Using the Server-Based Certificate Validation Protocol (SCVP) to Convey Long-Term Evidence Records
RFC 5276

Network Working Group                                         C. Wallace
Request for Comments: 5276                            Cygnacom Solutions
Category: Standards Track                                    August 2008

   Using the Server-Based Certificate Validation Protocol (SCVP) to
                   Convey Long-Term Evidence Records

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Abstract

   The Server-based Certificate Validation Protocol (SCVP) defines an
   extensible means of delegating the development and validation of
   certification paths to a server.  It can be used to support the
   development and validation of certification paths well after the
   expiration of the certificates in the path by specifying a time of
   interest in the past.  The Evidence Record Syntax (ERS) defines
   structures, called evidence records, to support the non-repudiation
   of the existence of data.  Evidence records can be used to preserve
   materials that comprise a certification path such that trust in the
   certificates can be established after the expiration of the
   certificates in the path and after the cryptographic algorithms used
   to sign the certificates in the path are no longer secure.  This
   document describes usage of the SCVP WantBack feature to convey
   evidence records, enabling SCVP responders to provide preservation
   evidence for certificates and certificate revocation lists (CRLs).

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  3
   2.  Concept of Operations  . . . . . . . . . . . . . . . . . . . .  4
   3.  Requests . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Responses  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  WantBacks  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  Evidence Record for a Complete Certification Path  . . . .  7
     5.2.  Evidence Record for a Partial Certification Path . . . . .  7
     5.3.  Evidence Record for a Public Key Certificate . . . . . . .  8
     5.4.  Evidence Record for Revocation Information . . . . . . . .  8
     5.5.  Evidence Record for Any replyWantBack  . . . . . . . . . .  8
     5.6.  Partial Certification Path . . . . . . . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  ASN.1 Module  . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   Digital signatures are frequently verified using public key
   infrastructure (PKI) artifacts, including public key certificates and
   certificate revocation information.  Verifiers construct and validate
   certification paths from a public key certificate containing the
   public key used to verify the signature to a trusted public key.
   Construction of a certification path may require the acquisition of
   different types of information generated by multiple PKIs.  To verify
   digital signatures many years after signature generation, additional
   considerations must be addressed.  For example, some necessary PKI
   artifacts may no longer be available, some may have expired, and the
   cryptographic algorithms or keys used in generating digital
   signatures may no longer provide the desired degree of security.

   SCVP [RFC5055] provides a means of delegating certification path
   construction and/or validation to a server, including the ability to
   request the status of a certificate relative to a time in the past.
   SCVP does not define a means of providing or validating long-term
   non-repudiation information.  ERS [RFC4998] defines a syntax for
   preserving materials over long periods of time through a regimen that
   includes periodic re-signing of relevant materials using newer keys
   and stronger cryptographic algorithms.  LTAP [LTANS-LTAP] defines a
   protocol for communicating with a long-term archive (LTA) server for
   the purpose of preserving evidence records and data.  Clients store,
   retrieve, and delete data using LTAP; LTAs maintain evidence records
   covering data submitted by clients.

   This document defines an application of SCVP to permit retrieval of
   an evidence record corresponding to information returned by the SCVP
   server by creating an association between an evidence record and
   information contained in an SCVP response.  The SCVP response can
   then in turn be used to verify archived data objects retrieved using
   LTAP.  Separating the preservation of the certification path
   information from the preservation of data enables the LTA to store
   archived data objects more efficiently, i.e., complete verification
   information need not be stored with each archived data object.
   Verifiers can more efficiently process archived data objects by
   reusing the same certification path information to verify multiple
   archived data objects of similar vintage without retrieving and/or
   validating the same PKI artifacts multiple times.

1.1.  Requirements Notation

   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.  Concept of Operations

   During certification path processing, active SCVP servers may
   encounter a large portion of the PKI artifacts generated by a
   particular PKI.  By storing and preserving these artifacts, an SCVP
   server can respond to queries for certificate status over very long
   periods of time.  Optionally, SCVP servers may actively seek PKI
   information for storage and preservation, even when no query is made,
   that requires the information during its period of validity in order
   to service future queries relative to any point in time.

