RFC 3565

Use of the Advanced Encryption Standard (AES) Encryption Algorithm in Cryptographic Message Syntax (CMS)

📅 published July 2003, from ietf

RFC 3565: Use of the Advanced Encryption Standard (AES) Encryption Algorithm in Cryptographic Message Syntax (CMS) 🎀

Okay bestie, the header says this one is by J. Schaad from Soaring Hawk Consulting 🦅, dated July 2003, and it's a Standards Track doc, so it's an actual Internet standard and not just a vibe 💅. The header doesn't list any Obsoletes or Updates, so nothing is replaced or changed by it 🌸.

Status of this Memo ✨

This document is an Internet standards track protocol spec 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) 📖. Distribution of this memo is unlimited 🌈.

Copyright (C) The Internet Society (2003). All Rights Reserved. 🔒

Abstract

This doc lays out how to use the Advanced Encryption Standard (AES) algorithm for encryption with the Cryptographic Message Syntax (CMS) 🔐. It's giving "here's the recipe, follow it" energy 🍓.

Conventions used in this document

Okay so the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be read as described in BCP 14, RFC 2119 [MUSTSHOULD] 🧠. Those capital words are the serious ones, so pay attention to them 👑.

1. Overview 🦋

Here's the main idea: this doc is about using the Advanced Encryption Standard (AES) content encryption algorithm with the Cryptographic Message Syntax [CMS] enveloped-data and encrypted-data content types 💜.

CMS values are generated using ASN.1 [X.208-88], using the Basic Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules (DER) [X.509-88] 🧚.

1.1. AES ✨

AES [AES] was made to replace DES [DES] 💅. The AES Federal Information Processing Standard (FIPS) Publication describes a cryptographic algorithm for use by U.S. Government organizations 🇺🇸. Honestly though, AES is also going to be used widely by organizations, institutions, and individuals outside of the U.S. Government, so it's not just a government thing 🌍.

Two researchers who developed and submitted the Rijndael algorithm for consideration are both cryptographers from Belgium 🇧🇪: Dr. Joan Daemen of Proton World International, and Dr. Vincent Rijmen, a postdoctoral researcher in the Electrical Engineering Department of Katholieke Universiteit Leuven 🎓.

The National Institute of Standards and technology (NIST) picked the Rijndael algorithm for AES because it has a combination of security, performance, efficiency, ease of implementation, and flexibility 🔥. Rijndael seems to be consistently a very good performer in both hardware and software across a wide range of computing environments, no matter whether it's used in feedback or non-feedback modes 💖. Its key setup time is excellent, and its key agility is good ⭐. Its very low memory requirements make it super well suited for restricted-space environments, and it also shows excellent performance there 🧚. The Rijndael algorithm operations are among the easiest to defend against power and timing attacks 🛡️. Also, it looks like some defense can be provided against those attacks without really hurting the algorithm's performance 🫶. Finally, the algorithm's internal round structure seems to have good potential to benefit from instruction-level parallelism 🌈.

The AES specifies three key sizes: 128, 192 and 256 bits 🔑.

2. Enveloped-data Conventions 💖

The CMS enveloped-data content type is made of encrypted content and wrapped content-encryption keys for one or more recipients 🎀. The AES algorithm is used to encrypt the content 🔐.

Compliant software MUST meet the requirements for constructing an enveloped-data content type stated in [CMS] Section 6, "Enveloped-data Content Type" 📋.

The AES content-encryption key MUST be randomly generated for each instance of an enveloped-data content type 🎲. The content-encryption key (CEK) is what's used to encrypt the content 🗝️.

AES can be used with the enveloped-data content type using any of the following key management techniques defined in [CMS] Section 6 🌸:

  1. Key Transport: The AES CEK is uniquely wrapped for each recipient using the recipient's public RSA key and other values. Section 2.2 provides additional details 💌.
  2. Key Agreement: The AES CEK is uniquely wrapped for each recipient using a pairwise symmetric key-encryption key (KEK). That KEK is generated using an originator's randomly generated private key (ES-DH [DH]) or previously generated private key (SS-DH [DH]), the recipient's public DH key, and other values. Section 2.3 provides additional details 🤝.
  3. Previously Distributed Symmetric KEK: The AES CEK is wrapped using a previously distributed symmetric KEK (such as a Mail List Key). The methods by which the symmetric KEK is generated and distributed are beyond the scope of this document. Section 2.4 provides additional details 📬.
  4. Password Encryption: The AES CEK is wrapped using a KEK derived from a password or other shared secret. Section 2.5 provides additional details 🔑.

