Request for Comments: 6033
Category: Standards Track
Algorithms for Cryptographic Message Syntax (CMS)
Encrypted Key Package Content Type
This document describes the conventions for using several cryptographic algorithms with the Cryptographic Message Syntax (CMS) encrypted key package content type. Specifically, it includes conventions necessary to implement EnvelopedData, EncryptedData, and AuthEnvelopedData.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6033.
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This document describes the conventions for using several cryptographic algorithms with the Cryptographic Message Syntax (CMS) encrypted key package content type [RFC6032]. Specifically, it includes conventions necessary to implement the following CMS content types: EnvelopedData [RFC5652], EncryptedData [RFC5652], and AuthEnvelopedData [RFC5083].
This document does not define any new algorithms; instead, it refers to previously defined algorithms.
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].
EnvelopedData [RFC5652] supports a number of key management techniques. Implementations that claim conformance to this document MUST support the key transport mechanisms and SHOULD support the key agreement mechanisms as defined below. Other techniques MAY be supported.
When key transport is used, RSA encryption [RFC3370] MUST be supported and RSA Encryption Scheme - Optimal Asymmetric Encryption Padding (RSAES-OAEP) [RFC3560] SHOULD be supported.
When key agreement is used, Ephemeral-Static Diffie-Hellman (DH) [RFC3370] MUST be supported.
Since the content type is used to carry a cryptographic key and its attributes, an algorithm that is traditionally used to encrypt one key with another is employed. Regardless of the key management technique choice, implementations MUST support AES-128 Key Wrap with Padding [RFC5649] as the content-encryption algorithm. Implementations SHOULD support AES-256 Key Wrap with Padding [RFC5649] as the content-encryption algorithm.
When key agreement is used, a key wrap algorithm is also specified to wrap the content-encryption key. If the content-encryption algorithm is AES-128 Key Wrap with Padding, then the key wrap algorithm MUST be AES-128 Key Wrap with Padding [RFC5649]. If the content-encryption algorithm is AES-256 Key Wrap with Padding, then the key wrap algorithm MUST be AES-256 Key Wrap with Padding [RFC5649].
EncryptedData [RFC5652] requires that keys be managed by other means; therefore, the only algorithm specified is the content-encryption algorithm. Since the content type is used to carry a cryptographic key and its attributes, an algorithm that is traditionally used to encrypt one key with another is employed. Implementations MUST support AES-128 Key Wrap with Padding [RFC5649]. Implementations SHOULD support AES-256 Key Wrap with Padding [RFC5649].
AuthEnvelopedData [RFC5083], like EnvelopedData, supports a number of key management techniques. The key management requirements for AuthEnvelopedData are the same as for EnvelopedData. The difference is the content-encryption algorithm. Implementations MUST support 128-bit AES-Galois/Counter Mode (AES-GCM) [RFC5084] and SHOULD support 256-bit AES-GCM [RFC5084]. Implementations MAY also support AES-Counter with CBC-MAC (AES-CCM) [RFC5084].
5. Signed Data
Implementations of SignedData [RFC5652] MUST support the signature scheme RSA [RFC3370] [RFC5754] and SHOULD support the signature schemes RSA Probabilistic Signature Scheme (RSASSA-PSS) [RFC4056] and Digital Signature Algorithm (DSA) [RFC3370] [RFC5754]. Additionally, implementations MUST support in concert with these signature schemes the hash function SHA-256 [RFC5754] and it SHOULD support the hash function SHA-1 [RFC3370].
6. Public Key Sizes
The easiest way to implement SignedData, EnvelopedData, and AuthEnvelopedData is with public key certificates [RFC5280]. If an implementation supports RSA, RSAES-OAEP, DH, RSASSA-PSS, or DSA, then it MUST support key lengths from 1024 bits to 2048 bits, inclusive.
7. Security Considerations
The security considerations from [RFC3370], [RFC3560], [RFC4056], [RFC5083], [RFC5084], [RFC5649], [RFC5652], [RFC5754], and [RFC6032] apply.
The choice of content-encryption algorithms for this document was based on [RFC5649]: "In the design of some high assurance cryptographic modules, it is desirable to segregate cryptographic keying material from other data. The use of a specific cryptographic mechanism solely for the protection of cryptographic keying material can assist in this goal". Unfortunately, there is no AES-GCM or AES-CCM mode that provides the same properties. If an AES-GCM and AES-CCM mode that provides the same properties is defined, then this document will be updated to adopt that algorithm.
[SP800-57] provides comparable bits of security for some algorithms and key sizes. [SP800-57] also provides time frames during which certain numbers of bits of security are appropriate, and some environments may find these time frames useful.
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3370] Housley, R., "Cryptographic Message Syntax (CMS) Algorithms", RFC 3370, August 2002. [RFC3560] Housley, R., "Use of the RSAES-OAEP Key Transport Algorithm in Cryptographic Message Syntax (CMS)", RFC 3560, July 2003. [RFC4056] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in Cryptographic Message Syntax (CMS)", RFC 4056, June 2005. [RFC5083] Housley, R., "Cryptographic Message Syntax (CMS) Authenticated-Enveloped-Data Content Type", RFC 5083, November 2007. [RFC5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated Encryption in the Cryptographic Message Syntax (CMS)", RFC 5084, November 2007. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, May 2008. [RFC5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard (AES) Key Wrap with Padding Algorithm", RFC 5649, September 2009. [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, September 2009. [RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic Message Syntax", RFC 5754, January 2010. [RFC6032] Turner, S. and R. Housley, "Cryptographic Message Syntax (CMS) Encrypted Key Package Content Type", RFC 6032, December 2010.
8.2. Informative References
[SP800-57] National Institute of Standards and Technology (NIST), Special Publication 800-57: Recommendation for Key Management - Part 1 (Revised), March 2007.
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