Re: (Another) new ID of potential interest

Network Working Group                                          J. Herzog
Internet-Draft                                                 R. Khazan
Intended status: Informational                    MIT Lincoln Laboratory
Expires: February 21, 2011                               August 20, 2010


             A set-key attribute for symmetric-key packages
                         draft-herzog-setkey-01

Abstract

   A set-key is a symmetric key (or set of keys) associated with an
   immutable set of participants.  This document defines a set-key
   attribute for use in the CMS-based symmetric-key package structure
   defined in in RFC XXXX. {{{ RFC Editor, please replace XXXX with the
   number assigned to draft-ietf-keyprov-symmetrickeyformat when it is
   published. }}}

Disclaimer

   This work is sponsored by the United States Air Force under Air Force
   Contract FA8721-05-C-0002.  Opinions, interpretations, conclusions
   and recommendations are those of the authors and are not necessarily
   endorsed by the United States Government.

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   This Internet-Draft will expire on February 21, 2011.




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Copyright Notice

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   document authors.  All rights reserved.

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   described in the BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Set-keys . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Symmetric key packages . . . . . . . . . . . . . . . . . .  3
     1.3.  Intended Usage . . . . . . . . . . . . . . . . . . . . . .  5
     1.4.  Requirements Terminology . . . . . . . . . . . . . . . . .  5
   2.  The set-key attribute  . . . . . . . . . . . . . . . . . . . .  5
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     5.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  ASN.1 Module  . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12




















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1.  Introduction

   This document defines a new set-key attribute for the symmetric-key
   package structure defined in [Housley-symmkey].

1.1.  Set-keys

   A 'set-key' is a symmetric key associated with (and to be limited to)
   a specific set of participants.  Such a key can be used in many ways,
   including:

   o  To secure broadcast or multicasts of e.g., pay-per-view movies,

   o  To secure group-communication such as chat rooms, or

   o  To secure data-at-rest which belongs to a group (such as on an
      encrypted file-system or on a server).

   The only requirement of a set-key is that it be associated with a
   specific and unchanging set of participants.  That is, we make a
   distinction between sets and groups.  Sets are immutable structures:
   a set S of participants is a mathematical entity and does not change.
   A group, on the other hand, is a mutable structure that might have a
   certain name or set of names; may be under the control of a given
   administration; and may be mapped to a sequence of sets over time
   according to the needs of some particular application.  This document
   considers only sets, and therefore will not consider issues such as
   group-administration, adding or removing members, revoking keys, and
   so on.  Set-keys can be used to build group-keying protocols, but
   such issues are outside the scope of this document.

   We note that the participant-set of a set-key can be partitioned into
   two sub-sets: the active participants and the passive participants.
   In some set-key applications, such as group-chat, all participants
   may be active: both sending and receiving.  In other applications,
   such as multicast of pay-per-view movies, most participants are
   passive: only receiving.  Becuase it may be important, in some
   applications, to know which partipants are active and which are
   passive, we will allow these two sub-sets to be explicitly
   distinguished from each other.

1.2.  Symmetric key packages

   The Cryptographic Message Syntax (CMS) [RFC5652] is a standard
   notation and representation for cryptographic messages.  CMS is based
   on ASN.1 [X.680], [X.681], [X.682], [X.683] and uses that notation to
   define a number of structures relevant to cryptography such as
   certificates, encrypted or signed messages, and so on.



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   A current Internet Draft [Housley-symmkey] uses CMS to define a
   structure for symmetric key packages: collections of symmetric keys
   which share common properties and are intended for the same
   participants.  The syntax for this structure follows:


   SymmetricKeyPackage ::= SEQUENCE {
     version           KeyPkgVersion DEFAULT v1,
     sKeyPkgAttrs  [0] SEQUENCE SIZE (1..MAX) OF Attribute
                                    {{ SKeyPkgAttributes }} OPTIONAL,
     sKeys             SymmetricKeys,
     ... }


   SymmetricKeys ::= SEQUENCE SIZE (1..MAX) OF OneSymmetricKey

   OneSymmetricKey ::= SEQUENCE {
     sKeyAttrs  SEQUENCE SIZE (1..MAX) OF Attribute
                                    {{ SKeyAttributes }}  OPTIONAL,
     sKey       OCTET STRING OPTIONAL }
     ( WITH COMPONENTS { ..., sKeyAttrs PRESENT } |
       WITH COMPONENTS { ..., sKey PRESENT } )

   KeyPkgVersion ::= INTEGER  { v1(1) } ( v1, ... )