Documents that define the use of the Other Recipient Info structure will need to define the proper use for the AES algorithm, if they want it 📝.

2.1. EnvelopedData Fields 🧩

The enveloped-data content type is ASN.1 encoded using the EnvelopedData syntax 🧾. The fields of the EnvelopedData syntax MUST be populated as follows:

The EnvelopedData version is determined based on a number of factors 🔢.

See [CMS] section 6.1 for the algorithm to determine this value 🧮.

The EnvelopedData recipientInfos CHOICE depends on the key management technique used. Section 2.2, 2.3, 2.4 and 2.5 provide additional information 💅.

The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm field MUST specify a symmetric encryption algorithm. Implementations MUST support content encryption with AES, but implementations MAY support other algorithms as well 🌟.

The EnvelopedData unprotectedAttrs MAY be present 🎈.

2.2. KeyTransRecipientInfo Fields 🗝️

The enveloped-data content type is ASN.1 encoded using the EnvelopedData syntax 🧾. The fields of the EnvelopedData syntax MUST be populated as follows:

The KeyTransRecipientInfo version MUST be either 0 or 2. If the RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the version MUST be 0. If the RecipientIdentifier is subjectKeyIdentifier, then the version MUST be 2 🔀.

The KeyTransRecipientInfo RecipientIdentifier gives two options for specifying the recipient's certificate, and therefore the recipient's public key 📜. The recipient's certificate MUST contain an RSA public key 🔓. The CEK is encrypted with the recipient's RSA public key 🔐. The issuerAndSerialNumber option identifies the recipient's certificate by the issuer's distinguished name and the certificate serial number 🪪. The subjectKeyIdentifier option identifies the recipient's certificate by the X.509 subjectKeyIdentifier extension value 🆔.

The KeyTransRecipientInfo keyEncryptionAlgorithm field specifies the key transport algorithm (i.e., RSAES-OAEP [RSA-OAEP]), and the associated parameters used to encrypt the CEK for the recipient 🚚.

The KeyTransRecipientInfo encryptedKey is the result of encrypting the CEK with the recipient's RSA public key 🎁.

2.3. KeyAgreeRecipientInfo Fields 🤝

This section describes the conventions for using ES-DH or SS-DH and AES with the CMS enveloped-data content type to support key agreement 🌷. When key agreement is used, then the RecipientInfo keyAgreeRecipientInfo CHOICE MUST be used 💫.

The KeyAgreeRecipient version MUST be 3 🔢.

The EnvelopedData originatorInfo field MUST be the originatorKey alternative. The originatorKey algorithm fields MUST contain the dh-public-number object identifier with absent parameters. The originatorKey publicKey MUST contain the originator's ephemeral public key 🌙.

The EnvelopedData ukm MAY be present 🎈.

The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH algorithm identifier [CMSALG] 🏷️.

2.3.1. ES-DH/AES Key Derivation 🧪

Generating the AES KEK, which gets used with the AES-key wrap algorithm, is done using the method described in [DH] 🪄.

2.3.1.1. Example 1 🍰

Here's a worked example, so you can see the bytes for yourself 👀:

ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
                   0a 0b 0c 0d 0e 0f 10 11 12 13

The key wrap algorithm is AES-128 wrap, so we need 128 bits (16 bytes) of keying material 🔑.

No partyAInfo is used 🙅.

So the input to SHA-1 is:

00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 1b
   30 11
      06 09 60 86 48 01 65 03 04 01 05           ; AES-128 wrap OID
      04 04
         00 00 00 01                             ; Counter
   a2 06
      04 04
      00 00 00 80                                ; key length

And the output is the 32 bytes:

d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64 49 08 50 f9

So, consequently:

K= d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64

2.3.1.2. Example 2 💎

Same ZZ, but this time it's the AES-256 key wrap, so we need 256 bits (32 bytes) of keying material 💪.

