bit snmpEngineBoots is converted to the first 4 octets (Most Significant Byte first) of our "salt". The 32-bit integer is then converted to the last 4 octets (Most Significant Byte first) of our "salt". The resulting "salt" is then XOR-ed with the pre-IV. The 8-octet "salt" is then put into the privParameters field, encoded as an OCTET STRING. After that, the "salt" integer gets modified. We recommend that it be incremented by one and wrap when it reaches the maximum value 🔁.
How exactly the value of the "salt" (and therefore of the IV) varies is an implementation issue, as long as measures are taken to avoid producing a duplicate IV 🫶.
The "salt" must be placed in the privParameters field so the receiving entity can compute the correct IV and decrypt the message. (Yes, MUST, I'm not joking 💀.)
The "salt" MUST be placed in the privParameters field to enable the receiving entity to compute the correct IV and to decrypt the message.
8.1.1.2. Data Encryption. 🍬
The data to be encrypted is treated as a sequence of octets. Its length should be an integral multiple of 8, and if it isn't, the data is padded at the end as necessary. The actual pad value is irrelevant 🤷♀️.
The data is encrypted in Cipher Block Chaining mode 🔗.
The plaintext is divided into 64-bit blocks.
The plaintext for each block is XOR-ed with the ciphertext of the previous block, the result is encrypted, and the output of the encryption is the ciphertext for that block. This procedure repeats until there are no more plaintext blocks 🔁.
For the very first block, the Initialization Vector is used instead of the ciphertext of the previous block 🌱.
8.1.1.3. Data Decryption 🔓
Before decryption, the encrypted data length is verified. If the length of the OCTET STRING to be decrypted is not an integral multiple of 8 octets, the decryption process is halted and an appropriate exception is noted. When decrypting, the padding is ignored 🙈.
The first ciphertext block is decrypted, the decryption output is XOR-ed with the Initialization Vector, and the result is the first plaintext block.
For each subsequent block, the ciphertext block is decrypted, the decryption output is XOR-ed with the previous ciphertext block, and the result is the plaintext block 🩷.
8.2. Elements of the DES Privacy Protocol 🧩
This section has the definitions you need to realize the privacy module defined by this memo ✨.
8.2.1. Users 👤
Data en/decryption with this Symmetric Encryption Protocol uses a defined set of userNames. For any user on whose behalf a message must be en/decrypted at a particular SNMP engine, that SNMP engine must know about that user. An SNMP engine that wants to talk to another SNMP engine must also know about a user known to that SNMP engine, including the applicable attributes of that user 💬.
A user and its attributes are defined as follows:
<userName>: An octet string representing the name of the user.<privKey>: A user's secret key to be used as input for the DES key and IV. The length of this key MUST be 16 octets.
8.2.2. msgAuthoritativeEngineID 🪪
The msgAuthoritativeEngineID value contained in an authenticated message specifies the authoritative SNMP engine for that particular message (see the definition of SnmpEngineID in the SNMP Architecture document [RFC2261]).
The user's (private) privacy key is normally different at each authoritative SNMP engine, so the snmpEngineID is used to select the proper key for the en/decryption process 🔑💖.
8.2.3. SNMP Messages Using this Privacy Protocol 📨
Messages using this privacy protocol carry a msgPrivacyParameters field as part of the msgSecurityParameters. For this protocol, the msgPrivacyParameters field is the serialized OCTET STRING representing the "salt" that was used to create the IV 🧂.
8.2.4. Services provided by the DES Privacy Module 🛎️
This section describes the inputs and outputs that the DES Privacy module expects and produces when the User-based Security module invokes it for services 🎀.
8.2.4.1. Services for Encrypting Outgoing Data 🔐
This DES privacy protocol assumes the caller selects the privKey and passes in the secret key to be used.
When it's done, the privacy module returns statusInformation and, if the encryption process was successful, the encryptedPDU and the msgPrivacyParameters encoded as an OCTET STRING. The abstract service primitive looks like this 👀:
statusInformation = -- success of failure
encryptData(
IN encryptKey -- secret key for encryption
IN dataToEncrypt -- data to encrypt (scopedPDU)
OUT encryptedData -- encrypted data (encryptedPDU)
OUT privParameters -- filled in by service provider
)The abstract data elements are:
statusInformation: An indication of the success or failure of the encryption process. In case of failure, it is an indication of the error.encryptKey: The secret key to be used by the encryption algorithm. The length of this key MUST be 16 octets.dataToEncrypt: The data that must be encrypted.encryptedData: The encrypted data upon successful completion.privParameters: The privParameters encoded as an OCTET STRING.