   SCVP permits clients to request as much or as little information as
   desired from the SCVP server.  Clients include zero or more Object
   Identifiers (OIDs) indicating the type(s) of information the server
   should include in the response.  By defining additional OID values,
   clients can request an evidence record for specific types of
   information returned by the SCVP server.  This document defines OIDs
   to permit the retrieval of evidence records for the following four
   types of information:

   o  end entity certificates.

   o  certification paths containing an end entity certificate up to a
      trust anchor.

   o  certification paths containing an intermediate certificate up to a
      trust anchor.

   o  revocation information.

   Additionally, an OID is defined to permit inclusion of a single OID
   indicating an evidence record is desired for all information
   requested via the WantBack mechanism.

   By associating evidence records with information maintained by an
   SCVP server, clients are able to determine the status of certificates
   over very long periods of time using SCVP without consulting
   additional resources.  The nature of SCVP servers is well suited to
   the preservation of infrastructure materials.  Additionally, the SCVP
   server's signature over an SCVP response can secure the transmission
   of trust anchors included in evidence records, allowing clients to
   refrain from establishing additional trust relationships with LTAs.

   The transactions used to verify an archived data object using LTAP
   and the SCVP WantBacks described in this document are as follows:

   o  Client retrieves a signed archived data object from an LTA using
      LTAP.

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   o  Client prepares an SCVP request to validate the signer's
      certificate at the time of interest and includes WantBacks for
      evidence records corresponding to the PKI artifacts required to
      validate the signer's certificate.

   o  SCVP server returns a response with status as of the time of
      interest and includes requested evidence records.

   o  Client processes the SCVP request, determines the status, and
      verifies the evidence records.

   o  Client verifies signatures in the archived data object using the
      validated signer's certificate.

3.  Requests

   Clients request long-term archive evidence records from an SCVP
   server by including one of the following OIDs in the wantBack field
   of a CVRequest sent to an SCVP server:

   o  id-swb-ers-best-cert-path

   o  id-swb-ers-partial-cert-path

   o  id-swb-ers-pkc-cert

   o  id-swb-ers-revocation-info

   o  id-swb-ers-all

   Additionally, id-swb-partial-cert-path is defined to permit clients
   to request a partial certification path consisting of the
   certification authority (CA) that issued the end entity certificate
   through a trust anchor.  This is similar to the id-swb-best-cert-path
   WantBack defined in SCVP except the resulting replyWantBack will
   contain a CertBundle containing the certification path minus the end
   entity certificate.

   For each id-swb-ers OID except id-swb-ers-all, an EvidenceRecord (as
   defined in [RFC4998]) covering the corresponding information in the
   response will be returned as a replyWantBack.  For example, if a
   client wishes to obtain a certification path and revocation
   information plus an evidence record for each, the SCVP request would
   include the following four replyWantBack OIDs: id-swb-best-cert-path,
   id-swb-pkc-revocation-info, id-swb-ers-best-cert-path, and id-swb-
   ers-revocation-info.

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   Alternatively, for id-swb-ers-all, an EvidenceRecordWantBacks
   structure will be returned containing an EvidenceRecord for each
   information item contained in the replyWantBacks field.  For example,
   if a client wishes to obtain a certification path and revocation
   information plus an evidence record for each, the SCVP request could
   include the following three replyWantBack OIDs: id-swb-best-cert-
   path, id-swb-pkc-revocation-info, and id-swb-ers-all.

