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/RFC 6033.
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
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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.
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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 [RFC 6032]. Specifically, it
includes conventions necessary to implement the following CMS content
types: EnvelopedData [RFC 5652], EncryptedData [RFC 5652], and
AuthEnvelopedData [RFC 5083].
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 [RFC 2119].
2. EnvelopedData
EnvelopedData [RFC 5652] 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 [RFC 3370] MUST be
supported and RSA Encryption Scheme - Optimal Asymmetric Encryption
Padding (RSAES-OAEP) [RFC 3560] SHOULD be supported.
When key agreement is used, Ephemeral-Static Diffie-Hellman (DH)
[RFC 3370] 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 [RFC 5649] as the content-encryption algorithm.
Implementations SHOULD support AES-256 Key Wrap with Padding
[RFC 5649] 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 [RFC 5649]. 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 [RFC 5649].
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RFC 6033 Algorithms for CMS Encrypted Key Packages December 2010
3. EncryptedData
EncryptedData [RFC 5652] 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 [RFC 5649]. Implementations
SHOULD support AES-256 Key Wrap with Padding [RFC 5649].
4. AuthEnvelopedData
AuthEnvelopedData [RFC 5083], 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) [RFC 5084] and SHOULD
support 256-bit AES-GCM [RFC 5084]. Implementations MAY also support
AES-Counter with CBC-MAC (AES-CCM) [RFC 5084].
5. Signed Data
Implementations of SignedData [RFC 5652] MUST support the signature
scheme RSA [RFC 3370] [RFC 5754] and SHOULD support the signature
schemes RSA Probabilistic Signature Scheme (RSASSA-PSS) [RFC 4056] and
Digital Signature Algorithm (DSA) [RFC 3370] [RFC 5754]. Additionally,
implementations MUST support in concert with these signature schemes
the hash function SHA-256 [RFC 5754] and it SHOULD support the hash
function SHA-1 [RFC 3370].
6. Public Key Sizes
The easiest way to implement SignedData, EnvelopedData, and
AuthEnvelopedData is with public key certificates [RFC 5280]. 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 [RFC 3370], [RFC 3560], [RFC 4056],
[RFC 5083], [RFC 5084], [RFC 5649], [RFC 5652], [RFC 5754], and [RFC 6032]
apply.
The choice of content-encryption algorithms for this document was
based on [RFC 5649]: "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
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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
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3370] Housley, R., "Cryptographic Message Syntax (CMS)
Algorithms", RFC 3370, August 2002.
[RFC 3560] Housley, R., "Use of the RSAES-OAEP Key Transport
Algorithm in Cryptographic Message Syntax (CMS)", RFC
3560, July 2003.
[RFC 4056] Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
Cryptographic Message Syntax (CMS)", RFC 4056, June 2005.
[RFC 5083] Housley, R., "Cryptographic Message Syntax (CMS)
Authenticated-Enveloped-Data Content Type", RFC 5083,
November 2007.
[RFC 5084] Housley, R., "Using AES-CCM and AES-GCM Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 5084, November 2007.
[RFC 5280] 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.
[RFC 5649] Housley, R. and M. Dworkin, "Advanced Encryption Standard
(AES) Key Wrap with Padding Algorithm", RFC 5649,
September 2009.
[RFC 5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD
70, RFC 5652, September 2009.
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RFC 6033 Algorithms for CMS Encrypted Key Packages December 2010
[RFC 5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010.
[RFC 6032] 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
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