Internet Engineering Task Force (IETF)                         S. Turner
Request for Comments: 6033                                          IECA
Category: Standards Track                                  December 2010
ISSN: 2070-1721


           Algorithms for Cryptographic Message Syntax (CMS)
                   Encrypted Key Package Content Type

Abstract

   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.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

1.  Introduction

   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.

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 [RFC2119].

2.  EnvelopedData

   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].

3.  EncryptedData

   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].

4.  AuthEnvelopedData

   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.  References

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.

Author's Address

   Sean Turner
   IECA, Inc.
   3057 Nutley Street, Suite 106
   Fairfax, VA 22031
   USA

   EMail: turners@ieca.com