8.2.4.2. Services for Decrypting Incoming Data 🔓
This DES privacy protocol assumes the caller selects the privKey and passes in the secret key to be used 🗝️.
When it's done, the privacy module returns statusInformation and, if the decryption process was successful, the scopedPDU in plain text. The abstract service primitive looks like this:
statusInformation =
decryptData(
IN decryptKey -- secret key for decryption
IN privParameters -- as received on the wire
IN encryptedData -- encrypted data (encryptedPDU)
OUT decryptedData -- decrypted data (scopedPDU)
)The abstract data elements are:
statusInformation: An indication of whether the data was successfully decrypted and, if not, an indication of the error.decryptKey: The secret key to be used by the decryption algorithm. The length of this key MUST be 16 octets.privParameters: The "salt" to be used to calculate the IV.encryptedData: The data to be decrypted.decryptedData: The decrypted data.
8.3. Elements of Procedure. 🎬
This section describes the procedures for the DES privacy protocol 💜.
8.3.1. Processing an Outgoing Message ✨
okay so this section is the step-by-step for what an SNMP engine does whenever it needs to encrypt part of an outgoing message using the usmDESPrivProtocol 🔐 it's giving recipe card energy, honestly
- The secret
cryptKeygets used to build the DES encryption key, the "salt", and the DES pre-IV (as described in section 8.1.1.1) 🗝️ - The
privParametersfield gets set to the serialization, following the rules in [RFC1906], of an OCTET STRING representing the the "salt" string 💅 - The
scopedPDUgets encrypted (as described in section 8.1.1.2), and the encrypted data is serialized as an OCTET STRING following the rules in [RFC1906] 🎀 - The serialized OCTET STRING holding the encrypted
scopedPDU, together withprivParametersand astatusInformationsaying success, gets returned to the calling module 🦋
8.3.2. Processing an Incoming Message 📥
this is the flip side, the decrypting part, and it's so important because you have to check stuff before you do anything 🧐
- If the
privParametersfield is not an 8-octet OCTET STRING, then an error indication (decryptionError) gets returned to the calling module 🙈 - The "salt" gets pulled out of the
privParametersfield 🧂 - The secret
cryptKeyand the "salt" are then used to build the DES decryption key and pre-IV (as described in section 8.1.1.1) 🔑 - The
encryptedPDUthen gets decrypted (as described in section 8.1.1.3) 🔓 - If the
encryptedPDUcan't be decrypted, then an error indication (decryptionError) gets returned to the calling module 💀 - The decrypted
scopedPDUand astatusInformationindicating success get returned to the calling module 👑
9. Intellectual Property 🌈
bestie, this part is all about the legal stuff, so here's the tea ☕
The IETF takes no position on whether any intellectual property or other rights that someone might claim over the tech in this document are valid or what their scope is, and it doesn't say whether any license under those rights would or wouldn't be available. It also doesn't claim it has tried to find any of those rights. 🤷♀️ If you want info on how the IETF handles rights for standards-track and standards-related docs, check out BCP-11. Copies of claimed rights that were made available for publication, any assurances of licenses that will be made available, or the result of an attempt to get a general license or permission for implementers or users to use those proprietary rights can all be requested from the IETF Secretariat 📬
The IETF also invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that might cover technology needed to practice this standard. Send that info to the IETF Executive Director 💌
10. Acknowledgements 🫶
this doc is the result of the efforts of the SNMPv3 Working Group, and we need to give some seriously special thanks to these SNMPv3 WG members ✨
- Dave Battle (SNMP Research, Inc.)
- Uri Blumenthal (IBM T.J. Watson Research Center)
- Jeff Case (SNMP Research, Inc.)
- John Curran (BBN)
- T. Max Devlin (Hi-TECH Connections)
- John Flick (Hewlett Packard)
- David Harrington (Cabletron Systems Inc.)