4.  Responses

   When a client request contains a WantBack request for an evidence
   record, the response generated MUST include the replyWantBack
   containing the requested information plus a replyWantBack containing
   the evidence record corresponding to that information.  For each id-
   swb-ers OID except id-swb-ers-pkc-cert and id-swb-ers-revocation-
   info, the evidence record MUST be calculated over the value of the
   value field in the corresponding replyWantBack; the tag and length
   bytes are not covered by the evidence record.  The targets for the
   id-swb-ers-pkc-cert and id-swb-ers-revocation-info replyWantBacks are
   described below.  For example, if a client request contains id-swb-
   pkc-best-cert-path and id-swb-ers-best-cert-path, the resulting
   response will contain a replyWantBack of each type where the evidence
   record covers the DER-encoded CertBundle returned in the id-swb-pkc-
   best-cert-path replyWantBack.  For id-swb-ers-pkc-cert, the evidence
   record MUST be calculated over the value of the cert field in the
   CertReply object.  For id-swb-ers-revocation-info, a sequence of
   evidence records is returned.  Each revocation information object
   contained in the id-swb-pkc-revocation-info replyWantBack is covered
   by an evidence record in the id-swb-ers-revocation-info
   replyWantBack.  A single evidence record may cover multiple
   revocation information objects.  The correct evidence record can be
   identified by locating the hash of the revocation information object
   in the first initial timestamp of the evidence record.

   If the server cannot return an EvidenceRecord for the requested
   information item, a replyWantBack of the appropriate type MUST be
   returned with an empty value field.  For example, if a client
   requests id-swb-ers-pkc-cert and the server cannot fulfill the
   request, the resulting response will contain a replyWantBack with the
   wb field set to id-swb-ers-pkc-cert and the value field empty, i.e.,
   zero length.

5.  WantBacks

   The following sections describe each WantBack defined in this
   document.  Each WantBack for an evidence record requires a
   corresponding WantBack for the object covered by the evidence record
   to be present in the request.  Upon receipt of a request missing the

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   corresponding WantBack for the object covered by a requested evidence
   record, the server MUST indicate wantBackUnsatisfied in the
   ReplyStatus.  Clients MAY ignore evidence record WantBacks when the
   WantBack for the corresponding object is not present.

5.1.  Evidence Record for a Complete Certification Path

   The id-swb-ers-best-cert-path OID is used to request an evidence
   record for a complete certification path.  It is used in conjunction
   with the id-swb-best-cert-path OID.  Requests containing id-swb-ers-
   best-cert-path as a WantBack MUST also contain id-swb-best-cert-path.
   Responses containing id-swb-ers-best-cert-path MUST also contain id-
   swb-best-cert-path.

   An SCVP server may maintain evidence records for complete
   certification paths, i.e., certification paths containing all
   certificates from end entity to trust anchor.  The evidence record
   MUST be calculated over the CertBundle returned via the id-swb-best-
   cert-path replyWantBack.  In such cases, a signature within the
   archived data object may be verified using an end entity certificate
   returned via SCVP.  The end entity certificate can be verified using
   SCVP using a request containing id-swb-ers-best-cert-path, id-swb-
   best-cert-path, id-swb-pkc-revocation-info, and id-swb-ers-
   revocation-info.

5.2.  Evidence Record for a Partial Certification Path

   The id-swb-ers-partial-cert-path OID is used to request an evidence
   record for a partial certification path.  It is used in conjunction
   with the id-swb-partial-cert-path OID.  Requests containing id-swb-
   ers-partial-cert-path as a WantBack MUST also contain id-swb-partial-
   cert-path.  Responses containing id-swb-ers-partial-cert-path MUST
   also contain id-swb-partial-cert-path.

   As an alternative to relying on SCVP to obtain evidence records for
   end entity certificates, the certificate could be included in the
   archived data object(s) submitted to an LTA.  In such cases, a
   signature within the archived data object may be verified using the
   included end entity certificate, which is protected by the evidence
   record covering the archived data object, including the certificate.
   The end entity certificate can be verified using SCVP using a request
   containing id-swb-partial-cert-path, id-swb-ers-partial-cert-path,
   id-swb-pkc-revocation-info, and id-swb-ers-revocation-info.  Unlike
   the partial certification path, the revocation information includes
   material that can be used to determine the status of the end entity
   certificate.

