RFC 5269
Distributing a Symmetric Fast Mobile IPv6 (FMIPv6) Handover Key Using SEcure Neighbor Discovery (SEND)
Distributing a Symmetric Fast Mobile IPv6 (FMIPv6) Handover Key Using SEcure Neighbor Discovery (SEND) 🦋
okay bestie, this is RFC 5269 💖 It's a Standards Track document from the Network Working Group, dated June 2008. The authors are J. Kempf (DoCoMo Labs USA) and R. Koodli (Starent Networks) 🫶 It doesn't obsolete or update any other RFCs, since there are no Obsoletes or Updates lines in its header.
Status of This Memo ✨
This document specifies an Internet standards track protocol for the Internet community, and it's asking for discussion and suggestions for improvements 🌸 For the standardization state and status of this protocol, check the current edition of the "Internet Official Protocol Standards" (STD 1). And distribution of this memo is unlimited, so share it with everyone 💅
Abstract 🎀
Okay so, Fast Mobile IPv6 needs a Fast Binding Update to be secured with a security association shared between an Access Router and a Mobile Node, or else certain attacks become possible 😭 This document defines a way to provision a shared key from the Access Router to the Mobile Node to protect that signaling 🔐
Here's how it goes. The Mobile Node makes a public/private key pair using the same public key algorithm as SEND (RFC 3971) 🧚 It sends the public key to the Access Router, and the Access Router encrypts a shared handover key with it and sends that back. The Mobile Node decrypts the handover key using the matching private key, and now it's ready to generate an authenticator on a Fast Binding Update 🦋 The whole exchange rides on the Router Solicitation for Proxy Advertisement and Proxy Router Advertisement from Fast Mobile IPv6.
The key exchange messages are required to have SEND security. That means the source address is a Cryptographically Generated Address (CGA), and the messages are signed with the CGA private key of the sending node 🔥 This lets the Access Router check that the Mobile Node is authorized to claim the address before it hands over the shared handover key. The previous care-of CGA in the Fast Binding Update can then act as the name of the key ✨
Table of Contents 🌈
1. Introduction ....................................................2
1.1. Terminology ................................................3
2. Overview of the Protocol ........................................3
2.1. Brief Review of SEND .......................................3
2.2. Protocol Overview ..........................................4
3. Handover Key Provisioning and Use ...............................4
3.1. Sending Router Solicitations for Proxy Advertisement .......4
3.2. Receiving Router Solicitations for Proxy
Advertisement and Sending Proxy Router Advertisements ......5
3.3. Receiving Proxy Router Advertisements ......................6
3.4. Sending FBUs ...............................................7
3.5. Receiving FBUs .............................................7
3.6. Key Generation and Lifetime ................................7
3.7. Protocol Constants .........................................8
4. Message Formats .................................................8
4.1. Handover Key Request Option ................................8
4.2. Handover Key Reply Option .................................10
5. Security Considerations ........................................11
6. IANA Considerations ............................................11
7. References .....................................................12
7.1. Normative References ......................................12
7.2. Informative References ....................................121. Introduction ✨
In Fast Mobile IPv6 (FMIPv6) [FMIP], a Fast Binding Update (FBU) is sent from a Mobile Node (MN), which is undergoing IP handover, to the previous Access Router (AR) 🦋 The FBU causes a routing change, so traffic sent to the MN's previous Care-of Address on the previous AR's link gets tunneled to the new Care-of Address on the new AR's link 🚇 Only an MN that's authorized to claim the address should be able to change the routing for the previous Care-of Address. If that authorization isn't established, an attacker can redirect a victim MN's traffic at will 💀 Scary, right?
