Internet Engineering Task Force (IETF) S. Turner
Request for Comments: 6160 IECA
Category: Standards Track April 2011
ISSN: 2070-1721
Algorithms for Cryptographic Message Syntax (CMS) Protection
of Symmetric Key Package Content Types
Abstract
This document describes the conventions for using several
cryptographic algorithms with the Cryptographic Message Syntax (CMS)
to protect the symmetric key package content type. Specifically, it
includes conventions necessary to implement SignedData,
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/rfc6160.
Copyright Notice
Copyright (c) 2011 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 6160 Algorithms for Symmetric Key Packages April 2011
1. Introduction
This document describes the conventions for using several
cryptographic algorithms with the Cryptographic Message Syntax (CMS)
[RFC5652] to protect the symmetric key package content type defined
in [RFC6031]. Specifically, it includes conventions necessary to
implement the following CMS content types: SignedData [RFC5652],
EnvelopedData [RFC5652], EncryptedData [RFC5652], and
AuthEnvelopedData [RFC5083]. Familiarity with [RFC5083], [RFC5652],
[RFC5753], and [RFC6031] is assumed.
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. SignedData
If an implementation supports SignedData, then it MUST support the
signature scheme RSA [RFC3370] and SHOULD support the signature
schemes RSA Probabilistic Signature Scheme (RSASSA-PSS) [RFC4056] and
Digital Signature Algorithm (DSA) [RFC3370]. Additionally,
implementations MUST support the hash function SHA-256 [RFC5754] in
concert with these signature schemes, and they SHOULD support the
hash function SHA-1 [RFC3370]. If an implementation supports
SignedData, then it MAY support Elliptic Curve Digital Signature
Algorithm (ECDSA) [RFC6090][RFC5753].
3. EnvelopedData
If an implementation supports EnvelopedData, then it MUST implement
key transport, and it MAY implement key agreement.
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, Diffie-Hellman (DH) ephemeral-static
[RFC3370] MUST be supported. When key agreement is used, Elliptic
Curve Diffie-Hellman (ECDH) [RFC6090][RFC5753] MAY be supported.
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RFC 6160 Algorithms for Symmetric Key Packages April 2011
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, the same key-wrap algorithm MUST be used
for both key and content encryption. 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].
4. EncryptedData
If an implementation supports EncryptedData, then it MUST implement
AES-128 Key Wrap with Padding [RFC5649] and SHOULD implement AES-256
Key Wrap with Padding [RFC5649].
NOTE: EncryptedData requires that keys be managed by other means;
therefore, the only algorithm specified is the content-encryption
algorithm.
5. AuthEnvelopedData
If an implementation supports AuthEnvelopedData, then it MUST
implement the EnvelopedData recommendations except for the content-
encryption algorithm, which, in this case, MUST be AES-GCM [RFC5084];
the 128-bit version MUST be implemented, and the 256-bit version
SHOULD be implemented. Implementations MAY also support AES-CCM
[RFC5084].
6. Public Key Sizes
The easiest way to implement SignedData, EnvelopedData, and
AuthEnvelopedData is with public key certificates [RFC5280]. If an
implementation supports RSA, RSASSA-PSS, DSA, RSAES-OAEP, or Diffie-
Hellman, then it MUST support key lengths from 1024-bit to 2048-bit,
inclusive. If an implementation supports ECDSA or ECDH, then it MUST
support keys on P-256.
7. Security Considerations
The security considerations from [RFC3370], [RFC3560], [RFC4056],
[RFC5083], [RFC5084], [RFC5649], [RFC5652], [RFC5753], [RFC5754], and
[RFC6031] apply.
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RFC 6160 Algorithms for Symmetric Key Packages April 2011
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.
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RFC 6160 Algorithms for Symmetric Key Packages April 2011
[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.
[RFC5753] Turner, S. and D. Brown, "Use of Elliptic Curve
Cryptography (ECC) Algorithms in Cryptographic Message
Syntax (CMS)", RFC 5753, January 2010.
[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010.
[RFC6031] Turner, S. and R. Housley, "Cryptographic Message Syntax
(CMS) Symmetric Key Package Content Type", RFC 6031,
December 2010.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental
Elliptic Curve Cryptography Algorithms", RFC 6090,
February 2011.
8.2. Informative Reference
[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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