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Versions: 00 01 02 03 04 05 06 07 RFC 6664

Network Working Group                                          J. Schaad
Internet-Draft                                   Soaring Hawk Consulting
Intended status: Informational                         November 19, 2010
Expires: May 23, 2011


             S/MIME Capabilities for Public Key Definitions
                     draft-ietf-pkix-pubkey-caps-00

Abstract

   This document defines a set of S/MIME Capability types for ASN.1
   encoding for the current set of public keys define in the PKIX
   working group.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on May 23, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Notation . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  RSA Public Keys  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Generic RSA Public Keys  . . . . . . . . . . . . . . . . .  5
     2.2.  RSASSA-PSS Signature Public Keys . . . . . . . . . . . . .  6
   3.  Diffie-Hellman Keys  . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Diffie-Hellman Signature Public Key  . . . . . . . . . . .  7
   4.  Elliptical Curve Keys  . . . . . . . . . . . . . . . . . . . .  9
     4.1.  Generic Elliptical Curve Keys  . . . . . . . . . . . . . .  9
   5.  RSASSA-PSS Signature Algorithm Capability  . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Appendix A.  2008 ASN.1 Module . . . . . . . . . . . . . . . . . . 15
   Appendix B.  Future Work . . . . . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19































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1.  Introduction

   In the process of dealing with the OCSP agility issues in
   [I-D.ietf-pkix-ocspagility] it was noted that we really wanted to
   describe some information to be used in selecting a public key, but
   we did not currently have any way of doing so at the current time.
   This document fills that hole by defining a set of S/MIME Capability
   types for a small set of public key representations.

1.1.  Notation

   The main body of the text is written using snippets of ASN.1 that are
   extracted from the ASN.1 2008 module in Appendix A.  This is because
   I am a strong advocate of moving the current versions of ASN.1 as
   they can contain meta-data which is not representable in the 1988
   version of ASN.1.  In keeping with the current policy of the PKIX
   working group, the 1988 module is still to be considered the
   normative module in the event of a conflict between the contents of
   the two modules.

   When reading this document, it is assumed that you will have a degree
   of familiarity with tthe basic object module that is presented in
   section 3 of RFC 5912 ([RFC5912]).  We use the SMIME-CAPS object in
   this document, it assoicates two fields together in a single object.

   SMIME-CAPS ::= CLASS {
       &id         OBJECT IDENTIFIER UNIQUE,
       &Type       OPTIONAL
   }
   WITH SYNTAX { [TYPE &Type] IDENTIFIED BY &id }

   These fields are:

   &id  contains an object identifier.  When placed in an objet set,
      this element is tagged so that no two elements can be placed in
      the set that have the same value in the &id field.

   &Type  optionally contains an ASN.1 type identifier.  If the field
      &Type is not defined then the optional parameters field of the
      AlgorithmIdentifier type would be omitted.

   The class also has a specialized syntax for how to define an object
   in this class.  The all upper case fields TYPE IDENTIFIER and BY are
   syntaxtic sugar to make it easier to read and identify what pieces
   are manditory and which are optional.

   One of the things that can be done is to reference the fields of an
   object while defining other objects.  This means that if an object



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   call foo has a field name &value, the same value can be directly
   referenced as foo.&value.  This means that we would automatically get
   any updates to values or types and we do not need to do any
   replication of the data.















































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2.  RSA Public Keys

   There are currently three different public key object identifiers for
   RSA public keys.  These are RSA, RSA-OCSP and RSA-PSS.

2.1.  Generic RSA Public Keys

   Almost all RSA keys that are contained in certificates today use the
   generic RSA public key format and identifier.  This allows for the
   public key to be used both for key transport and for signature
   validation (assuming it is compatible with the bits in the key usage
   extension).  The only reason for using one of more specific public
   key identifiers is if the user wants to restrict the usage of the RSA
   public key with a specific algorithm.

   For the generic RSA public key, the S/MIME capibility that is
   advertised is a request for a specific key size to be used.  This
   would normally be used for dealing with a request on the key to be
   used for a signature that the client would then verify.  In general
   the user would provide a specific key when a key transport algorithm
   is being considered.