In this document, there's a lightweight mechanism for the AR to provision a shared handover key on the MN, which is what secures FMIP 🌸 It uses SEND [SEND] plus an extra public/private key pair. The MN generates that pair with the same public key algorithm as SEND, and it's used to encrypt and decrypt the shared handover key that goes from the AR to the MN 🔐 The key provisioning can happen at some arbitrary time before handover, so it doesn't add any performance overhead to the handover itself ⭐
The message exchange between the MN and the AR that provisions the handover key is required to be protected by SEND. In other words, the source address for the key provisioning messages must be a CGA, and the messages must be signed with the CGA private key 🫶 This lets the AR establish the MN's authorization to operate on the CGA, and the AR uses that CGA to name the handover key. The SEND key pair is independent from both the handover encryption/decryption key pair and the actual handover key, so they're three separate things, bestie 🧚 Once the shared handover key is established and the MN goes through IP handover, the MN generates an authorization Message Authentication Code (MAC) on the FBU. The previous care-of CGA included in the FBU is what the AR uses to find the right handover key for checking that authorization 👑
Handover keys are an instantiation of the purpose built key architectural principle [PBK] 💜
1.1. Terminology 🎀
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 RFC 2119 [RFC2119] 📖
Here's the extra terminology we're using:
- CGA public key: the public key used to generate the CGA according to RFC 3972 [CGA] 🔑
- CGA private key: the private key that corresponds to the CGA public key 🗝️
- Handover key encryption public key: a public key generated by the MN and sent to the current AR so it can encrypt the shared handover key 🦋
- Handover key encryption private key: the private key that corresponds to the handover key encryption public key, and the MN holds onto it 🌸
2. Overview of the Protocol 🌈
2.1. Brief Review of SEND 💅
SEND protects against a variety of threats to local link address resolution (also known as Neighbor Discovery) and last hop router (AR) discovery in IPv6 [RFC3756] 🛡️ These threats aren't exclusive to wireless networks, but they're generally easier to pull off on certain wireless networks, because the link between the access point and the MN can't be physically secured 😤
SEND uses CGAs to secure Neighbor Discovery signaling [CGA]. Briefly, a CGA is formed by hashing together the IPv6 subnet prefix for a node's subnet, a random nonce, and an RSA public key, which is called the CGA public key 🧮 The CGA private key signs a Neighbor Advertisement (NA) message that's sent to resolve the link-layer address to the IPv6 address ✍️ The combination of the CGA and the signature on the NA proves to a receiving node that the sender is authorized to claim the address. The node can opportunistically generate one or several keys specifically for SEND, or it can use a certified key that it distributes more widely 🎀
2.2. Protocol Overview 🦋
The protocol uses the SEND secured Router Solicitation for Proxy Advertisement (RtSolPr)/Proxy Router Advertisement (PrRtAdv) [FMIP] exchange between the MN and the AR to carry an encrypted, shared handover key from the AR to the MN 📬 First, the MN generates the key pair it needs, plus the associated CGA addresses, so it can use SEND 🔐 Then the MN generates a public/private key pair for encrypting and decrypting the shared handover key, using the same public key algorithm that SEND uses 🧚
The MN then sends an RtSolPr message with a Handover Key Request Option that contains the handover key encryption public key. The source address of the RtSolPr message is the MN's care-of CGA on the AR's link. The RtSolPr message is signed with the MN's CGA key and contains the CGA Parameters option, following RFC 3971 [SEND] ✍️ The AR verifies the message using SEND. Then it uses the handover key encryption public key to encrypt a shared handover key, which goes back with the PrRtAdv in the Handover Key Reply Option 💌 The MN decrypts the shared handover key and uses it to set up an authorization MAC when it sends an FBU to the previous AR 🚀
3. Handover Key Provisioning and Use 👑
3.1. Sending Router Solicitations for Proxy Advertisement 📮
At some time before handover, the MN MUST generate a handover key encryption public/private key pair, using exactly the same public key algorithm with exactly the same parameters (key size, etc.) as SEND [SEND] 🔑 The MN can reuse that key pair on different access routers, but it MUST NOT use the key pair for any other encryption or for signature operations 🙅♀️ To prevent cryptanalysis, the key pair SHOULD be discarded after either a duration of HKEPK-LIFETIME or HKEPK-HANDOVERS number of handovers, whichever happens first ⏳ See Section 3.7 for definitions of protocol constants 📚
The MN MUST send a Router Solicitation for Proxy Advertisement (RtSolPr) that contains a Handover Key Request Option with the handover encryption public key 💌 A CGA for the MN MUST be the source address on the packet, and the MN MUST include the SEND CGA Option and SEND Signature Option with the packet, as specified in [SEND] 🫶 The SEND signature covers all fields in the RtSolPr, including the 128-bit source and destination addresses and the ICMP checksum as described in RFC 3971, except for the Signature Option itself 🧾 The MN also sets the handover authentication Algorithm Type (AT) extension field in the Handover Key Request Option to the MN's preferred FBU authentication algorithm 🎯 The SEND Nonce MUST also be included for anti-replay protection 🛡️✨
3.2. Receiving Router Solicitations for Proxy Advertisement and Sending Proxy Router Advertisements ✨
Okay so, when an FMIPv6 capable AR with SEND gets an RtSolPr from an MN that's protected with SEND and includes a Handover Key Request Option, the AR MUST first validate that RtSolPr using SEND, as described in RFC 3971. 💅 If the RtSolPr can not be validated, the AR MUST NOT include a Handover Key Reply Option in the reply. The AR also MUST NOT change any existing key record for the address, because that message could be an attacker trying to mess things up for a legit MN 🙈. The AR SHOULD respond to the RtSolPr but MUST NOT perform handover key provisioning.