   A key-package (SymmetricKeyPackage) contains some meta-information,
   including attributes (sKeyPkgAttrs) [Housley-symmkey], and a sequence
   of key-structures.  Each individual key-structure (OneSymmetricKey)
   contains some attributes (sKeyAttrs) and some actual keying material
   (sKey).  Attributes in the sKeyPkgAttrs field apply to every key in
   the package, while attributes in a sKeyAttrs field apply only to the
   key in the containing OneSymmetricKey structure.  The same attribute
   cannot appear at both the package-level and the key-level.  That is,
   a given attribute can:

   o  appear in the sKeyPkgAttrs field but not in the sKeyAttrs field of
      any OneSymmetricKey structure,

   o  appear in the sKeyAttrs field of one or more OneSymmetricKey
      structures but not the sKeyPkgAttrs field, or

   o  not be included in the key-package at all.

   Also, it is not required that every attribute be applicable at both
   levels.  That is, [Housley-symmkey] allows a given attribute to be
   valid for only the sKeyPkgAttrs field or for only the sKeyAttrs
   field.




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   The grammar for attributes is quite complex and given in
   [Housley-symmkey].  We intend to use only a small subset of this
   grammar, however, and therefore do not reproduce it here.  We note,
   however, that it can be used to define a wide variety of attributes,
   such as attributes to represent the expiration date of a key or key-
   package, the intended cryptographic algorithm for a key, an
   identifier for a key or key-package, or the creator of a key or key-
   package.  As such attributes do not apply only to set-keys, however,
   we will not consider them futher in this document.

1.3.  Intended Usage

   The attribute defined in this document is meant to be embedded in a
   SymmetricKeyPackage structure, perhaps with other attributes as well.
   The resulting value will then hold a set-key in a common and standard
   format.  This format is appropriate both for transporting set-keys
   and for holding set-keys computed from some higher-level protocol.
   That is, one could create and distribute set-keys in this format
   through some distribution protocol, or one could use this format to
   hold set-keys computed by some protocol as Logical Key Heirarchy
   [RFC2627].

1.4.  Requirements 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.  The set-key attribute

   The set-key attribute is used to associate a participant-set with a
   symmetric key-package, where the participant-set MAY be divided into
   the active and passive participants.  It has the following syntax:

















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   aa-setkey-information ATTRIBUTE ::= {
        TYPE SetKeyInformation
        IDENTIFIED BY id-aa-setkey-information }

   id-aa-setkey-information OBJECT IDENTIFIER ::= {
     iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
            smime(16) aa(2) XX }

   SetKeyInformation::= SEQUENCE {
        version    SetKeyAttrVersion DEFAULT v1,
        active     SetKeyParticipantSet,
        passive    SetKeyParticipantSet OPTIONAL }

   SetKeyAttrVersion ::= INTEGER  { v1(1) } ( v1, ... )

   SetKeyParticipantSet ::= CHOICE {
        union        [0] SEQUENCE SIZE (2..MAX) OF SetKeyParticipantSet,
        intersection [1] SEQUENCE SIZE (2..MAX) OF SetKeyParticipantSet,
        setdiff      [2] SEQUENCE {
                          orig SetKeyParticipantSet,
                          without SetKeyParticipantSet },
        community    [3] Community,
        groupID      [4] OCTET STRING,
        explicit     [5] SEQUENCE SIZE (1..MAX) OF SetMember }

   SetMember ::= CHOICE {
        issuerAndSerialNumber  [0] IssuerAndSerialNumber,
        publicKey              [1] SubjectPublicKeyInfo,
        participantID          [2] OCTET STRING }


   This attribute SHOULD NOT appear in the sKeyAttrs field of a
   OneSymmetricKey structure.  That is, it is defined to apply only to
   symmetric-key packages, not individual keys.

   The SetKeyInformation fields are used as follows:

   o  The 'version' value identifies version of the set-key
      attributestructure.  For this version of the specification, the
      default value, v1, MUST be used.  Implementations MUST gracefully
      handle unfamiliar (future) versions as if the entire attribute was
      unfamiliar.

   o  The 'active' and 'passive' fields jointly identify the
      participant-set of this set-key.  The 'active' field identifies
      the active members of this set, meaning those which may send
      messages secured by the set-key.  The 'passive' field identifies
      the passive members of this set, meaning those participants who



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      are expected to never send a message secured by this key.  (The
      internal structures of these sets are described below.)  Any
      member which could possibly send messages under some scenario
      SHOULD be in the 'active' field.  A member MAY appear in both
      fields, in which case it SHOULD be considered to be active.

