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Network Working Group                                          J. Schaad
Internet-Draft                                   Soaring Hawk Consulting
Intended status: Informational                         December 12, 2010
Expires: June 15, 2011


             S/MIME Capabilities for Public Key Definitions
                     draft-ietf-pkix-pubkey-caps-01

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 June 15, 2011.

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
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   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  ASN.1 Notation . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Requirements Terminology . . . . . . . . . . . . . . . . .  4
   2.  RSA Public Keys  . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Generic RSA Public Keys  . . . . . . . . . . . . . . . . .  5
     2.2.  RSASSA-PSS Signature Public Keys . . . . . . . . . . . . .  6
     2.3.  RSA ES-OAEP Key Transport Public Keys  . . . . . . . . . .  6
   3.  Diffie-Hellman Keys  . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  DSA Signature Public Key . . . . . . . . . . . . . . . . .  8
     3.2.  DH Key Agreement Keys  . . . . . . . . . . . . . . . . . .  9
   4.  Elliptical Curve Keys  . . . . . . . . . . . . . . . . . . . . 10
     4.1.  Generic Elliptical Curve Keys  . . . . . . . . . . . . . . 10
     4.2.  Elliptical Curve DH Keys . . . . . . . . . . . . . . . . . 10
     4.3.  Elliptical Curve MQV Keys  . . . . . . . . . . . . . . . . 11
   5.  RSASSA-PSS Signature Algorithm Capability  . . . . . . . . . . 12
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 16
   Appendix A.  2008 ASN.1 Module . . . . . . . . . . . . . . . . . . 17
   Appendix B.  Future Work . . . . . . . . . . . . . . . . . . . . . 20
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 21


























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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 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.

   S/MIME Capabilities where originally defined in [SMIMEv3-MSG] as a
   way for the sender of an S/MIME message to tell the recipient of the
   message the set of encryption algorithms that were supported by the
   senders system.  In the beginning, the focus was primarily on
   communicating the set of encryption algorithms that were supported.
   Over time it was expanded to allow for an S/MIME client to say that
   it supported the compression data type and binary contents.  As
   originally defined it was targeted towards supporting items with a
   small number of possible parameters.  For the RC2 encryption
   algorithm only two values from the entire range of values were ever
   use.  The object of restricting the set of values was so that a
   client could do a simple binary comparison without having to decode
   the S/MIME capability.  This was especially easy since most just
   consisted of the object identifier for the algorithm.

   Given that we are assigning different data types to the algorithm
   descriptors here, and many of the algorithm descriptors are the same
   as are used in signature, key transport or key agreement algorithms,
   the public key versions of these structures MUST NOT be placed in the
   same locations as the other versions.  It is expected that the places
   where one needs S/MIME capabilities for public keys is going to be
   vastly different than for the other values.

1.1.  ASN.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 to 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 the basic object module that is presented in
   section 3 of RFC 5912 ([RFC5912]).  We use the SMIME-CAPS object in
   this document, it associates two fields together in a single object.




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   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 object 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.  Note that this
      is not a restriction which says that only a single object can
      exist with a single object identifier.

   &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 words TYPE IDENTIFIER and BY are
   syntactic sugar to make it easier to read.  The square brackets
   defined optional pieces of the syntax.

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

1.2.  Requirements Terminology

   When capitalized 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.  RSA Public Keys

   There are currently three different public key object identifiers for
   RSA public keys.  These are RSA, RSA ES-OCSP and RSA SSA-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 capability 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 |
                              4096 | 8192, ...)


   In the above ASN.1 we have defined the following:

   scap-pk-rsa  is a new SMIME-CAP object.  This object associates the
      existing object identifier (rsaEncryption) used for the public key
      in certificates (defined in [RFC3279] and [RFC5912]) with a new
      type defined in this document.






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   RSAKeyCapabilities  carries the set of desired capabilities for an
      RSA key.  The fields of this type are:

      minKeySize  contains the minimum length of the RSA modulus to be
         used.  This field SHOULD NOT contain a value less than 1024.

      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.  This value is normally selected so as
         not to cause the current code to run unacceptably long when
         processing signatures.

   RSAKeySize  provides a set of suggested values to be used.  The
      values 1024, 2048, 3072, 7680 and 15360 are from the NIST guide on
      signature sizes [NIST-SIZES] while the others are common powers of
      two that would be used.  The list is not closed and other values
      can be used.