If the RtSolPr can be validated, the AR MUST then figure out whether the CGA already has a shared handover key 🔑. If the CGA is associated with an existing handover key, the AR MUST return that existing handover key to the MN. If the CGA does not have a shared handover key, the AR MUST construct one as described in Section 3.6. The AR MUST encrypt the handover key with the handover key encryption public key included in the Handover Key Request Option. Then the AR MUST insert the encrypted handover key into a Handover Key Reply Option and MUST attach that Handover Key Reply Option to the PrRtAdv. The lifetime of the key, HK-LIFETIME, MUST also be included in the Handover Key Reply Option.
The AR SHOULD set the AT field of the Handover Key Option to the MN's preferred algorithm type, which is indicated in the AT field of the Handover Key Request Option, if that algorithm is supported. Otherwise, the AR MUST select an authentication algorithm that is of equivalent strength or stronger, and set the field to that one. The AR MUST also include the SEND nonce from the RtSolPr for anti-replay protection 🛡️.
The AR MUST have a certificate suitable for a SEND-capable router, support SEND certificate discovery, and include a SEND CGA Option and a SEND Signature Option in the PrRtAdv messages it sends. Similarly, the mobile nodes MUST be configured with one or more SEND trust anchors so that they can verify these messages. The SEND signature covers all fields in the PrRtAdv, including the 128-bit source and destination addresses and ICMP checksum as described in RFC 3971, except for the Signature Option itself. The PrRtAdv is then unicast back to the MN at the MN's care-of CGA, which was the source address on the RtSolPr 💌.
Last but not least, the handover key MUST be stored by the AR for future use, indexed by the CGA. The authentication algorithm type (i.e., the resolution of the AT field processing) and HK-LIFETIME MUST be recorded with the key 📝.
3.3. Receiving Proxy Router Advertisements 🦋
Upon receipt of one or more PrRtAdvs that are secured with SEND and have the Handover Key Reply Option, the MN MUST first validate the PrRtAdvs as described in RFC 3971. Normally the MN will already have the router's certification path to validate an RA, which it got before sending the PrRtSol. Even so, the MN MUST check to ensure that the key used to sign the PrRtAdv is the router's certified public key. If the MN does not have the router's certification path cached, it MUST use the SEND CPS/CPA messages to obtain the certification path and validate the key. If a certified key from the router was not used to sign the message, the message MUST be dropped 🚫.
From the messages that validate, the MN SHOULD choose one with an AT flag in the Handover Key Reply Option that indicates an authentication algorithm the MN supports 💜. From that message, the MN MUST determine which handover key encryption public key to use in case the MN has more than one. The MN finds the right public key by matching the SEND nonce from the RtSolPr. If no such match occurs, the MN MUST drop the PrRtAdv. The MN MUST use the matching private key to decrypt the handover key with its handover key encryption private key, and then store the handover key for later use. It gets named with the AR's CGA, along with the algorithm type and HK-LIFETIME. The MN MUST use the returned algorithm type indicated in the PrRtAdv.
The MN MUST index the handover keys with the AR's IPv6 address, which is where the MN later sends the FBU, and with the MN's CGA that the handover key applies to. This lets the MN pick the proper key when it's talking to a previous AR 🧚. Before HK-LIFETIME expires, the MN MUST request a new key from the AR if FMIPv6 service is still required from the router ⏰.
If more than one router responds to the RtSolPr, the MN MAY keep track of all of those keys. If none of the PrRtAdvs contains an algorithm type indicator for an algorithm the MN supports, the MN MAY re-send the RtSolPr requesting a different algorithm. But to prevent bidding down attacks from compromised routers, the MN SHOULD NOT request an algorithm that is weaker than its original request 🛡️💀.