   Both the 'active' and 'passive' field will be values of type
   SetKeyParticipantSet.  These values can be any of:

   o  A 'union' of other SetKeyParticipantSet values.  The sequence of
      SetKeyParticipentSet values MUST contain at least two members.
      The semanitics of this sequence is as follows: a participant is in
      the union if and only if it is any of the SetKeyParticipantSet
      values of the sequence.  If any set in the sequence is non-empty,
      then the union will be non-empty.  If all sets in the union
      sequence are empty, then the union is empty too.

   o  The 'intersection' of other SetKeyParticipantSet values.  The
      sequence of SetKeyParticipentSet values MUST contain at least two
      members.  The semantics of this sequence is as follows: a
      participant is in intersection if and only if it is all of the
      SetKeyParticipantSet values of the sequence.  If any set in the
      sequence is empty, the intersection is empty, too.  If all sets in
      the sequence are non-empty, the intersection might still be empty
      (which it would be if there is no element common to all sets in
      the sequence).

   o  The set-difference ('setdiff') of two other SetKeyParticipantSet
      values.  The semantics of this is as follows: a participant is in
      the set-difference if and only if it is in the set identified in
      the 'orig' field and not in the set identified by the 'without'
      field.  If the set identified in the 'orig' field is empty is
      empty, then the set-difference will be empty.  If the set
      identified in the 'orig' field is a subset of the set identified
      in the 'without' field, then the set-difference will be empty.  If
      neither of the two previous cases apply, the set-difference will
      be non-empty.

   o  A community, which is used byt the Trust Anchor Management
      Protocol (TAMP) [Housley-TAMP] to identify a set of 'cryptographic
      modules.'

   o  A 'groupID', which is interpreted in an application-independent
      way.  One possible use for this option is to name pre-established
      groups such as organizational departments or roles.  The details
      of establishing or using such a name-space are outside the scope
      of this document.




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   o  An 'explicit' list of SetMember values.  In this sequence, order
      is not given any meaning.  This sequence MAY NOT be empty.

   If a SetKeyParticipantSet is not defined in terms of a community or
   groupID (that is, these options do not appear in the parse-tree of
   the SetKeyParticipantSet) then the set is static and unchanging.  In
   this case, the attribute-creator SHOULD process the
   SetKeyParticipantSet to ensure that the resulting set is non-empty.
   The set of active participants SHOULD NOT be empty.  If the set of
   passive participants is empty, then it SHOULD be omitted.  If a
   SetKeyParticipantSet is defined in terms of a community or groupID,
   these may refer to participant-sets that evolve over time.  Thus, it
   may not be possible to determine if the described set will be empty
   for all points in the future.  The attribute creator MAY pre-process
   the SetKeyParticipantSet to ensure the set is non-empty, but MUST NOT
   conclude the set to be empty unless that conclusion results from the
   set-theoretic semantics described above and independently of any
   specific interpretation of community-identifiers or groupID values.
   A SetMember value, meant to identify a specific unique participant,
   can be any of:

   o  An IssuerAndSerialNumber value, defined in [RFC5911],

   o  A SubjectPublicKeyInfo, defined in [RFC5912], or

   o  A freeform ('participantID') octet-string, which is interpreted in
      an application-dependent way.  One possible use of this option is
      to use pre-established names for participants.  The details of
      establishing or using such a name-space are outside the scope of
      this document.

   This list allows a certified key to be identified in two different
   ways: using the IssuerAndSerialNumber option, or the
   SubjectPublicKeyInfo option.  This can cause ambiguity when parsing a
   SetKeyParticipantSet structure.  Suppose that a given key is
   sometimes identified using the IssuerAndSerialNumber form (value 'I')
   and sometimes identified using the SubjectPublicKeyInfor form (value
   'S').  If these two distinct values are interpreted as identifying
   distinct keys, then the following structures may be misinterpreted:

   o  The 'explicit' sequence ...I, S... may be interpreted as having
      more elements than it actually does, due to the participant in
      question being counted twice (one for the I value, one for the S
      value).

   o  Similarly, the structure "union {...I...} {...S...}" may be
      interpreted as containgin one more participant than it actually
      does.



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   o  Likewise, the set "setdiff {...I...} {...S...}" may be interpreted
      as being non-empty when it is empty.

   o  On the other hand, the set "intersection {...I...} {...S...}" may
      be interpreted as being empty when it should not.  (It will always
      contain at least one element: the participant indicated by both I
      and S.)

   For these reasons, implementations that wish to name a certified key
   SHOULD use the IssuerAndSerialNumber form rather than the
   SubjectPublicKeyInfo form.  However, implementations that receive and
   process a SetKey attribute SHOULD NOT regard different names as
   always indicating different participants.  If the set-key attribute
   is present, it MUST contain a single value.


