PKIX Working Group T. Gindin
Internet Draft IBM
Document: <draft-ietf-pkix-technr-01.txt> July 2000
Category: Informational
Internet X.509 Public Key Infrastructure
Technical Requirements for a non-Repudiation Service
<draft-ietf-pkix-technr-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026[1].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft expires on January 14, 2001. Comments and
suggestions on this document are encouraged. Comments on this
document should be sent to the PKIX working group discussion list:
<ietf-pkix@imc.org>
or directly to the author, at tgindin@us.ibm.com.
This Internet-Draft represents the views of its author, and not
necessarily those of his employer.
1. Abstract
This document describes those features of a service which processes
signed documents which must be present in order for that service to
constitute a "technical non-repudiation" service. A technical
non-repudiation service must permit an independent verifier to
determine whether a given signature was applied to a given data
object by the private key associated with a given valid certificate,
at a time later than the signature. The features of a technical non-
repudiation service are expected to be necessary for a full non-
repudiation service, although they may not be sufficient.
This document is intended to clarify the definition of the
"non-repudiation" service in RFC 2459. It should thus serve as a
guide to when the nonRepudiation bit of the keyUsage extension should
be set and to when a Certificate Authority is required to archive
CRL's.
2. Introduction
RFC 2459 [2] specifies a bit within the KeyUsage extension called the
nonRepudiation bit which is "asserted when the subject public key is
used to verify digital signatures used to provide a non-repudiation
Gindin Informational - January 2001 1
Technical Requirements for a non-Repudiation Service July 2000
service which protects against the signing entity falsely denying
some action, excluding certificate or CRL signing." Extensive
discussions in the PKIX WG have revealed that the description of the
non-repudiation service contained in this passage is not widely
enough understood or agreed upon to characterize any given service as
providing or not providing a non-repudiation service. Two major
categories of service have been proposed as potentially providing a
non-repudiation service: the technical non-repudiation service, which
this draft attempts to define with greater precision, and a full non-
repudiation service which is intended to prevent all possible
repudiations of a signed object or document. Since a full non-
repudiation service is required to meet all the requirements of this
technical non-repudiation service as a prerequisite, the technical
non-repudiation service's definition is necessary for both.
2.1 Definitions
Signing Certificate: A certificate containing the public key
component of a key pair whose private key was used to create the
signature being verified.
Signer: The party who created the signature being verified.
It is outside the scope of these requirements to distinguish between
the actual signer and the subject of the signing certificate.
Relying Party: The party who received the signature being verified,
and initially verified it.
Verifier: An entity independent of both the signer and the relying
party who is verifying that the supplied signature, data object, and
certificate are consistent with each other.
1-way NR: A service in which the relying party preserves sufficient
evidence to permit the verifier to perform a verification, and may
submit it for verification by his or her own action. This service is
not expected to be extended in such a way that it may be used as a
basis for legal agreements.
2-way NR: A service in which the relying party submits sufficient
evidence to permit the verifier to perform a verification to a third
party, known as the "escrow holder". Either the relying party or the
signer may request that this data be submitted for verification.
Future extensions of this service would be necessary to permit it to
serve as a basis for legal agreements.
Escrow holder: The party responsible for preserving signature
evidence in 2-way NR. The escrow holder may also be, but need not
be, the verifier.
Escrow package: The data submitted from the relying party to the
escrow holder, in 2-way NR. The escrow holder may add certain
auditing and tracking information to this package before storage.
NR service: The technical nonRepudiation service referenced above.
keyUsage extension: A standard extension within X.509v3 [3]
certificates with object identifier { 2 5 29 15 }, consisting of a
series of enumerated bits.
NR bit: The nonRepudiation bit (offset 1) of the keyUsage
extension.
Gindin Informational - January 2001 2
Technical Requirements for a non-Repudiation Service July 2000
2.2 Scope and caveats
The NR service is expected to provide evidence that a given object
was signed by the private key corresponding to a given certificate
which was valid at the time of signature. It is not anticipated that
the use of the NR service will ordinarily constitute execution of a
contract, or acceptance of any other legal obligation. It is
anticipated that any use of this service in accepting legal
obligations would be the subject of legislation or judicial decision
in various jurisdictions, which are likely to lay additional
technical burdens upon the provision of such a service to such an
extent as to constitute another, larger service which need not be the
same in all jurisdictions. It is outside the scope of the definition
of this service to provide evidence that the signer and the subject
of the signing certificate are the same, that the signer has been
adequately informed of the content which is signed, that the signer
is not acting under duress, etc.
