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Keywords: [--------], ERS, Evidence Record, SCVP, Server-based Certificate Validation Protocol, PKI artifact preservation
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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