Algorithms for Cryptographic Message Syntax (CMS) Encrypted Key Package Content Type
Which encryption, key management and signature algorithms to support when protecting cryptographic keys in CMS.
Turner, S., "Algorithms for Cryptographic Message Syntax (CMS) Encrypted Key Package Content Type", RFC 6033, DOI 10.17487/RFC6033, December 2010, <https://www.rfc-editor.org/info/rfc6033>.
@misc{rfc6033,
series = {Request for Comments},
number = 6033,
howpublished = {RFC 6033},
publisher = {RFC Editor},
doi = {10.17487/RFC6033},
url = {https://www.rfc-editor.org/info/rfc6033},
author = {S. Turner},
title = {{Algorithms for Cryptographic Message Syntax (CMS) Encrypted Key Package Content Type}},
pagetotal = 5,
year = 2010,
month = dec,
} RFC text
Contents
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.
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
AI ELI5
On this page
§1Introduction
RFC 6033 gives conventions for using cryptographic algorithms with the CMS encrypted key package content type from RFC 6032. It covers EnvelopedData and EncryptedData (RFC 5652) and AuthEnvelopedData (RFC 5083), and later adds SignedData and key sizes.
It does not define any new algorithms. It points to algorithms defined elsewhere and says which are required (MUST), recommended (SHOULD) or optional (MAY).
It is a shopping list of locks, with must-have, should-have and nice-to-have items.
§2EnvelopedData and EncryptedData
For EnvelopedData, implementations MUST support key transport, with RSA encryption required and RSAES-OAEP recommended. They SHOULD support key agreement, with Ephemeral-Static Diffie-Hellman required if key agreement is used.
Whatever key management is chosen, AES-128 Key Wrap with Padding MUST be supported as the content-encryption algorithm, and AES-256 Key Wrap with Padding SHOULD be. With key agreement, the key wrap algorithm for the content-encryption key must match the AES size of the content-encryption algorithm. EncryptedData leaves key management to other means, so only the content-encryption algorithm applies, with the same MUST and SHOULD.
Keys are wrapped in a special key-only lock. Which lock size you use for the key box decides the lock size for the inner key.
| CMS type | MUST support | SHOULD support |
|---|---|---|
| EnvelopedData | AES-128 Key Wrap with Padding | AES-256 Key Wrap with Padding |
| EncryptedData | AES-128 Key Wrap with Padding | AES-256 Key Wrap with Padding |
| AuthEnvelopedData | 128-bit AES-GCM | 256-bit AES-GCM |
§3AuthEnvelopedData and SignedData
AuthEnvelopedData has the same key management requirements as EnvelopedData; the difference is the content-encryption algorithm. Implementations MUST support 128-bit AES-GCM and SHOULD support 256-bit AES-GCM. They MAY also support AES-CCM.
For SignedData, implementations MUST support RSA signatures and SHOULD support RSASSA-PSS and DSA. They MUST support SHA-256 together with these schemes and SHOULD support SHA-1.
Some boxes also carry a tamper seal, and signed notes need a stamp. The list says which seals and stamps must work.
§4Key Sizes and Security
If an implementation supports RSA, RSAES-OAEP, DH, RSASSA-PSS or DSA, it MUST support key lengths from 1024 bits to 2048 bits, inclusive.
The security considerations of the referenced algorithm documents apply. The key wrap choice follows RFC 5649, which favours a mechanism used only to protect keying material. The RFC says no AES-GCM or AES-CCM mode provides that property, and that it will be updated if one is defined.
Locks must come in sizes from medium to big. Key-only locks are preferred because they keep keys apart from other data.