- N.C. Hien (IBM T.J. Watson Research Center)
- Dave Levi (SNMP Research, Inc.)
- Louis A Mamakos (UUNET Technologies Inc.)
- Paul Meyer (Secure Computing Corporation)
- Keith McCloghrie (Cisco Systems)
- Russ Mundy (Trusted Information Systems, Inc.)
- Bob Natale (ACE*COMM Corporation)
- Mike O'Dell (UUNET Technologies Inc.)
- Dave Perkins (DeskTalk)
- Peter Polkinghorne (Brunel University)
- Randy Presuhn (BMC Software, Inc.)
- David Reid (SNMP Research, Inc.)
- Shawn Routhier (Epilogue)
- Juergen Schoenwaelder (TU Braunschweig)
- Bob Stewart (Cisco Systems)
- Bert Wijnen (IBM T.J. Watson Research Center)
the document is based on recommendations from the IETF Security and Administrative Framework Evolution for SNMP Advisory Team 🧠 the members of that Advisory Team were:
- David Harrington (Cabletron Systems Inc.)
- Jeff Johnson (Cisco Systems)
- David Levi (SNMP Research Inc.)
- John Linn (Openvision)
- Russ Mundy (Trusted Information Systems) chair
- Shawn Routhier (Epilogue)
- Glenn Waters (Nortel)
- Bert Wijnen (IBM T. J. Watson Research Center)
as the Advisory Team and the SNMPv3 Working Group Charter recommended, the design pulls in as much as is practical from previous RFCs and drafts 🍓 so special thanks are also due to the authors of the earlier designs known as SNMPv2u and SNMPv2*:
- Jeff Case (SNMP Research, Inc.)
- David Harrington (Cabletron Systems Inc.)
- David Levi (SNMP Research, Inc.)
- Keith McCloghrie (Cisco Systems)
- Brian O'Keefe (Hewlett Packard)
- Marshall T. Rose (Dover Beach Consulting)
- Jon Saperia (BGS Systems Inc.)
- Steve Waldbusser (International Network Services)
- Glenn W. Waters (Bell-Northern Research Ltd.)
11. Security Considerations 🔒
this is the serious section, so it's giving main character drama 😭 but the stakes are real, so read every bit of it
11.1. Recommended Practices 🛡️
this section lists practices that help the mechanisms in this memo run securely and effectively 💖
- An SNMP engine must discard SNMP Response messages that do not correspond to any currently outstanding Request message. It is the responsibility of the Message Processing module to take care of this. For example it can use a msgID for that.
An SNMP Command Generator Application must discard any Response PDU for which there is no currently outstanding Request PDU. For example, for SNMPv2 [RFC1905] PDUs, the request-id component in the PDU can be used to correlate Responses to outstanding Requests.
Although it would be typical for an SNMP engine and an SNMP Command Generator Application to do this as a matter of course, when using these security protocols it is significant because of the possibility of message duplication (malicious or otherwise) 👀
- If an SNMP engine uses a msgID for correlating Response messages to outstanding Request messages, then it MUST use different msgIDs in all such Request messages that it sends out during a Time Window (150 seconds) period.
A Command Generator or Notification Originator Application MUST use different request-ids in all Request PDUs that it sends out during a TimeWindow (150 seconds) period.
This must be done to protect against the possibility of message duplication (malicious or otherwise) 🚨
For example, starting operations with a msgID and/or request-id value of zero is not a good idea. Initializing them with an unpredictable number (so they do not start out the same after each reboot) and then incrementing by one would be acceptable 🔢
- An SNMP engine should perform time synchronization using authenticated messages in order to protect against the possibility of message duplication (malicious or otherwise) ⏰
- When sending state altering messages to a managed authoritative SNMP engine, a Command Generator Application should delay sending successive messages to that managed SNMP engine until a positive acknowledgement is received for the previous message or until the previous message expires.