   The ASN.1 that is used for the generic RSA public key is defined as
   below:

      scap-pk-rsa SMIME-CAPS ::= {
        TYPE RSAKeyCapabilities
        IDENTIFIED BY pk-rsa.&id
      }

      RSAKeyCapabilities ::= SEQUENCE {
         minKeySize        RSAKeySize,
         maxKeySize        RSAKeySize OPTIONAL
      }

      RSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360)


   From the above we can see that:

   o  We use the same object identifer as the public key to identify the
      S/MIME capability field.

   o  We define a new type RSAKeyCapabilities that is used as the type
      field for the S/MIME capability.

   For the structure RSAKeyCapabilities, the fields are used as follows:




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   minKeySize  contains the minimum length of the RSA modulus to be
      used.

   maxKeySize  contains the maximum length of the RSA modules that
      should be used.  If this field is absent then no maximum length is
      requested/expected.

2.2.  RSASSA-PSS Signature Public Keys

   While most of the time one will use the generic RSA public key
   identifier in a certificate, the RSA SSA-PSS identifier can be used
   if the owner of the key desires to restrict the usage of the key to
   just this algorithm.

   The ASN.1 that is used for the RSA SSA-PSS public key is defined
   below:

      scap-pk-rsaSSA-PSS SMIME-CAPS ::= {
        TYPE RSAKeyCapabilities
        IDENTIFIED BY pk-rsaSSA-PSS.&id
      }

   From the above we can see that:

   o  We use the same object identifier as the public key to identify
      the S/MIME capability field.

   o  We use the same type for the S/MIME capability as is used above
      for the generic RSA public key.






















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3.  Diffie-Hellman Keys

   There is current two Diffie-Hellman public key object identifiers.
   These are DH and DSA.

3.1.  Diffie-Hellman Signature Public Key

   This public key type is used for the validation of DSA signatures.

   The ASN.1 that is used for DSA keys is defined below:

      scap-pk-dsa ::= {
        TYPE DSAKeyCapabilities
        IDENTIFIED BY pk-dsa.&id
      }

      DSAKeyCapabilites ::= CHOICE {
          keySizes         [0] SEQUENCE {
             minKeySize            DSAKeySize,
             maxKeySize            DSAKeySize OPTIONAL
          },
          keyParams        [1] pk-dsa.&Type
      }

      DSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360 )

   From the above we can see that:

   o  We use the same object identifer as the public key to identify the
      S/MIME capability field.

   o  We define a new type DSAKeyCapabilities that is used as the type
      field for the S/MIME capability.

   For the structure DSAKeyCapabilities, the fields are used as follows:

   keySizes  is used when only a key size is needed to be specified and
      not a specific group.  It is expected that ths would be the most
      commonly used of the two options.  In key sizes the fields are
      used as follows:

      minKeySize  contains the minimum length of the DSA modulus to be
         used.

      maxKeySize  contains the maximum length of the DSA modules that
         should be used.  If this field is absent then no maximum length
         is requested/expected.




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   keyParams  contains the exact set of DSA for the key used to sign the
      message.

   NOTE: In the original discussions the option keyParams would not have
   existed in this structure, and they may not exist in a future version
   of the structure.  The issue is that we really only need to have the
   key size fields, but there seems to be a mis-match between this
   structure and that used for ECC where we don't specify anything about
   key sizes, but do specify the exact group to be used.  We should
   probably have a discussion about rationalizing these together.









































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4.  Elliptical Curve Keys

   There are currently three Eliptical Curve public key object
   identifiers.  These are EC, EC-DH and EC-MQV

4.1.  Generic Elliptical Curve Keys

   All most all ECC keys that are contained in certificates today use
   the generic ECC public key format and identifier.  This allows for
   the public key to be used both for key agreement and for signature
   validation (assuming the appropriate bits are in the certificate).
   The only reason for using one of the more specific public key
   identifier is if the user wants to restrict the usage of the ECC
   public key with a specific algorithm.

   For the generic ECC public key, the S/MIME capability that is
   advertized is a request for a specific group to be used.

   The ASN.1 that is used for the generic ECC public key is defined as
   below:

      scap-pk-ec SMIME-CAPS ::= {
        TYPE pk-ec.&Type
        IDENTIFIED BY pk-ec.&id
      }

   From the above we can see that:

   o  We use the same object identifier as the public key to identify
      the S/MIME capability field.

   o  We use the same data type for the S/MIME capability as is used for
      the public key.


