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.  This algorithm does have the ability to place a
   set of algorithm parameters in the public key info structure, they
   have not been included in this location s as the same information
   should be carried in the signature S/MIME capabilities instead.

   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
      }

   In the above ASN.1 we have defined the following:

   scap-pk-rsaSSA-PSS  is a new SMIME-CAP object.  This object
      associates the existing object identifier (id-RSASSA-PSS) used for
      the public key certificates (defined in [RFC4055] and [RFC5912])
      with type RSAKeyCapabilities.

2.3.  RSA ES-OAEP Key Transport Public Keys

   While most of the time one will use the generic RSA public key
   identifier in a certificate, the RSA ES-OAEP identifier can be used
   if the owner of the key desires to restrict the usage of the key to



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   just this algorithm.  This algorithm does have the ability to place a
   set of algorithm parameters in the public key info structure, they
   have not been included in this location s as the same information
   should be carried in the key transport S/MIME capabilities instead.

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

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

   In the above ASN.1 we have defined the following:

   scap-pk-rsaES-OAEP  is a new SMIME-CAP object.  This object
      associates the existing object identifier (id-RSAES-OAEP) used for
      the public key certificates (defined in [RFC4055] and [RFC5912])
      with type RSAKeyCapabilities.
































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3.  Diffie-Hellman Keys

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

3.1.  DSA 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 SMIME-CAPS ::= {
        TYPE DSAKeyCapabilities
        IDENTIFIED BY pk-dsa.&id
      }

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

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

   In the above ASN.1 we have defined the following:

   scap-pk-dsa  is a new SMIME-CAP object.  This object associated the
      existing object identifier (id-dsa) used for the public key in
      certificates (defined in [RFC3279] and [RFC5912]) with a new type
      defined here, DSAKeyCapabilities.

   DSAKeyCapabilities  carries the desired set of capabilities for the
      DSA key.  The fields of this type are:

      keySizes  is used when only a key size is needed to be specified
         and not a specific group.  It is expected that this 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.

3.2.  DH Key Agreement Keys

   This public key type is used with the Diffie-Hellman key agreement
   algorithm.

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

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

   In the above ASN.1 we have defined the following:

   scap-pk-dh  is a new SMIME-CAP object.  This object associates the
      existing object identifier (id-dh) used for the public key
      algorithm in the certificates (defined in [RFC3279] and [RFC5912])
      with a new type defined above, DSAKeyCapabilities.






















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4.  Elliptical Curve Keys

   There are currently three Elliptical 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
   advertised 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
      }

   In the above ASN.1 we have defined the following:

   scap-pk-ec  is a new SMIME-CAP object.  This object associated the
      existing object identifier (id-ecPublicKey) used for the public
      key algorithm in the certificates (defined in [RFC3279] and
      [RFC5912]) with the same type used for the public key (ECPoint).

4.2.  Elliptical Curve DH Keys

   This public key type is used with the Elliptical Curve Diffie-Hellman
   key agreement algorithm.

   The ASN.1 that is used for EC-DH keys is defined below:

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

   In the above ASN.1 we have defined the following:





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   scap-ec-dh  is a new SMIME-CAP object.  This object associated the
      existing object identifier (id-??) used for the public key
      algorithm in the certificate (defined in [RFC3279] and [RFC5912])
      with the same type structure used for public keys.

4.3.  Elliptical Curve MQV Keys

   This public key type is used with the Elliptical Curve MQV key
   agreement algorithm.

   The ASN.1 that is used for EC-MQV keys is defined below:

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

   In the above ASN.1 we have defined the following:

   scap-ec-MQV  is a new SMIME-CAP object.  This object associated the
      existing object identifier (id-??) used for the public key
      algorithm in the certificate (defined in [RFC3279] and [RFC5912])
      with the same type structure used for public keys.




























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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 possibility that
   the publisher could accidently publish a point in the matrix that is
   not supported.