No message ordering is imposed by the SNMP. Messages may be received in any order relative to their time of generation, and each will be processed in the order received. Note that when an authenticated message is sent to a managed SNMP engine, it will be valid for a period of time of approximately 150 seconds under normal circumstances, and it is subject to replay during this period. Indeed, an SNMP engine and SNMP Command Generator Applications must cope with the loss and re-ordering of messages resulting from anomalies in the network as a matter of course 🌪️
However, a managed object, snmpSetSerialNo [RFC1907], is specifically defined for use with SNMP Set operations, in order to provide a mechanism that ensures SNMP messages are processed in a specific order ✨
- The frequency with which the secrets of a User-based Security Model user should be changed is indirectly related to the frequency of their use.
Protecting the secrets from disclosure is critical to the overall security of the protocols. Frequent use of a secret provides a continued source of data that may be useful to a cryptanalyst in exploiting known or perceived weaknesses in an algorithm. Frequent changes to the secret avoid this vulnerability 🔐
Changing a secret after each use is generally regarded as the most secure practice, but a significant amount of overhead may be associated with that approach 😮💨
Note, too, that in a local environment the threat of disclosure may be less significant, so changing secrets may be less frequent. However, when public data networks are used as the communication paths, more caution is prudent 🌐
11.2. Defining Users ✨
Okay so, "users" are the whole vibe here 💅. The mechanisms in this document send messages on behalf of users, and how you define a user is up to the network administration's security policy 🎀. A user could be an individual (like "joe" or "jane"), a role (like "operator" or "administrator"), or a combo (like "joe-operator", "jane-operator" or "joe-admin") 🦋. And a user can even be a logical entity, like one SNMP Application or a whole set of them, acting for an individual, a role, a set of individuals, a set of roles, or any combination of those 👑.
Appendix A has an algorithm that turns a user "password" into a 16 octet value 🔑. That value can be the user's authentication key, their privacy key, or both. ngl, using the same password (and therefore the same key) for both auth and privacy is very poor security practice 💀, and it should be strongly discouraged 🙅♀️. Humans generate, remember and type in passwords, and those can be shorter than the 16 octets the protocols need. Brute force attacks can be pretty easy on a short ASCII character set 😭. So the Appendix A algorithm performs a transformation on the password. If you use the Appendix A algorithm, SNMP implementations (and SNMP configuration applications) must ensure that passwords are at least 8 characters in length 💖.
Because the Appendix A algorithm uses these passwords (nearly) directly, it is very important that they not be easily guessed 🔐. The suggestion is to mix uppercase and lowercase alphanumeric and punctuation characters, and avoid words or phrases you'd find in a dictionary 🌸. Longer passwords make the system more secure ✨. Users can also type in multiword phrases, which makes the string longer and still memorable 🧚.
Here's the tricky bit: it's basically impossible for humans to keep a different password for every SNMP engine, but security requirements strongly discourage using the same key on more than one SNMP engine 😩. So the User-based Security Model uses a compromise proposed in [Localized-key]. It derives the user keys for each SNMP engine from the user's password, in a way that makes it practically impossible to either work out the user's password, or the user's key for another SNMP engine, from any combination of the user's keys on SNMP engines 🦋.
Note however, that if the user's password is disclosed, key localization won't help, and network security may be compromised in this case. So a user's password or non-localized key MUST NOT be stored on a managed device/node. Instead, the localized key SHALL be stored (if at all), so that if a device does get compromised, no other managed or managing devices get compromised 🚨.
11.3. Conformance 🎀
To be termed a "Secure SNMP implementation" based on the User-based Security Model, an SNMP implementation MUST:
- implement one or more Authentication Protocol(s). The HMAC-MD5-96 and HMAC-SHA-96 Authentication Protocols defined in this memo are examples of such protocols.
- to the maximum extent possible, prohibit access to the secret(s) of each user about which it maintains information in a Local Configuration Datastore (LCD) under all circumstances except as required to generate and/or validate SNMP messages with respect to that user.
- implement the key-localization mechanism.
- implement the SNMP-USER-BASED-SM-MIB.
In addition, an authoritative SNMP engine SHOULD provide initial configuration in accordance with Appendix A.1 🌈.
Implementing a Privacy Protocol is optional. The DES Symmetric Encryption Protocol defined in this memo is one such protocol 🔒.
12. References 📚
[RFC1321] Rivest, R., "Message Digest Algorithm MD5", RFC 1321, April 1992.
[RFC1903] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Textual Conventions for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1903, January 1996.
[RFC1905] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Protocol Operations for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1905, January 1996.