3.  Security Considerations

   As with the entire symmetric-key package, the set-key attribute is
   not protected.  The symmetric key package content type can be
   combined with a security protocol to protect the contents of the
   attribute.


4.  Acknowledgements

   The authors would like to thank Jim Schaad for his helpful comments
   and suggestions.


5.  References

5.1.  Normative References

   [Housley-TAMP]
              Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Management Protocol (TAMP)", Internet
              Draft draft-ietf-pkix-tamp-08.txt, April 2010.

   [Housley-symmkey]
              Turner, S. and R. Housley, "Symmetric Key Package Content
              Type", Internet
              Draft draft-ietf-keyprov-symmetrickeyformat-10.txt,
              August 2010.

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




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   [RFC5911]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for
              Cryptographic Message Syntax (CMS) and S/MIME", Request
              For Comments 5911, June 2010.

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

   [X.680]    ITU-T, "Information Technology - Abstract Syntax Notation
              One", Recommendation X.680, ISO/IEC 8824-1:2002, 2002.

   [X.681]    ITU-T, "Information Technology - Abstract Syntax Notation
              One: Information Object Specification",
              Recommendation X.681, ISO/IEC 8824-2:2002, 2002.

   [X.682]    ITU-T, "Information Technology - Abstract Syntax Notation
              One: Constraint Specification", Recommendation X.682, ISO/
              IEC 8824-3:2002, 2002.

   [X.683]    ITU-T, "Information Technology - Abstract Syntax Notation
              One: Parameterization of ASN.1 Specifications",
              Recommendation X.683, ISO/IEC 8824-4:2002, 2002.

5.2.  Informative References

   [RFC2627]  Wallner, D., Harder, E., and R. Agee, "Key Management for
              Multicast: Issues and Architectures", RFC 2627, June 1999.


Appendix A.  ASN.1 Module

   This appendix provides the normative ASN.1 definitions for the
   structures described in this specification using ASN.1 as defined in
   [X.680] through [X.683].

SetKeyAttributev1
  { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
    smime(16) modules(0) XX }

DEFINITIONS IMPLICIT TAGS ::=

BEGIN

-- EXPORTS ALL

IMPORTS

   ATTRIBUTE



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     FROM PKIX-CommonTypes-2009
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkixCommon-02(57)}
   IssuerAndSerialNumber
     FROM CryptographicMessageSyntax-2009
       { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
         smime(16) modules(0) id-mod-cms-2004-02(41)}
   SubjectPublicKeyInfo
     FROM PKIX1Explicit-2009
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-pkix1-explicit-02(51) }
   Community
     FROM TAMP-Protocol-v2
       { joint-iso-ccitt(2) country(16) us(840) organization(1)
         gov(101) dod(2) infosec(1) modules(0) 30 }
   ;

aa-setkey-information ATTRIBUTE ::= {
     TYPE SetKeyInformation
     IDENTIFIED BY id-aa-setkey-information }

id-aa-setkey-information OBJECT IDENTIFIER ::= {
  iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
         smime(16) aa(2) XX }

SetKeyInformation::= SEQUENCE {
     version    SetKeyAttrVersion DEFAULT v1,
     active     SetKeyParticipantSet,
     passive    SetKeyParticipantSet OPTIONAL }

SetKeyAttrVersion ::= INTEGER  { v1(1) } ( v1, ... )

SetKeyParticipantSet ::= CHOICE {
     union        [0] SEQUENCE SIZE (2..MAX) OF SetKeyParticipantSet,
     intersection [1] SEQUENCE SIZE (2..MAX) OF SetKeyParticipantSet,
     setdiff      [2] SEQUENCE {
                       orig SetKeyParticipantSet,
                       without SetKeyParticipantSet },
     community    [3] Community,
     groupID      [4] OCTET STRING,
     explicit     [5] SEQUENCE SIZE (1..MAX) OF SetMember }

SetMember ::= CHOICE {
     issuerAndSerialNumber  [0] IssuerAndSerialNumber,
     publicKey              [1] SubjectPublicKeyInfo,
     participantID          [2] OCTET STRING }




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END


Authors' Addresses

   Jonathan C. Herzog
   MIT Lincoln Laboratory
   244 Wood St.
   Lexington, MA  02144
   USA

   Email: [email protected]


   Roger Khazan
   MIT Lincoln Laboratory
   244 Wood St.
   Lexington, MA  02144
   USA

   Email: [email protected]






























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