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Internet Engineering Task Force (IETF)                         S. TurnerRequest for Comments: 5959                                          IECACategory: Standards Track                                    August 2010ISSN: 2070-1721Algorithms for Asymmetric Key Package Content TypeAbstract   This document describes the conventions for using several   cryptographic algorithms with the EncryptedPrivateKeyInfo structure,   as defined inRFC 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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 20101.  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], EncryptedDataTurner                       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 01 306.  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.  References7.1.  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)",RFC3560, 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 2010Author's Address   Sean Turner   IECA, Inc.   3057 Nutley Street, Suite 106   Fairfax, VA 22031   USA   EMail: turners@ieca.comTurner                       Standards Track                    [Page 7]