[RFC1906] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Transport Mappings for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1906, January 1996.
[RFC1907] Case, J., McCloghrie, K., Rose, M. and S. Waldbusser, "Management Information Base for Version 2 of the Simple Network Management Protocol (SNMPv2)", RFC 1907 January 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
[RFC2028] Hovey, R., and S. Bradner, "The Organizations Involved in the IETF Standards Process", BCP 11, RFC 2028, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2261] Harrington, D., Presuhn, R., and B. Wijnen, "An Architecture for describing SNMP Management Frameworks", RFC 2261, January 1998.
[RFC2262] Case, J., Harrington, D., Presuhn, R., and B. Wijnen, "Message Processing and Dispatching for the Simple Network Management Protocol (SNMP)", RFC 2262, January 1998.
[Localized-Key] U. Blumenthal, N. C. Hien, B. Wijnen "Key Derivation for Network Management Applications" IEEE Network Magazine, April/May issue, 1997.
[DES-NIST] Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 46-1. Supersedes FIPS Publication 46, (January, 1977; reaffirmed January, 1988).
[DES-ANSI] Data Encryption Algorithm, American National Standards Institute. ANSI X3.92-1981, (December, 1980).
[DESO-NIST] DES Modes of Operation, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 81, (December, 1980).
[DESO-ANSI] Data Encryption Algorithm - Modes of Operation, American National Standards Institute. ANSI X3.106-1983, (May 1983).
[DESG-NIST] Guidelines for Implementing and Using the NBS Data Encryption Standard, National Institute of Standards and Technology. Federal Information Processing Standard (FIPS) Publication 74, (April, 1981).
[DEST-NIST] Validating the Correctness of Hardware Implementations of the NBS Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-20.
[DESM-NIST] Maintenance Testing for the Data Encryption Standard, National Institute of Standards and Technology. Special Publication 500-61, (August, 1980).
[SHA-NIST] Secure Hash Algorithm. NIST FIPS 180-1, (April, 1995) http://csrc.nist.gov/fips/fip180-1.txt (ASCII) http://csrc.nist.gov/fips/fip180-1.ps (Postscript)
13. Editors' Addresses 💌
Uri Blumenthal
- IBM T. J. Watson Research
- 30 Saw Mill River Pkwy,
- Hawthorne, NY 10532
- USA
- EMail: uri@watson.ibm.com
- Phone: +1-914-784-7064
Bert Wijnen
- IBM T. J. Watson Research
- Schagen 33
- 3461 GL Linschoten
- Netherlands
- EMail: wijnen@vnet.ibm.com
- Phone: +31-348-432-794
APPENDIX A - Installation 🛠️
A.1. SNMP engine Installation Parameters 🌸
During installation, an authoritative SNMP engine SHOULD (in the meaning as defined in [RFC2119]) be configured with several initial parameters. These include:
- A security posture
The choice of security posture determines whether initial configuration is implemented and, if so, how. One of three possible choices is selected:
- minimum-secure,
- semi-secure,
- very-secure (i.e., no-initial-configuration)
In the case of a very-secure posture, there is no initial configuration, so the following steps are irrelevant 🙈.
- one or more secrets
These are the authentication/privacy secrets for the first user to be configured.
One way to accomplish this is to have the installer enter a "password" for each required secret. The password is then algorithmically converted into the required secret by:
- forming a string of length 1,048,576 octets by repeating the value of the password as often as necessary, truncating accordingly, and using the resulting string as the input to the MD5 algorithm [MD5]. The resulting digest, termed "digest1", is used in the next step.
- a second string is formed by concatenating digest1, the SNMP engine's snmpEngineID value, and digest1. This string is used as input to the MD5 algorithm [MD5].
The resulting digest is the required secret (see Appendix A.2) 🔑.