3.4. Sending FBUs 📤
When the MN needs to signal the Previous AR (PAR) using an FMIPv6 FBU, the MN MUST use the handover key and the corresponding authentication algorithm to generate an authenticator for the message. The MN MUST select the right key for the PAR using the PAR's CGA and the MN's previous care-of CGA on the PAR's link. As defined by the FMIPv6 [FMIP], the MN MUST generate the authentication MAC using the handover key and the appropriate algorithm, and MUST include that MAC in the FBU message. As specified by FMIPv6, the MN MUST include the old care-of CGA in a Home Address Option. The FMIPv6 document has more detail about how the authenticator on the FBU is built 📚.
3.5. Receiving FBUs 📥
When the PAR receives an FBU message containing an authenticator, the PAR MUST find the matching handover key, using the MN's care-of CGA from the Home Address Option as the index. If a handover key is found, the PAR MUST use the handover key and the appropriate algorithm to verify the authenticator. If the handover key is not found, the PAR MUST NOT change forwarding for the care-of CGA 🙅♀️. The FMIPv6 document [FMIP] has more detail on how the AR processes an FBU containing an authenticator.
3.6. Key Generation and Lifetime 🗝️
The AR MUST randomly generate a key with sufficient strength to match the authentication algorithm. Some authentication algorithms specify a required key size. The AR MUST generate a unique key for each CGA public key, and SHOULD take care that key generation is uncorrelated between handover keys, and between handover keys and CGA keys. The actual algorithm used to generate the key doesn't matter for interoperability, since only the AR generates the key and the MN simply uses it 🤷♀️.
A PAR SHOULD NOT discard the handover key immediately after use if it is still valid. The MN could move really fast to another AR before the Mobile IPv6 binding update finishes on the PAR 🏃♀️💨, and the MN MAY as a consequence start another, subsequent handover optimization to move traffic from the PAR to another new AR. The default time for keeping the key valid matches the default time that forwarding from the PAR to the new AR is performed for FMIP. The FMIPv6 document [FMIP] has more detail about the FMIP forwarding time default.
If the MN returns to a PAR before the handover key expires, the PAR MAY send, and the MN MAY receive, the same handover key that was returned before, as long as the MN generates the same CGA for its Care-of Address. However, the MN MUST NOT assume it can keep using the old key without actually receiving the handover key again from the PAR 🙅. The MN SHOULD discard the handover key after the MIPv6 binding update is complete on the new AR. The PAR MUST discard the key after FMIPv6 forwarding for the previous Care-of Address times out, or when HK-LIFETIME expires ⏳.
3.7. Protocol Constants 🧮
The following are protocol constants with suggested defaults:
HKEPK-LIFETIME: The maximum lifetime for the handover key
encryption public key. Default is 12 hours.
HKEPK-HANDOVERS: The maximum number of handovers for which the
handover key encryption public key should be
reused. Default is 10.
HK-LIFETIME: The maximum lifetime for the handover key. Default
is 12 hours (43200 seconds).4. Message Formats 🎀
4.1. Handover Key Request Option 🌸
The Handover Key Request Option is a standard IPv6 Neighbor Discovery [RFC4861] option in TLV format. It gets included in the RtSolPr message along with the SEND CGA Option, RSA Signature Option, and Nonce Option.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Pad Length | AT |Resrvd.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Handover Key Encryption Public Key .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Here are the fields, bestie 👇
Type: The value is27.Length: The length of the option in units of 8 octets, including theTypeandLengthfields. The value0is invalid. The receiver MUST discard a message that contains this value 🚫.Pad Length: The number of padding octets beyond the end of theHandover Key Encryption Public Keyfield but within the length specified by theLengthfield. Padding octets MUST be set to zero by senders and ignored by receivers 🧸.AT: A 4-bit algorithm type field describing the algorithm used by FMIPv6 to calculate the authenticator. See [FMIP] for details.Resrvd.: A 4-bit field reserved for future use. The value MUST be initialized to zero by the sender and MUST be ignored by the receiver 🔒.Handover Key Encryption Public Key: The handover key encryption public key. The key MUST be formatted according to the same specification as the CGA key in the CGA Parameters Option [CGA] of the message, and MUST have the same parameters as the CGA key 🔐.Padding: A variable-length field that makes the option length a multiple of 8, containing as many octets as specified in thePad Lengthfield 🎀.
