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rfc:rfc5959

Internet Engineering Task Force (IETF) S. Turner Request for Comments: 5959 IECA Category: Standards Track August 2010 ISSN: 2070-1721

         Algorithms for Asymmetric Key Package Content Type

Abstract

 This document describes the conventions for using several
 cryptographic algorithms with the EncryptedPrivateKeyInfo structure,
 as defined in RFC 5958.  It also includes conventions necessary to
 protect the AsymmetricKeyPackage content type with SignedData,
 EnvelopedData, EncryptedData, AuthenticatedData, 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/rfc5959.

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 5959 Algorithms for Asymmetric Key Packages August 2010

1. Introduction

 This document describes the conventions for using several
 cryptographic algorithms with the EncryptedPrivateKeyInfo structure
 [RFC5958].  The EncryptedPrivateKeyInfo is used by [P12] to encrypt
 PrivateKeyInfo [RFC5958].  It is similar to EncryptedData [RFC5652]
 in that it has no recipients, no originators, and no content
 encryption keys and requires keys to be managed by other means.
 This document also includes conventions necessary to protect the
 AsymmetricKeyPackage content type [RFC5958] with Cryptographic
 Message Syntax (CMS) protecting content types: SignedData [RFC5652],
 EnvelopedData [RFC5652], EncryptedData [RFC5652], AuthenticatedData
 [RFC5652], and AuthEnvelopedData [RFC5083].  Implementations of
 AsymmetricKeyPackage do not require support for any CMS protecting
 content type; however, if the AsymmetricKeyPackage is CMS protected
 it is RECOMMENDED that conventions defined herein be followed.
 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. EncryptedPrivateKeyInfo

 The de facto standard used to encrypt the PrivateKeyInfo structure,
 which is subsequently placed in the EncryptedPrivateKeyInfo
 encryptedData field, is Password Based Encryption (PBE) based on PKCS
 #5 [RFC2898] and PKCS #12 [P12].  The major difference between PKCS
 #5 and PKCS #12 is the supported encoding for the password: ASCII for
 PKCS #5 and Unicode for PKCS #12, encoded as specified in Section B.1
 of [P12].  [RFC2898] specifies two PBE Schemes (PBES) 1 and 2;
 [RFC2898] recommends PBES2 for new specification.  PBES2 with a key
 derivation algorithm of PBKDF2 using HMAC with SHA-256 [RFC5754] and
 an encryption algorithm of AES Key Wrap with Padding as defined in
 [RFC5649] MUST be supported.  AES-256 Key Wrap with Padding [RFC5649]
 MAY also be supported as an encryption algorithm.

3. AsymmetricKeyPackage

 As noted in Asymmetric Key Packages [RFC5958], CMS can be used to
 protect the AsymmetricKeyPackage.  The following provides guidance
 for SignedData [RFC5652], EnvelopedData [RFC5652], EncryptedData

Turner Standards Track [Page 2] RFC 5959 Algorithms for Asymmetric Key Packages August 2010

 [RFC5652], AuthenticatedData [RFC5652], and AuthEnvelopedData
 [RFC5083].

3.1. SignedData

 If an implementation supports SignedData, then it MUST support the
 signature scheme RSA [RFC3370] [RFC5754] and SHOULD support the
 signature schemes RSASSA-PSS [RFC4056] and DSA [RFC3370] [RFC5754].
 Additionally, implementations MUST support in concert with these
 signature schemes the hash function SHA-256 [RFC5754] and SHOULD
 support the hash function SHA-1 [RFC3370].

3.2. 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 RSAES-OAEP (RSA Encryption Scheme - Optimal Asymmetric
 Encryption Padding) [RFC3560] SHOULD be supported.
 When key agreement is used, Diffie-Hellman (DH) ephemeral-static
 [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].

3.3. 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].

Turner Standards Track [Page 3] RFC 5959 Algorithms for Asymmetric Key Packages August 2010

 NOTE: EncryptedData 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.