With these configured parameters, the SNMP engine instantiates the following usmUserEntry in the usmUserTable:
no privacy support privacy support
------------------ ---------------
usmUserEngineID localEngineID localEngineID
usmUserName "initial" "initial"
usmUserSecurityName "initial" "initial"
usmUserCloneFrom ZeroDotZero ZeroDotZero
usmUserAuthProtocol usmHMACMD5AuthProtocol usmHMACMD5AuthProtocol
usmUserAuthKeyChange "" ""
usmUserOwnAuthKeyChange "" ""
usmUserPrivProtocol none usmDESPrivProtocol
usmUserPrivKeyChange "" ""
usmUserOwnPrivKeyChange "" ""
usmUserPublic "" ""
usmUserStorageType anyValidStorageType anyValidStorageType
usmUserStatus active activeA.2. Password to Key Algorithm 🗝️
A sample code fragment (section A.2.1) demonstrates the password to key algorithm, which can be used when mapping a password to an authentication or privacy key using MD5. The reference source code of MD5 is available in [RFC1321] 💜.
Another sample code fragment (section A.2.2) demonstrates the password to key algorithm, which can be used when mapping a password to an authentication or privacy key using SHA (documented in SHA-NIST) 🦋.
An example of the results of a correct implementation is provided (section A.3), which an implementor can use to check whether their implementation produces the same result ✨.
A.2.1. Password to Key Sample Code for MD5 💻
void password_to_key_md5(
u_char *password, /* IN */
u_int passwordlen, /* IN */
u_char *engineID, /* IN - pointer to snmpEngineID */
u_int engineLength /* IN - length of snmpEngineID */
u_char *key) /* OUT - pointer to caller 16-octet buffer */ {
MD5_CTX MD;
u_char *cp, password_buf[64];
u_long password_index = 0;
u_long count = 0, i;
MD5Init (&MD); /* initialize MD5 */
/**********************************************/
/* Use while loop until we've done 1 Megabyte */
/**********************************************/
while (count < 1048576) {
cp = password_buf;
for (i = 0; i < 64; i++) {
/*************************************************/
/* Take the next octet of the password, wrapping */
/* to the beginning of the password as necessary.*/
/*************************************************/
*cp++ = password[password_index++ % passwordlen];
}
MD5Update (&MD, password_buf, 64);
count += 64;
}
MD5Final (key, &MD); /* tell MD5 we're done */
/*****************************************************/
/* Now localize the key with the engineID and pass */
/* through MD5 to produce final key */
/* May want to ensure that engineLength <= 32, */
/* otherwise need to use a buffer larger than 64 */
/*****************************************************/
memcpy(password_buf, key, 16);
memcpy(password_buf+16, engineID, engineLength);
memcpy(password_buf+engineLength, key, 16);
MD5Init(&MD);
MD5Update(&MD, password_buf, 32+engineLength);
MD5Final(key, &MD);
return;
}Okay so that's the end of the MD5 function, and it's giving full "keep looping until we hit 1 Megabyte, then mix in the engineID" energy 💅✨. The loop just keeps grabbing the next octet of the password and wraps back around to the start when it runs out, so the password gets stretched out over the whole megabyte 🌸. Then the final key gets localized by sandwiching it around the engineID and hashing again 🫶.
A.2.2. Password to Key Sample Code for SHA
The SHA version is literally the same vibe, just with SHA instead of MD5 and a bigger buffer, so you know it's a sibling situation 🦋.
void password_to_key_sha(
u_char *password, /* IN */
u_int passwordlen, /* IN */
u_char *engineID, /* IN - pointer to snmpEngineID */
u_int engineLength /* IN - length of snmpEngineID */
u_char *key) /* OUT - pointer to caller 20-octet buffer */
{
SHA_CTX SH;
u_char *cp, password_buf[72];
u_long password_index = 0;
u_long count = 0, i;
SHAInit (&SH); /* initialize SHA */
/**********************************************/
/* Use while loop until we've done 1 Megabyte */
/**********************************************/
while (count < 1048576) {
cp = password_buf;
for (i = 0; i < 64; i++) {
/*************************************************/
/* Take the next octet of the password, wrapping */
/* to the beginning of the password as necessary.*/
/*************************************************/
*cp++ = password[password_index++ % passwordlen];
}
SHAUpdate (&SH, password_buf, 64);
count += 64;
}
SHAFinal (key, &SH); /* tell SHA we're done */
/*****************************************************/
/* Now localize the key with the engineID and pass */
/* through SHA to produce final key */
/* May want to ensure that engineLength <= 32, */
/* otherwise need to use a buffer larger than 72 */
/*****************************************************/
memcpy(password_buf, key, 20);
memcpy(password_buf+20, engineID, engineLength);
memcpy(password_buf+engineLength, key, 20);
SHAInit(&SH);
SHAUpdate(&SH, password_buf, 40+engineLength);
SHAFinal(key, &SH);
return;
}Same 1 Megabyte loop, same wrap-around trick, same localize-then-hash finish 🎀. The only real differences are the 20-octet output buffer and the 72-octet password_buf, which is the one to watch because of that engineLength note 👀.