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   By maintaining an evidence record for a partial certification path,
   SCVP servers can achieve greater storage efficiency.

5.3.  Evidence Record for a Public Key Certificate

   The id-swb-ers-pkc-cert OID is used to request an evidence record for
   an individual public key certificate.  It is used in conjunction with
   the id-swb-pkc-cert OID.  Requests containing id-swb-ers-pkc-cert as
   a WantBack MUST also contain id-swb-pkc-cert.  Responses containing
   id-swb-ers-pkc-cert MUST also contain id-swb-pkc-cert.

   SCVP servers may maintain evidence records for individual
   certificates.  This enables clients to omit the signer's certificate
   from archived data object(s) submitted to an LTA.  In such cases, a
   signature within the archived data object may be verified using an
   end entity certificate returned via SCVP.  The end entity certificate
   can be verified using SCVP using a request containing id-swb-pkc-
   cert, id-swb-ers-pkc-cert, id-swb-partial-cert-path, id-swb-ers-
   partial-cert-path, id-swb-pkc-revocation-info, and id-swb-ers-
   revocation-info.

5.4.  Evidence Record for Revocation Information

   The id-swb-ers-revocation-info OID is used to request evidence
   records for a set of revocation information.  It is used in
   conjunction with the id-swb-revocation-info OID.  Requests containing
   id-swb-ers-revocation-info as a WantBack MUST also contain id-swb-
   revocation-info.  Responses containing id-swb-ers-revocation-info
   MUST also contain id-swb-revocation-info.  A sequence of evidence
   records is returned, with one evidence record provided for each
   element in id-swb-revocation-info.

     EvidenceRecords ::= SEQUENCE SIZE (1..MAX) OF EvidenceRecord

   An SCVP server may maintain evidence records for revocation
   information.  Revocation information may be provided in the form of
   CRLs or Online Certificate Status Protocol (OCSP) responses.
   Cumulative CRLs may be generated for archiving to simplify evidence
   record maintenance.

5.5.  Evidence Record for Any replyWantBack

   An SCVP server may maintain evidence records for additional types of
   information that can be returned using the wantBack mechanism, e.g.,
   attribute certificate information.  The id-swb-ers-all OID provides a
   shorthand means for clients to request evidence records for all
   information returned via the replyWantBacks field.  Since id-swb-ers-
   all can result in the return of multiple evidence records in the

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   response, a mechanism is needed to associate an evidence record with
   the type of information covered by the evidence record.  The
   EvidenceRecordWantBacks structure provides a flexible means of
   conveying an evidence record for different types of information.

   EvidenceRecordWantBack ::= SEQUENCE
   {
       targetWantBack    OBJECT IDENTIFIER,
       evidenceRecord    EvidenceRecord OPTIONAL
   }

   EvidenceRecordWantBacks ::=
       SEQUENCE SIZE (1..MAX) OF EvidenceRecordWantBack

   EvidenceRecordWantBacks is a SEQUENCE OF EvidenceRecordWantBack
   structures.  The targetWantBack field indicates the type of
   replyWantBack covered by the associated EvidenceRecord.  The
   evidenceRecord field, if present, contains an EvidenceRecord
   structure calculated over the replyWantBack indicated by the
   targetWantBack field.  Where EvidenceRecordWantBacks is used, there
   MUST be a one-to-one correspondence between other replyWantBack
   objects and objects in the EvidenceRecordWantBacks collection.  If a
   server does not have an EvidenceRecord for a particular replyWantBack
   object, an EvidenceRecordWantBack with the evidenceRecord field
   absent should be included in the EvidenceRecordWantBacks collection.

5.6.  Partial Certification Path

   The id-swb-partial-cert-path is an alternative to id-swb-best-cert-
   path.  This is the only OID defined in this document for which an
   EvidenceRecord is not returned in the response.  For efficiency, SCVP
   servers that maintain evidence records for certification paths may
   only do so for partial paths instead of maintaining one or more paths
   for each end entity certificate.