The partyAInfo used is the 64 bytes 🌈:

01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01

So the input to the first invocation of SHA-1 is:

00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 5f
   30 11
      06 09 60 86 48 01 65 03 04 01 2d            ; AES-256 wrap OID
      04 04
         00 00 00 01                              ; Counter
   a0 42
      04 40
         01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo

         01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
         01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
         01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
   a2 06
      04 04
         00 00 01 00                              ; key length

And the output is the 20 bytes:

88 90 58 5C 4E 28 1A 5C 11 67 CA A5 30 BE D5 9B 32 30 D8 93

The input to the second invocation of SHA-1 is:

00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
30 5f
   30 11
      06 09 60 86 48 01 65 03 04 01 2d            ; AES-256 wrap OID
      04 04
         00 00 00 02                              ; Counter
   a0 42
      04 40
         01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo

         01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
         01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
         01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
   a2 06
      04 04
         00 00 01 00                              ; key length

And the output is the 20 bytes:

CB A8 F9 22 BD 1B 56 A0 71 C9 6F 90 36 C6 04 2C AA 20 94 37

So, consequently:

K = 88 90 58 5C 4E 28 1A 5C 11 67 CA A5 30 BE D5 9B
    32 30 D8 93 CB A8 F9 22 BD 1B 56 A0

To be continued in part 2 🧚💖

2.3.2. AES CEK Wrap Process 🎀

okay so the AES key wrap algorithm takes one AES key and encrypts it inside another AES key 🔐✨ the output comes out 64-bits longer than the input AES CEK, and those extra bits are a checksum 🧁 it runs 6*n AES encryption/decryption operations, where n is the number of 64-bit blocks in the AES CEK 💅 if you want the full tea on how the algorithm works, it's all in [AES-WRAP] 📚

NIST assigned the following OIDs to define the AES key wrap algorithm 🦋

id-aes128-wrap OBJECT IDENTIFIER ::= { aes 5 }
id-aes192-wrap OBJECT IDENTIFIER ::= { aes 25 }
id-aes256-wrap OBJECT IDENTIFIER ::= { aes 45 }

in every case the parameters field MUST be absent 🙅‍♀️ the OID tells you the KEK key size, but it says nothing about how big the wrapped AES CEK is 🤷‍♀️ implementations MAY use different KEK and CEK sizes 💜 implements MUST support the CEK and the KEK having the same length. 💖 if different lengths are supported, the KEK MUST be of equal or greater length than the CEK 🫶

2.4. KEKRecipientInfo Fields 🔑

this section is all about how to use AES with the CMS enveloped-data content type when you're using a symmetric KEK that was already distributed beforehand 🌸 when a previously distributed symmetric KEK is used to wrap the AES CEK, the RecipientInfo KEKRecipientInfo CHOICE MUST be used 💅 how you generate and distribute that symmetric KEK is out of scope for this doc 🙈 one possible way to distribute keys is written up in [SYMKEYDIST] ✨

the KEKRecipientInfo fields MUST be populated as specified in [CMS] Section 6.2.3, KEKRecipientInfo Type 🎀

the KEKRecipientInfo keyEncryptionAlgorithm algorithm field MUST be one of the OIDs defined in section 2.3.2, which tells everyone that the AES wrap function is being used to wrap the AES CEK 🦋 and the KEKRecipientInfo keyEncryptionAlgorithm parameters field MUST be absent 🙅‍♀️

the KEKRecipientInfo encryptedKey field MUST include the AES CEK wrapped using the previously distributed symmetric KEK as input to the AES wrap function 🔐💖

2.5. PasswordRecipientInfo Fields 🗝️

this section is the same kind of deal, but for password-based key management with the CMS enveloped-data content type 🌈

when a password derived KEK is used to wrap the AES CEK, then the RecipientInfo PasswordRecipientInfo CHOICE MUST be used 🙌

the keyEncryptionAlgorithm algorithm field MUST be one of the OIDs defined in section 2.3.2, indicating that the AES wrap function is used to wrap the AES CEK ✨ and the keyEncryptionAlgorithm parameters field MUST be absent 🙅‍♀️

the encryptedKey field MUST be the result of the AES key wrap algorithm applied to the AES CEK value 💜