A.3. Password to Key Sample Results
Okay now the fun part, where we get to see what the algorithm actually spits out 🔥. These examples all use the same password, so you can compare them side by side and see how the localization changes things 🧚.
A.3.1. Password to Key Sample Results using MD5
The following shows a sample output of the password to key algorithm for a 16-octet key using MD5.
With a password of "maplesyrup" the output of the password to key algorithm before the key is localized with the SNMP engine's snmpEngineID is:
'9f af 32 83 88 4e 92 83 4e bc 98 47 d8 ed d9 63'HAfter the intermediate key (shown above) is localized with the snmpEngineID value of:
'00 00 00 00 00 00 00 00 00 00 00 02'Hthe final output of the password to key algorithm is:
'52 6f 5e ed 9f cc e2 6f 89 64 c2 93 07 87 d8 2b'HSee how the intermediate key and the final key look totally different even though the password is the same? That's the localization doing its thing 💜
A.3.2. Password to Key Sample Results using SHA
The following shows a sample output of the password to key algorithm for a 20-octet key using SHA.
With a password of "maplesyrup" the output of the password to key algorithm before the key is localized with the SNMP engine's snmpEngineID is:
'f1 be a9 ae 66 7f 4f b6 34 1e 51 af 06 80 7e 91 e4 3b 01 ac'HAfter the intermediate key (shown above) is localized with the snmpEngineID value of:
'00 00 00 00 00 00 00 00 00 00 00 02'Hthe final output of the password to key algorithm is:
'8a a3 d9 9e 3e 30 56 f2 bf e3 a9 ee f3 45 d5 39 54 91 12 be'HSame password, same engineID, but this one's 20 octets long because it's SHA 🌈
A.4. Sample encoding of msgSecurityParameters
The msgSecurityParameters in an SNMP message are represented as an OCTET STRING, and this OCTET STRING should be considered opaque outside a specific Security Model 🙈. So basically the other parts of the message just treat it as a blob 🫧.
The User-based Security Model defines the contents of the OCTET STRING as a SEQUENCE (see section 2.4) 💖.
Given these two properties, the following is an example of the msgSecurityParameters for the User-based Security Model, encoded as an OCTET STRING:
04 <length>
30 <length>
04 <length> <msgAuthoritativeEngineID>
02 <length> <msgAuthoritativeEngineBoots>
02 <length> <msgAuthoritativeEngineTime>
04 <length> <msgUserName>
04 0c <HMAC-MD5-96-digest>
04 08 <salt>Here is the example once more, but now with real values (except for the digest in msgAuthenticationParameters and the salt in msgPrivacyParameters, which depend on variable data that we have not defined here) 🦋:
Hex Data Description
-------------- -----------------------------------------------
04 39 OCTET STRING, length 57
30 37 SEQUENCE, length 55
04 0c 80000002 msgAuthoritativeEngineID: IBM
01 IPv4 address
09840301 9.132.3.1
02 01 01 msgAuthoritativeEngineBoots: 1
02 02 0101 msgAuthoritativeEngineTime: 257
04 04 62657274 msgUserName: bert
04 0c 01234567 msgAuthenticationParameters: sample value
89abcdef
fedcba98
04 08 01234567 msgPrivacyParameters: sample value
89abcdefB. Full Copyright Statement 💜
Copyright (C) The Internet Society (1997). All Rights Reserved.
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The limited permissions above are perpetual, and they will not be revoked by the Internet Society or its successors or assigns 💅.
Now the disclaimer, which is giving all caps energy for a reason 😭. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 🔥
📦 where this page comes from
- ietf: rfc2264 1998-01
- 💖 girlified by claude-haiku-5-5 on : a language model's rewrite of the original, under its terms