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5.  RSASSA-PSS Signature Algorithm Capability

   This document defines a new S/MIME Capability for the RSA-SSA-PSS
   signature algorithm.  THere already exists one in [RFC4055] where the
   parameters field is not used.

   When the S/MIME group defined a S/MIME Capability for the RSA-SSA-PSS
   signature algorithm, it was done so in the context of how S/MIME
   defines and uses S/MIME Capabilities.  When placed in an S/MIME
   message [RFC3851] or in a certificate [RFC4262] it is always placed
   in a sequence of capabilities.  This meant that one can place the
   identifier for RSA-SSA-PSS in the sequence along with the identifier
   for MD5, SHA-1 and SHA-256.  The assumption was then made that one
   could compute the matrix of all answers and the publisher would
   support all elements in the matrix.  This has the possiblity that the
   publisher could accendently publish a point in the matrix that is not
   actually supported.

   In this situation, there is only a single item that is published.
   This means that we need to publish all of the assoicated information
   along with the identifier for the signature algorrithm in a single
   entity.  For this reason we now define a new parameter type to be
   used as the S/MIME capaiblity type which contains a hash identifier
   and a mask identifier.  The actual ASN.1 used for this is as follows:

      scap-sa-rsaSSA-PSS SMIME-CAPS ::= {
         TYPE RsaSsa-Pss-sig-caps
         IDENTIFIED BY sa-rsaSSA-PSS.&id
      }

      RsaSsa-Pss-sig-caps ::= SEQUENCE {
         hashAlg  SMimeCapability{ HashAlgorithmSet },
         maskAlg  SMimeCapability{ MaskAlgorithmSet },
         trailerField INTEGER DEFAULT 1
      }

   From the above we can see that:

   o  We use the same object identifier as the public key to identify
      the S/MIME capability field.

   o  We define a new type RsaSsa-Pss-sig-caps which contains the
      information that needs to defined to describe a specific instance
      of the signature algorithm.

   For the type RsaSsa-Pss-sig-caps, the fields are used as follows:





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   hashAlg  contains the S/MIME capability for the hash algorithm we are
      declaring we support with the RSA-SSA-PSS signature algorithm.

   maskAlg  contains the S/MIME capability for the mask algorithm we are
      declaring we support with the RSA-SSA-PSS signature algorithm.

   trailerField  specifies which trailer field algorithm is being
      supported.  This MUST be the value 1.

   NOTE: In at least one iteration of the design we used a sequence of
   hash identifiers and a sequence of masking functions and again made
   the assumption that entire matrix would be supported.  This has been
   removed at this point since the original intent of S/MIME
   capabilities is that one should be able to do a binary comparison of
   the DER encoding of the field and determine a specific capability was
   published.  We could return back to using the sequence if we wanted
   to lose the ability to do a binary compare but needed to shorten the
   encodings.  This does not currently appear to be an issue at this
   point.
































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6.  Security Considerations

   To Be Supplied.
















































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7.  IANA Considerations

   This document has no IANA considerations.
















































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8.  References

8.1.  Normative References

   [RFC3279]  Bassham, L., Polk, W., and R. Housley, "Algorithms and
              Identifiers for the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 3279, April 2002.

   [RFC3370]  Housley, R., "Cryptographic Message Syntax (CMS)
              Algorithms", RFC 3370, August 2002.

   [RFC4055]  Schaad, J., Kaliski, B., and R. Housley, "Additional
              Algorithms and Identifiers for RSA Cryptography for use in
              the Internet X.509 Public Key Infrastructure Certificate
              and Certificate Revocation List (CRL) Profile", RFC 4055,
              June 2005.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              June 2010.

8.2.  Informative References

   [I-D.ietf-pkix-ocspagility]
              Hallam-Baker, P. and S. Santesson, "OCSP Algorithm
              Agility", draft-ietf-pkix-ocspagility-08 (work in
              progress), March 2010.

   [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, July 2004.

   [RFC4262]  Santesson, S., "X.509 Certificate Extension for Secure/
              Multipurpose Internet Mail Extensions (S/MIME)
              Capabilities", RFC 4262, December 2005.