   In this situation, there is only a single item that is published.
   This means that we need to publish all of the associated information
   along with the identifier for the signature algorithm in a single
   entity.  For this reason we now define a new parameter type to be
   used as the S/MIME capability type which contains a hash identifier
   and a mask identifier.  The 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{{ HashAlgorithms }},
         maskAlg  SMIMECapability{{ MaskAlgorithmSet }} OPTIONAL,
         trailerField INTEGER DEFAULT 1
      }

      scap-mf-mgf1 SMIME-CAPS ::= {
         TYPE SMIMECapability{{ HashAlgorithms }}
         IDENTIFIED BY id-mgf1
      }

      MaskAlgorithmSet SMIME-CAPS ::= {scap-mf-mgf1, ...}

   In the above ASN.1 we have defined the following:







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   scap-sa-rsaSSA-PSS  is a new SMIME-CAP object.  This object
      associates the existing object identifier (id-RSASSA-PSS) used for
      the signature algorithm (defined in [RFC4055] and [RFC5912]) with
      the new type RsaSsa-Pss-sig-caps.

   RsaSsa-Pss-sig-caps  carries the desired set of capabilities for the
      RSA SSA-PSS signature algorithm.  The fields of this type are:

      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

   This document provides new fields that can be placed in an S/MIME
   capabilities sequence.  There are number of considerations that need
   to be taking into account when doing this.

   As mentioned above, there are a small number of cases where the same
   object identifier may be used to identify a public key and an
   algorithm.  This is the case for many years with the OID
   rsaEncryption where it identifies both a public key and the RSA v1.5
   key transport algorithm.  This means that when an S/MIME capabilities
   sequence is defined care needs to be taken to specify the types of
   algorithms and/or public keys that are to be specified in that
   sequence.  In general, it is expected that algorithms and public keys
   will be segregated.

   The more detailed the information that is communicated, the better
   the end results are going to be.  If you can state you do RSA v1.5,
   EC-DSA, SHA-1 and SHA-256, then it would imply that all four values
   are supported.  It may be however that EC-DSA with SHA-1 is not
   supported.  Not including the SHA-1 hash algorithm could lead to
   problems as RSA with SHA-1 could be the only point of intersection,
   but including it means that a result may be returned that cannot be
   processed.

   The more information passed the better.  The more choices that are
   passed, the better the odds that both parties will be able to agree
   on a common algorithm.

   The less information passed the better.  Passing too much information
   can lead to computational issues in trying to deal with the
   possibilities.  This becomes acute when a negotiation over algorithms
   is going on between multiple parties (such as sending an encrypted
   S/MIME message) where the amount of memory and processing time can be
   greatly expanded if there are a large number of choices for each
   recipient.

   Ordering of preference of algorithms is not always supported by all
   places where S/MIME capabilities are used.  The addition of
   preference ordering greatly complicates the decisions to be used,
   especially as it is expected that not all parties will agree on the
   same ordering.









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7.  IANA Considerations

   This document has no IANA considerations.
















































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8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

   [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.

   [SMIMEv3-MSG]
              Ramsdell, B., "S/MIME Version 3 Message Specification",
              RFC 2633, June 1999.

   [NIST-SIZES]
              Barker, E., Barker, W., Burr, W., Polk, W., and M. Smid,
              "Recommendation for Key Management -- Part 1: General",
              NIST Special Publication 800-57, March 2007.






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Appendix A.  2008 ASN.1 Module

   PUBLIC-KEY-SMIME-CAPIBLITIES
      { iso(1) identified-organization(3) dod(6) internet(1)
        security(5) mechanisms(5) pkix(7) id-mod(0) TBD5 }
   DEFINITIONS ::=
   BEGIN
      IMPORTS
      SMIME-CAPS, PUBLIC-KEY, SMIMECapability
      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-ecDH, 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,
      HashAlgorithms, id-mgf1
      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 ::= {
         PubKeys-SMimeCaps |
         scap-sa-rsaSSA-PSS
      }

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

      --
      --  We defined RSA keys from the modules RFC3279 and RFC4055
      --




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      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 |
                              4096 | 8192, ...)


      scap-pk-rsaES-OAEP SMIME-CAPS ::= {
        TYPE RSAKeyCapabilities
        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{{ HashAlgorithms }},
         maskAlg  SMIMECapability{{ MaskAlgorithmSet }} OPTIONAL,
         trailerField INTEGER DEFAULT 1
      }

      scap-mf-mgf1 SMIME-CAPS ::= {
         TYPE SMIMECapability{{ HashAlgorithms }}
         IDENTIFIED BY id-mgf1
      }

      MaskAlgorithmSet SMIME-CAPS ::= {scap-mf-mgf1, ...}

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

      scap-pk-dsa SMIME-CAPS ::= {
        TYPE DSAKeyCapabilities



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        IDENTIFIED BY pk-dsa.&id
      }

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

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

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

      --
      --  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 definition.  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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