   SCVP clients can include id-swb-partial-cert-path in a request when a
   partial certification path is required.  This would typically be
   included along with id-swb-ers-partial-cert-path to account for the
   fact that some SCVP servers only produce evidence records for partial
   paths for storage and computational efficiency reasons.  In such
   cases, a separate evidence record may be available for the end entity
   certificate by including id-swb-pkc-cert and id-swb-ers-pkc-cert in
   the request.

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6.  Security Considerations

   For security considerations specific to SCVP, see [RFC5055].  For
   security considerations specific to ERS, see [RFC4998].

   The signature on the SCVP response containing one or more ERS
   structures must be verified using a public key trusted by the relying
   party.  The response may contain trust anchors used to verify
   interior layers of an ERS structure.  The trust anchors are protected
   by the SCVP server's signature covering the response.  The relying
   party may elect to use the trust anchors conveyed in the response or
   ignore the trust anchors in favor of trust anchors retrieved out of
   band.  Relying parties SHOULD ignore trust anchors contained in
   unsigned SCVP responses.

7.  References

7.1.  Normative References

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

   [RFC4998]     Gondrom, T., Brandner, R., and U. Pordesch, "Evidence
                 Record Syntax (ERS)", RFC 4998, August 2007.

   [RFC5055]     Freeman, T., Housley, R., Malpani, A., Cooper, D., and
                 W. Polk, "Server-Based Certificate Validation Protocol
                 (SCVP)", RFC 5055, December 2007.

7.2.  Informative References

   [LTANS-LTAP]  Jerman-Blazic, A., Sylvester, P., and C. Wallace,
                 "Long-term Archive Protocol (LTAP)", Work in Progress,
                 February 2008.

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Appendix A.  ASN.1 Module

   The following ASN.1 module defines object identifiers used to
   identify six new forms of SCVP WantBacks and three new structures.
   EvidenceRecordWantBack and EvidenceRecordWantBacks are used in
   conjunction with the id-swb-ers-all WantBack to correlate evidence
   records with WantBacks.  EvidenceRecords is used in conjunction with
   the id-swb-ers-revocation-info WantBack to return evidence records
   for individual revocation information objects.

   LTANS-SCVP-EXTENSION
   { iso(1) identified-organization(3) dod(6) internet(1)
      security(5) mechanisms(5) ltans(11) id-mod(0) id-mod-ers-scvp(5)
      id-mod-ers-scvp-v1(1) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   IMPORTS

   id-swb
   FROM SCVP
   { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0) 21 }

   EvidenceRecord
   FROM ERS
   {iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) ltans(11) id-mod(0) id-mod-ers88(2)
       id-mod-ers88-v1(1) };

   id-swb-partial-cert-path        OBJECT IDENTIFIER ::= {id-swb 15 }

   id-swb-ers-pkc-cert             OBJECT IDENTIFIER ::= {id-swb 16 }
   id-swb-ers-best-cert-path       OBJECT IDENTIFIER ::= {id-swb 17 }
   id-swb-ers-partial-cert-path    OBJECT IDENTIFIER ::= {id-swb 18 }
   id-swb-ers-revocation-info      OBJECT IDENTIFIER ::= {id-swb 19 }
   id-swb-ers-all                  OBJECT IDENTIFIER ::= {id-swb 20 }

   EvidenceRecordWantBack ::= SEQUENCE
   {
       targetWantBack    OBJECT IDENTIFIER,
       evidenceRecord    EvidenceRecord OPTIONAL
   }

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   EvidenceRecordWantBacks ::=
       SEQUENCE SIZE (1..MAX) OF EvidenceRecordWantBack

   EvidenceRecords ::= SEQUENCE SIZE (1..MAX) OF EvidenceRecord

   END

Author's Address

   Carl Wallace
   Cygnacom Solutions
   Suite 5200
   7925 Jones Branch Drive
   McLean, VA  22102

   EMail: cwallace@cygnacom.com

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