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Appendix A.  2008 ASN.1 Module

PUBLIC-KEY-SMIME-CAPIBLITIES --TBD Add module number--
DEFINITIONS ::=
BEGIN
   SMIME-CAPS, PUBLIC-KEY
   FROM AlgoritrithmInformation-2009
      {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0)
       id-mod-algorithmInformation-02(58)}

   pk-rsa, pk-dsa, pk-dh, pk-ec, pk-edDH, pk-ecMQV
   FROM PKIXAlgs-2009 { iso(1) identified-organization(3) dod(6)
     internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
     id-mod-pkix1-algorithms2008-02(56) }

   pk-rsaSSA-PSS, pk-rsaES-OAEP, sa-rsaSSA-PSS
   FROM PKIX1-PSS-OAEP-Algorithms-2009
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-rsa-pkalgs-02(54)}

   ;

   --
   --  Define a set containing all of the S/MIME capabilties defined
   --  by this document
   --

   SMimeCaps SMIME-CAPS ::= {
      scap-pk-rsa | scap-pk-rsaSSA-PSS |
      scap-pk-dsa |
      scap-pk-ec | scap-pk-ecDH |
      scap-sa-rsaSSA-PSS
   }

   --
   --  We defined RSA keys from the modules RFC3279 and
   --

   scap-pk-rsa SMIME-CAPS ::= {
     TYPE RSAKeyCapabilities
     IDENTIFIED BY pk-rsa.&id
   }

   RSAKeyCapabilities ::= SEQUENCE {
      minKeySize        RSAKeySize,
      maxKeySize        RSAKeySize OPTIONAL
   }



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   RSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360)


   scap-pk-rsaES-OAEP SMIME-CAPS ::= {
     TYPE NULL
     IDENTIFIED BY pk-rsaES-OAEP.&id
   }

   scap-pk-rsaSSA-PSS SMIME-CAPS ::= {
     TYPE RSAKeyCapabilities
     IDENTIFIED BY pk-rsaSSA-PSS.&id
   }

   scap-sa-rsaSSA-PSS SMIME-CAPS ::= {
      TYPE RsaSsa-Pss-sig-caps
      IDENTIFIED BY sa-rsaSSA-PSS.&id
   }

   RsaSsa-Pss-sig-caps ::= SEQUENCE {
      hashAlg  SMimeCapability{ HashAlgorithmSet },
      maskAlg  SMimeCapability{ MaskAlgorithmSet },
      trailerField INTEGER DEFAULT 1
   }

   --
   --  we define DH/DSA keys from the module RFC3279
   --

   scap-pk-dsa ::= {
     TYPE DSAKeyCapabilities
     IDENTIFIED BY pk-dsa.&id
   }

   DSAKeyCapabilites ::= CHOICE {
       keySizes         [0] SEQUENCE {
          minKeySize            DSAKeySize,
          maxKeySize            DSAKeySize OPTIONAL
       },
       keyParams        [1] pk-dsa.&Type
   }

   DSAKeySize ::= INTEGER (1024 | 2048 | 3072 | 7680 | 15360 )

   scap-pk-dh ::= {
     TYPE INTEGER
     IDENTIFIED BY pk-dh.&id
   }




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   --
   --  we define Eliptical Curve keys from the module RFC3279
   --

   scap-pk-ec SMIME-CAPS ::= {
     TYPE pk-ec.&Type
     IDENTIFIED BY pk-ec.&id
   }

   scap-pk-ecDH SMIME-CAPS ::= {
     TYPE pk-ecDH.&Type
     IDENTIFIED BY pk-ecDH.&id
   }

   scap-pk-ecMQV SMIME-CAPS ::= {
     TYPE pk-ecMQV.&Type
     IDENTIFIED BY pk-ecMQV.&id
   }

END































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Appendix B.  Future Work

   A future revision of [RFC5912] should be done at some point which
   expands the definition of the PUBLIC-KEY class and allows for an
   S/MIME Capability to be included in the class defintion.  This would
   encourage people to think about this as an issue when defining new
   public key structures in the future.












































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Author's Address

   Jim Schaad
   Soaring Hawk Consulting

   Email: jimsch@augustcellars.com













































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