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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
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Turner Standards Track [Page 1] RFC 6033 Algorithms for CMS Encrypted Key Packages December 2010

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].

Turner Standards Track [Page 2] RFC 6033 Algorithms for CMS Encrypted Key Packages December 2010

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

Turner Standards Track [Page 3] RFC 6033 Algorithms for CMS Encrypted Key Packages December 2010

 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.

Turner Standards Track [Page 4] RFC 6033 Algorithms for CMS Encrypted Key Packages December 2010

 [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

Turner Standards Track [Page 5]