3. Encrypted-data Conventions 🔒

the CMS encrypted-data content type is encrypted content with implicit key management 🫧 the AES algorithm is what encrypts the content 💅

compliant software MUST meet the requirements for constructing an enveloped-data content type stated in [CMS] Section 8, "Encrypted-data Content Type" 🎀

the encrypted-data content type is ASN.1 encoded using the EncryptededData syntax 🧚 the fields of the EncryptedData syntax MUST be populated as follows:

the EncryptedData version is determined based on a number of factors 🌸 see [CMS] section 9.1 for the algorithm that works out this value 🧮

the EncryptedData encryptedContentInfo contentEncryptionAlgorithm field MUST specify a symmetric encryption algorithm 🔥 implementations MUST support encryption using AES, but implementations MAY support other algorithms as well 🦋

the EncryptedData unprotectedAttrs MAY be present ✨

4. Algorithm Identifiers and Parameters 🆔

this section specified algorithm identifiers for the AES encryption algorithm 💖

4.1. AES Algorithm Identifiers and Parameters 🧩

the AES algorithm is defined in [AES] 📖 and RSAES-OAEP [RSA-OAEP] MAY be used to transport AES keys 🫶

AES is added to the set of symmetric content encryption algorithms defined in [CMSALG] 🌈 the AES content-encryption algorithm, in Cipher Block Chaining (CBC) mode, comes in three key sizes, and each one is identified by the following object identifiers 👑

id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 }
id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 }
id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }

the AlgorithmIdentifier parameters field MUST be present, and the parameters field MUST contain an AES-IV 🎀

AES-IV ::= OCTET STRING (SIZE(16))

content encryption algorithm identifiers live in the EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm field and the EncryptedData EncryptedContentInfo contentEncryptionAlgorithm field 🗂️

content encryption algorithms are used to encrypt the content found in the EnvelopedData EncryptedContentInfo encryptedContent field and the EncryptedData EncryptedContentInfo encryptedContent field 🔐✨

5. SMIMECapabilities Attribute Conventions 📣

an S/MIME client SHOULD announce the set of cryptographic functions it supports by using the S/MIME capabilities attribute 💅 this attribute gives a partial list of object identifiers for cryptographic functions, and it MUST be signed by the client ✍️ the algorithm OIDs SHOULD be logically separated into functional categories, and they MUST be ordered by preference 🗂️💜

RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed attribute (a SEQUENCE of SMIMECapability SEQUENCEs) that's used to specify a partial list of algorithms the software announcing the SMIMECapabilities can support 🦋

5.1. AES S/MIME Capability Attributes 🌸

if an S/MIME client is required to support symmetric encryption with AES, the capabilities attribute MUST contain the AES object identifier specified above in the category of symmetric algorithms 🔑 the parameter with this encoding MUST be absent 🙅‍♀️

the encodings for the mandatory key sizes are:

      Key Size                   Capability
       128          30 0B 06 09 60 86 48 01 65 03 04 01 02
       196          30 0B 06 09 60 86 48 01 65 03 04 01 16
       256          30 0B 06 09 60 86 48 01 65 03 04 01 2A

when a sending agent creates an encrypted message, it has to decide which type of encryption algorithm to use 🤔 in general that decision uses info from the capabilities lists in messages received from the recipient, plus other stuff like private agreements, user preferences, legal restrictions, and so on 🧠 if users require AES for symmetric encryption, the S/MIME clients on both the sending and receiving side MUST support it, and it MUST be set in the user preferences 💖

6. Security Considerations 🚨

⚠️ this part is seriously important, so read it carefully bestie 😭

if RSA-OAEP [PKCS#1v2.0] and RSA PKCS #1 v1.5 [PKCS#1v1.5] are both used to transport the same CEK, then an attacker can still use the Bleichenbacher attack against the RSA PKCS #1 v1.5 encrypted key 💀 it's generally unadvisable to mix RSA-OAEP and RSA PKCS#1 v1.5 in the same set of recipients 🙈

implementations must protect the RSA private key and the CEK 🔐 compromise of the RSA private key may result in the disclosure of all messages protected with that key 😱 compromise of the CEK may result in disclosure of the associated encrypted content 💔

the generation of AES CEKs relies on random numbers 🎲 using inadequate pseudo-random number generators (PRNGs) to generate these values can result in little or no security 💀 an attacker may find it much easier to reproduce the PRNG environment that produced the keys and then search the resulting small set of possibilities, instead of brute force searching the whole key space 😭 generating quality random numbers is difficult 🥺 RFC 1750 [RANDOM] offers important guidance in this area 📚

when wrapping a CEK with a KEK, the KEK MUST always be at least the same length as the CEK 🔒 an attacker will generally work at the weakest point in an encryption system 🎯 for a brute force attack, that's the smaller of the two key sizes 💅