3.4. AuthenticatedData

 If an implementation supports AuthenticatedData, then it MUST
 implement SHA-256 [RFC5754] and SHOULD support SHA-1 [RFC3370] as the
 message digest algorithm.  Additionally, HMAC with SHA-256 [RFC4231]
 MUST be supported and HMAC with SHA-1 [RFC3370] SHOULD be supported.

3.5. 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 for AES-CCM
 [RFC5084].

4. Public Key Sizes

 The easiest way to implement the SignedData, EnvelopedData, and
 AuthEnvelopedData is with public key certificates [RFC5280].  If an
 implementation support RSA, RSASSA-PSS, DSS, RSAES-OAEP, or DH, then
 it MUST support key lengths from 1024-bit to 2048-bit, inclusive.

5. SMIMECapabilities Attribute

 [RFC5751] defines the SMIMECapabilities attribute as a mechanism for
 recipients to indicate their supported capabilities including the
 algorithms they support.  The following are values for the
 SMIMECapabilities attribute for AES Key Wrap with Padding [RFC5649]
 when used as a content encryption algorithm:
 AES-128 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 08
 AES-192 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 1C
 AES-256 KW with Padding: 30 0d 06 09 60 86 48 01 65 03 04 01 30

6. Security Considerations

 The security considerations from [RFC3370], [RFC3560], [RFC4056],
 [RFC4231], [RFC5083], [RFC5084], [RFC5649], [RFC5652], [RFC5754], and
 [RFC5958] apply.

Turner Standards Track [Page 4] RFC 5959 Algorithms for Asymmetric Key Packages August 2010

 The strength of any encryption scheme is only as good as its weakest
 link, which in the case of a PBES is the password.  Passwords need to
 provide sufficient entropy to ensure they cannot be easily guessed.
 The U.S. National Institute of Standards and Technology (NIST)
 Electronic Authentication Guidance [SP800-63] provides some
 information on password entropy.  [SP800-63] indicates that a user-
 chosen 20-character password from a 94-character keyboard with no
 checks provides 36 bits of entropy.  If the 20-character password is
 randomly chosen, then the amount of entropy is increased to roughly
 131 bits of entropy.  The amount of entropy in the password does not
 correlate directly to bits of security but in general the more than
 the better.
 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.

7. References

7.1. Normative References

 [P12]       RSA Laboratories, "PKCS #12 v1.0: Personal Information
             Exchange Syntax", June 1999.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2898]   Kaliski, B., "PKCS #5: Password-Based Cryptography
             Specification Version 2.0", RFC 2898, September 2000.
 [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.

Turner Standards Track [Page 5] RFC 5959 Algorithms for Asymmetric Key Packages August 2010

 [RFC4056]   Schaad, J., "Use of the RSASSA-PSS Signature Algorithm in
             Cryptographic Message Syntax (CMS)", RFC 4056, June 2005.
 [RFC4231]   Nystrom, M., "Identifiers and Test Vectors for HMAC-
             SHA-224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
             RFC 4231, December 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.
 [RFC5751]   Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
             Mail Extensions (S/MIME) Version 3.2 Message
             Specification", RFC 5751, January 2010.
 [RFC5754]   Turner, S., "Using SHA2 Algorithms with Cryptographic
             Message Syntax", RFC 5754, January 2010.
 [RFC5958]   Turner, S., "Asymmetric Key Packages", RFC 5958, August
             2010.

7.2. Informative References

 [SP800-57]  National Institute of Standards and Technology (NIST),
             Special Publication 800-57: Recommendation for Key
             Management - Part 1 (Revised), March 2007.
 [SP800-63]  National Institute of Standards and Technology (NIST),
             Special Publication 800-63: Electronic Authentication
             Guidance, April 2006.

Turner Standards Track [Page 6] RFC 5959 Algorithms for Asymmetric Key Packages August 2010

Author's Address

 Sean Turner
 IECA, Inc.
 3057 Nutley Street, Suite 106
 Fairfax, VA 22031
 USA
 EMail: turners@ieca.com

Turner Standards Track [Page 7]

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