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 RFC-2119 [4].
3. Requirements for both 1-way and 2-way NR
3.1 The signer MUST submit, with the signature, the signing
certificate or an unambiguous identifier of that certificate.
Unambiguous identifiers of certificates include the combination of a
certificate serial number with an issuer name and the combination of
issuer name, subject name, and subject key identifier.
3.2 The signer MUST submit, with the signature, the content being
signed or an unambiguous reference to that content. It is explicitly
contemplated that a URI constitutes an unambiguous reference to its
content.
3.3 The signer MUST include, in the base over which the signature
is calculated, the time at which the signature was created. There is
no obligation that this time be verified with an independent
authority.
3.4 The signer MUST include, in the base over which the signature
is calculated, the content being signed or the message digest of that
content. This object therefore includes all those fields which are
both mandatory and variable on a per-object basis within a Non-
repudiation of origin token using asymmetric keys according to ISO
13888[5].
3.5 The relying party MUST, before accepting the signature, verify
that the signing certificate is valid. This verification SHOULD
include a CRL check or other online certificate verification using
such mechanisms as OCSP or SCVP.
3.6 The relying party MUST, before accepting the signature, verify
the signature of the data object being submitted.
3.7 The validity of the signer's certificate is considered to have
been finally judged once a validation has been performed in which the
effective time of the certificate status is no earlier than the later
Gindin Informational - January 2001 3
Technical Requirements for a non-Repudiation Service July 2000
of the signing time and the relying party's time. There is no need
to check for a later revocation with an invalidityDate CRL Entry
extension prior to the signature, because that extension's value is
purely advisory and not validated by anyone other than the
certificate subject. In the case where a certificate status is
derived from a CRL, the effective time of the certificate status is
the value of the "thisUpdate" field of the CRL. In the case where a
certificate status is represented by a basic OCSP response, the
effective time of the certificate status is the value of the
"thisUpdate" field of the status for that certificate.
4. Requirements for 1-way NR
4.1 The relying party MUST save a copy of the content being
signed.
4.2 The relying party MUST save the identity of the signing
certificate, along with the signature itself and any signature
attributes. If the relying party has verified the certificate using a
server supporting a "signed-status-response" protocol such as OCSP or
SCVP, the relying party MUST store the status responses with the data
submitted. If the relying party has verified the certificate using a
CRL, the relying party MAY store that CRL with the data submitted.
The relying party MAY also include the chain of issuer certificates
back to his (or her) trusted root.
4.3 The relying party MUST check that the signing certificate
contains a keyUsage extension and an extendedKeyUsage extension. If
the keyUsage extension is not present or does not contain the
nonRepudiation bit, unless the extendedKeyUsage extension is present
and contains a specific key purpose involving non-repudiation or the
version of the certificate is lower than v3, the submission MUST be
rejected.
4.4 The relying party SHOULD create, and save with the data
submitted by the signer, a package containing a current time stamp
signed by an independent authority, which may be a Time Stamp
Authority. This package signed by the independent authority SHOULD
include the time stamp, the identity of the signing certificate, and
at least one of the following: a countersignature created by the
relying party, a copy of the "signature block" of the submitted
document, or the entire submitted document.
4.5 The relying party MAY return a receipt to the signer. This
receipt, if it exists, SHOULD contain the package time stamped and
signed by an independent authority referred to in the preceding
subsection. The relying party SHOULD sign the receipt and include
both the current time and an unambiguous identifier of his (or her)
signing certificate or that certificate itself. This receipt
therefore includes all those fields which are both mandatory and
variable on a per-object basis within a Non-repudiation of delivery
token using asymmetric keys according to ISO 13888[6].
4.6 The relying party SHOULD, at various fixed intervals (not
herein defined) after the acceptance of the package, perform
certificate verifications for the purpose of finding a revocation of
the signing certificate, especially one having a revocation date
which is no later than the later of the originator's time (see
section 3.3) and the independent authority's time (see section 4.4).
If such a revocation is found evidence of the revocation MUST be
Gindin Informational - January 2001 4
Technical Requirements for a non-Repudiation Service July 2000
preserved with the package. Such evidence SHOULD be in the form of a
CRL or a signed response from a certificate verification service.