Normative References 📚

  • [AES] National Institute of Standards. FIPS Pub 197: Advanced Encryption Standard (AES). 26 November 2001.
  • [CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369, August 2002.
  • [AES-WRAP] Schaad, J. and R. Housley, "Advanced Encryption Standard (AES) Key Wrap Algorithm", RFC 3394, September 2002.
  • [CMSALG] Housley, R., "Cryptographic Message Syntax (CMS) Algorithms, RFC 3370, August 2002.
  • [DES] National Institute of Standards and Technology. FIPS Pub 46: Data Encryption Standard. 15 January 1977.
  • [DH] Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC 2631, June 1999.
  • [MUSTSHOULD] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
  • [RSA-OAEP] Housley, R. "Use of the RSAES-OAEP Key Transport Algorithm in the Cryptographic Message Syntax (CMS)", RFC 3560, July 2003.
  • [X.208-88] CCITT. Recommendation X.208: Specification of Abstract Syntax Notation One (ASN.1). 1988.
  • [X.209-88] CCITT. Recommendation X.209: Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1). 1988.
  • [X.509-88] CCITT. Recommendation X.509: The Directory - Authentication Framework. 1988.

Informational References 🧾

  • [MSG] Ramsdell, B., Editor, "S/MIME Version 3 Message Specification", RFC 2633, June 1999.
  • [PKCS#1v1.5] Kaliski, B., "PKCS #1: RSA Encryption, Version 1.5", RFC 2313, March 1998.
  • [PKCS#1v2.0] Kaliski, B., "PKCS #1: RSA Encryption, Version 2.0", RFC 2437, October 1998.
  • [RANDOM] Eastlake, D., Crocker, S. and J. Schiller, "Randomness Recommendations for Security", RFC 1750, December 1994.
  • [SYMKEYDIST] Turner, S., "CMS Symmetric Key Management and Distribution", Work in Progress, January 2003.

Acknowledgements 🫶

this document is the result of contributions from tons of professionals 🙌 we appreciate all the hard work from every member of the IETF S/MIME Working Group 💜

Appendix A ASN.1 Module 🧚

CMSAesRsaesOaep {iso(1) member-body(2) us(840) rsadsi(113549)
      pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-aes(19) }


DEFINITIONS IMPLICIT TAGS ::=
BEGIN

-- EXPORTS ALL --
IMPORTS
    -- PKIX
      AlgorithmIdentifier
          FROM PKIXExplicit88 {iso(1) identified-organization(3) dod(6)
              internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
              id-pkix1-explicit(18)};

-- AES information object identifiers --

aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840)
               organization(1) gov(101) csor(3)_ nistAlgorithms(4)  1 }

-- AES using CBC-chaining mode for key sizes of 128, 192, 256

id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 }
id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 }
id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }

-- AES-IV is a the parameter for all the above object identifiers.

AES-IV ::= OCTET STRING (SIZE(16))


-- AES Key Wrap Algorithm Identifiers  - Parameter is absent

id-aes128-wrap OBJECT IDENTIFIER ::= { aes 5 }
id-aes192-wrap OBJECT IDENTIFIER ::= { aes 25 }
id-aes256-wrap OBJECT IDENTIFIER ::= { aes 45 }


END

Author's Address 🖊️

Jim Schaad Soaring Hawk Consulting

EMail: jimsch@exmsft.com

okay bestie, time for the legal bit 🙈 this is the copyright part, and it's giving: Copyright (C) The Internet Society (2003). All Rights Reserved. ✨

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BUT the document itself can't be changed in any way 🙅‍♀️ you can't remove the copyright notice, and you can't remove the references to the Internet Society or other Internet organizations. there are two exceptions:

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