Algorithms for Asymmetric Key Package Content Type
draft-turner-asymmetrickeyformat-algs-01

Network Working Group                                 Sean Turner, IECA 
Internet Draft                                         February 1, 2010 
Intended Status: Standard Track 
Expires: August 1, 2010 
 
 
                                      
            Algorithms for Asymmetric Key Package Content Type 
               draft-turner-asymmetrickeyformat-algs-01.txt 

Abstract 

   This document describes the conventions for using several 
   cryptographic algorithms with the EncryptedPrivateKeyInfo structure, 
   as defined in RFC TBD1.  It also includes conventions necessary to 
   protect the AsymmetricKeyPackage content type with SignedData, 
   EnvelopedData, EncryptedData, AuthenticatedData, and 
   AuthEnvelopedData. 

Status of this Memo 

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1. Introduction 

   This document describes the conventions for using several 
   cryptographic algorithms with the EncryptedPrivateKeyInfo structure 
   [RFCTBD1]. The EncryptedPrivateKeyInfo is used by [P12] to encrypt 
   PrivateKeyInfo [RFCTBD1]. It is similar to EncryptedData [RFC5652] 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 [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]. 

 
 
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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.  [RFC2898] specifies two 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 [RFC5652], EnvelopedData [RFC5652], EncryptedData 
   [RFC5652], AuthenticatedData [RFC5652], and AuthEnvelopedData 
   [RFC5083]. 

3.1. SignedData 

   If an implementation supports SignedData, then it MUST support the 
   signature scheme RSA [RFC3370] and SHOULD support the signature 
   schemes RSASSA-PSS [RFC4056] and DSA [RFC3370].  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]. 

 
 
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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. 

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

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

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 

 
 
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   SHOULD be implemented.  Implementations MAY also support for AES-CCM 
   [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.  

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], [RFC3394], [RFC3560], 
   [RFC5652], [RFC4056], [RFC4231], [RFC5083], [RFC5084], [RFC5649], 
   [RFC5754], and [RFCTBD1] 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 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-CCM or AES-
 
 
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   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.   

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. 

   [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. 

 
 
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   [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, August 2009. 

   [RFC5652]   Housley, R., "Cryptographic Message Syntax (CMS)", RFC 
               5652, September 2009. 

   [RFC5751]   Turner, S., and B. Ramsdell, "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. 

   [RFCTBD1]   Turners, S., "Asymmetric Key Packages", draft-turner-
               asymmetrickeyformat-03.txt, work-in-progress. 

   /** 
   RFC Editor: Please replace "RFCTBD1" with "RFC####" where #### is the 
   number of the published RFC.  Please do this in both the references 
   and the text. 
   **/ 

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. 

 
 
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Authors' Addresses 

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

   EMail: turners@ieca.com 

 
 
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