Algorithms for Asymmetric Key Package Content Type
RFC 5959

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
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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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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

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   [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].

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   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.

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   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.

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   [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.

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Author's Address

   Sean Turner
   IECA, Inc.
   3057 Nutley Street, Suite 106
   Fairfax, VA 22031
   USA

   EMail: turners@ieca.com

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