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Versions: 00 01 RFC 5959

Network Working Group                                 Sean Turner, IECA
Internet Draft                                         November 9, 2009
Intended Status: Standard Track
Expires: March 9, 2010



            Algorithms for Asymmetric Key Package Content Type
               draft-turner-asymmetrickeyformat-algs-00.txt


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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
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   Please review these documents carefully, as they describe your rights
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Abstract

   This document describes the conventions for using several
   cryptographic algorithms with the EncryptedPrivateKeyInfo structure,
   as defined in RFC 5208.  It also includes conventions necessary to
   protect the AsymmetricKeyPackage content type with SignedData,
   EnvelopedData, EncryptedData, AuthenticatedData, and
   AuthEnvelopedData.

1. Introduction

   This document describes the conventions for using several
   cryptographic algorithms with the EncryptedPrivateKeyInfo structure
   [RFC5208]. The EncryptedPrivateKeyInfo is used by [P12] to encrypt
   PrivateKeyInfo [RFCTBD1]. It is similar to EncryptedData [RFC3852] in
   that it has no recipients, no originators, and no content encryption
   keys and requires keys be managed by other means.

   This document also includes conventions necessary to protect the
   AsymmetricKeyPackage content type [RFCTBD1] with Cryptographic
   Message Syntax (CMS) protecting content types: SignedData [RFC3852],
   EnvelopedData [RFC3852], EncryptedData [RFC3852], AuthenticatedData
   [RFC3852], 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


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   PKCS#5 [RFC2898] and PKCS#12 [P12]. The major difference between PKCS
   #5 and PKCS #12 being the supported encoding for the password: ASCII
   for PKCS #5 and Unicode for PKCS #12.  [RFC2898] specifies two
   mechanisms PBE Schemes (PBES) 1 and 2, the defacto is PBES 1.  The
   notation for the PBES 1 is: PBEWith<digest>And<encryption>.  The
   following schemes are defined in PKCS #5: PBEWithMD2AndDES-CBC,
   PBEWithMD2AndRC2, PBEWithMD5AndDES-CBC, PBEWithMD5AndRC2,
   PBEWithSHA1AndDES-CBC, PBEWithSHA1AndRC2.  The following schemes are
   defined in PKCS #12: PBEWithSHAAnd3-KeyTripleDES-CBC, PBEWithSHAAnd2-
   KeyTripleDES-CBC, PBEWithSHAAnd128BitRC2-CBC, PBEWithSHAAnd40BitRC2-
   CBC, PBEWithSHAAnd128BitRC4, and PBEWithSHAAnd40BitRC4.
   Implementation defaults vary.

   The PBES 1 algorithms require salt and iteration count values. The
   salt length in PKCS #5 is 8-octets while there is no restriction on
   the length of the salt in PKCS #12, but PKCS #12 recommends the salt
   be as long as the digest algorithms output (e.g., 20-octets for SHA-
   1).  The iteration count in PKCS #5 is recommended to be at least
   1000 and PKCS #12 recommends at least 1024.

   It is RECOMMENDED that implementations support AES-128 Key Wrap with
   Padding [RFC5649] or AES-256 Key Wrap with Padding [RFC5649].

3. AsymmetricKeyPackage

   As noted in Asymmetric Key Packages [RFCTBD1], CMS can be used to
   protect the AsymmetricKeyPackage.  The following provides guidance
   for SignedData [RFC3852], EnvelopedData [RFC3852], EncryptedData
   [RFC3852], AuthenticatedData [RFC3852], and AuthEnvelopedData
   [RFC5083].

3.1. SignedData

   If an implementation supports SignedData, then it MUST support RSA
   [RFC3370], SHOULD support RSASSA-PSS [RFC4056], and SHOULD support
   DSA [RFC3370].  Additionally, implementations MUST support SHA-256
   [RFCTBD3] and SHOULD support SHA-1 [RFC3370].

3.2. EnvelopedData

   If an implementation supports EnvelopedData, then it MUST implement
   the key transport and it MAY implement the key agreement mechanism.

   When key transport is used, RSA encryption [RFC3370] MUST be
   supported and RSAES-OAEP [RFC3560] SHOULD be supported.




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   When key agreement is used, Diffie-Hellman ephemeral-static [RFC3370]
   SHOULD be supported.

   Regardless of the key management technique choice, implementations
   MUST support AES-128 Key Wrap with Padding [RFC5649].
   Implementations SHOULD support AES-256 Key Wrap with Padding
   [RFC5649].

   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 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 MAY implement AES-256 Key
   Wrap with Padding [RFC5649].

   NOTE: EncryptedData requires that keys be managed by means other than
   EncryptedData; therefore, the only algorithm specified is the content
   encryption algorithm.

3.4. AuthenticatedData

   If an implementation supports AuthenticatedData, then it MUST
   implement SHA-256 [RFCTBD3] 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 AuthenticatedData, then it MUST
   implement the EnvelopedData recommendations except for the content
   encryption algorithm, which in this case is MUST be either 128-bit
   AES-CCM or AES-GCM [RFC5084] or SHOULD BE 256-bit AES-CCM or AES-GCM
   [RFC5084].

4. Public Key Sizes

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



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5. SMIMECapabilities Attribute

   [RFCTBD4] 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], [RFC3394], [RFC3560],
   [RFC3852], [RFC4056], [RFC4231], [RFC5083], [RFC5084], [RFC5649],
   [RFCTBD1], and [RFCTBD3] apply.

   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 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-CCM or AES-
   GCM mode that provides the same properties.  If an AES-CCM and AES-
   GCM 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.




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7. IANA Considerations

   None.  Please remove this section prior to publication as an RFC.

8. References

8.1. 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.

   [RFC3394]   Housley, R., and J. Schaad, "Advanced Encryption Standard
               (AES) Key Wrap Algorithm", RFC 3394, September 2002.

   [RFC3560]   Housley, R., "Use of the RSAES-OAEP Key Transport
               Algorithm in the Cryptographic Message Syntax (CMS)", RFC
               3560, July 2003.

   [RFC3852]   Housley, R., "Cryptographic Message Syntax (CMS)", RFC
               3852, July 2004.

   [RFC4056]   Schaad, J., "Use of 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.





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   [RFC5208]   Kaliski, B., "Public-Key Cryptography Standards (PKCS)
               #8: Private-Key Information Syntax Specification Version
               1.2", RFC 5208, May 2008.

   [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, August 2009.

   [RFCTBD1]   Turners, S., "Asymmetric Key Packages", draft-turner-
               asymmetrickeyformat-02.txt, work-in-progress.

   [RFCTBD3]   Turner, S., "Using SHA2 Algorithms with Cryptographic
               Message Syntax", draft-ietf-smime-sha2-11.txt, work-in-
               progress.

   [RFCTBD4]   Turner, S., and B. Ramsdell, "Secure/Multipurpose
               Internet Mail Extensions (S/MIME) Version 3.2 Message
               Specification", draft-ietf-smime-3851bis-11.txt, work-in-
               progress.

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.

   [SP800-63]  National Institute of Standards and Technology (NIST),
               Special Publication 800-63: Electronic Authentication
               Guidance, April 2006.

Authors' Addresses

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

   EMail: turners@ieca.com





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