4.2. Handover Key Reply Option 💖
Okay so the Handover Key Reply Option is a standard IPv6 Neighbor Discovery [RFC4861] option, and it's in TLV format 🎀 It rides along inside the PrRtAdv message with the SEND CGA Option, RSA Signature Option, and Nonce Option. Main character energy ✨
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Pad Length | AT |Resrvd.|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Lifetime | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
. .
. Encrypted Handover Key .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Here's the breakdown of each field, bestie 🧚
Type: it's 28 💅Length: the length of the option in units of 8 octets, counting theTypeandLengthfields too. A value of 0 is invalid, and the receiver MUST discard any message that has this value. Not me getting rejected over a zero 💀Pad Length: the number of padding octets sitting after the end of theEncrypted Handover Keyfield but still inside the length set by theLengthfield. Padding octets MUST be set to zero by senders and ignored by receivers 🫶AT: a 4-bit algorithm type field that describes the algorithm FMIPv6 uses to calculate the authenticator. Check [FMIP] for the deets 🔥Resrvd.: a 4-bit field that's reserved for future use. The value MUST be initialized to zero by the sender and MUST be ignored by the receiver. Reserved means hands off, iykyk 🙈Key Lifetime: the lifetime of the handover key, HK-LIFETIME, in seconds ⏳Encrypted Handover Key: the shared handover key, encrypted with the MN's handover key encryption public key, using the RSAES-PKCS1-v1_5 format [RFC3447]. It's giving secret-keeping energy 🔐Padding: a variable-length field that makes the option length a multiple of 8. It holds as many octets as thePad Lengthfield says 🌈
5. Security Considerations 🔥
Okay, this section is serious, so grab your snacks and listen up 🍓 This document describes a shared key provisioning protocol for the FMIPv6 handover optimization protocol. The key provisioning protocol uses a public key generated with the same public key algorithm as SEND to bootstrap a shared key. That shared key is used to authorize changes caused by handover that are tied to the MN's former address on the PAR.
General security considerations about CGAs apply to the protocol in this document too. For a discussion of security considerations around CGAs, see [CGA] 👑
This protocol faces the same risks from replay attacks and denial-of-service (DoS) attacks using the RtSolPr as the SEND protocol [SEND] does for RS. It's the same vibe, honestly. The measures recommended in RFC 3971 for mitigating replay attacks and DoS attacks apply here as well 🛡️
Here's the extra one to watch. You have to think about when to generate the handover key on the AR. To avoid state depletion attacks, the handover key SHOULD NOT be generated prior to SEND processing that verifies the originator of RtSolPr. Don't make the key before you know who's knocking, periodt 💀
State depletion attacks can be handled with techniques such as rate limiting RtSolPr, restricting the amount of state reserved for unresolved solicitations, and clever cache management. These techniques are the same as the ones used when implementing Neighbor Discovery, so you already know this stuff, bestie 🧠
For other FMIPv6 security considerations, please check the FMIPv6 document [FMIP] ✨
6. IANA Considerations 🌸
IANA has assigned IPv6 Neighbor Discovery option type codes for the two new IPv6 Neighbor Discovery options defined in this document. The Handover Key Request Option gets 27, and the Handover Key Reply Option gets 28 🦋
7. References 🧚
7.1. Normative References
- [FMIP] Koodli, R., Ed., "Mobile IPv6 Fast Handovers", RFC 5268, June 2008.
- [SEND] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005.
- [CGA] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC 3972, March 2005.
- [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
- [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007.
- [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003.
7.2. Informative References
- [RFC3756] Nikander, P., Ed., Kempf, J., and E. Nordmark, "IPv6 Neighbor Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
- [PBK] Bradner, S., Mankin, A., and Schiller, J., "A Framework for Purpose-Built Keys (PBK)", Work in Progress, June 2003. Progress.
Authors' Addresses 💌
James Kempf
DoCoMo Labs USA
3240 Hillview Avenue
Palo Alto, CA 94303
USA
Phone: +1 650 496 4711
EMail: kempf@docomolabs-usa.com
Rajeev Koodli
Starent Networks
30 International Place
Tewksbury, MA 01876
USA
Phone: +1 408 735 7679
EMail: rkoodli@starentnetworks.comFull Copyright Statement 📜
Copyright (C) The IETF Trust (2008).
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