Once a CRL is found with an effective time no earlier than both of
the relevant times above, the status is final and the CRL MUST be
preserved with the package. Once a status response is found with an
effective time no earlier than both of the relevant times above, the
status is final and the status reponse MUST be preserved with the
package.
4.7 If either the issuer of the submitter's certificate or the
third party which signed the package in 4.4 ceases operation and the
public key which they used can no longer be obtained from a source
independent of the relying party, the transaction is considered to be
unverifiable. If one or more of them cease operation, but their
public keys can still be obtained from independent sources, the
transaction does not become unverifiable.
5. Requirements for 2-way NR
5.1 The relying party MUST submit to the escrow holder a copy of
the content being signed, the identity of the signing certificate,
and the signature. If the relying party has verified the certificate
using a server supporting a "signed-status-response" protocol such as
OCSP or SCVP, the relying party MUST include the status responses in
the escrow package. If the relying party has verified the
certificate using a CRL, the relying party MAY include that CRL in
the escrow package. The relying party SHOULD also include the chain
of issuer certificates back to his (or her) trusted root.
5.2 The relying party MUST sign the submission to the escrow
holder. The relying party SHOULD include, in the base over which
that signature is calculated, the current time and the identity of
the signing certificate. This time will be between the time when the
signer submitted the signature and the time when the package is
submitted. The signed object submitted is known as the escrow
package.
5.3 The relying party MUST check whether or not the signing
certificate contains a keyUsage extension and an extendedKeyUsage
extension. If the keyUsage extension is present and the
nonRepudiation bit is not set, unless the extendedKeyUsage extension
contains a specific key purpose involving non-repudiation the
submission MUST be rejected. Larger services extending this one MAY
dispense from this requirement by explicit statements in their
service definition.
5.4 The relying party MAY return a receipt to the signer. This
receipt, if it exists, SHOULD contain the "signature block" of the
escrow package. This receipt therefore includes all those fields
which are both mandatory and variable on a per-object basis within a
Non-repudiation of delivery token using asymmetric keys according to
ISO 13888[7].
5.5 The escrow holder SHOULD, at various fixed intervals (not
herein defined) after the acceptance of the package, perform
certificate verifications for the purpose of finding a revocation of
the signing certificate, especially one having a revocation date
which is no later than the earlier of the originator's time (see
section 3.3) and the relying party's time (see section 5.2). If such
a revocation is found, the relying party SHOULD be informed, and
Gindin Informational - January 2001 5
Technical Requirements for a non-Repudiation Service July 2000
evidence of the revocation MUST be preserved with the escrow package.
Such evidence SHOULD be in the form of a CRL or a signed response
from a certificate verification service. Once a CRL is found with an
effective time no earlier than both of the relevant times above, the
status is final and the CRL MUST be preserved with the package. Once
a status response is found with an effective time no earlier than
both of the relevant times above, the status is final and the status
response MUST be preserved with the package.
6. Security Considerations
Most of this memo deals with security mechanisms. All messages
exchanged between identified parties consist mainly of signed
components.
7. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9,
RFC 2026, October 1996.
2 R. Housley, W. Ford, W. Polk, and D. Solo "Internet X.509 Public
Key Infrastructure Certificate and CRL Profile", RFC 2459, January
1999.
3 International Telecommunications Union, "ITU-T Recommendation X.509
- Information Technology - Open Systems Interconnection - The
Directory: Authentication Framework", June 1997.
4 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
5 International Standards Organization, "Information Technology -
Security techniques - Non-repudiation", ISO 13888, December 1997.
6 International Standards Organization, "Information Technology -
Security techniques - Non-repudiation", ISO 13888, December 1997.
7 International Standards Organization, "Information Technology -
Security techniques - Non-repudiation", ISO 13888, December 1997.
8. Acknowledgments
I would like to thank Tony Bartoletti, Ed Gerck, Steve Kent, Aram
Perez, Andreas Schmidt, and John Wray for their many suggestions for
revisions to earlier versions of this document.
9 Author's Addresses
Thomas Gindin
IBM Corporation
6710 Rockledge Drive
Bethesda, MD 20817
USA
Email: tgindin@us.ibm.com
Full Copyright Statement
"Copyright (C) The Internet Society (2000). All Rights Reserved. This
document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
Gindin Informational - January 2001 6
Technical Requirements for a non-Repudiation Service July 2000
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
Gindin Informational - January 2001 7