Internet Draft C. Adams(Entrust Technologies)
PKIX Working Group P. Cain (BBN)
expires in six months D. Pinkas (Bull)
R. Zuccherato(Entrust Technologies)
October 1999
Internet X.509 Public Key Infrastructure
Time Stamp Protocol (TSP)
<draft-ietf-pkix-time-stamp-04.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
Internet-Drafts are working documents of the Internet Engineering
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Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
A time stamping service allows to prove that a datum existed before
a particular time and can be used as a Trusted Third Party (TTP) as
one component in building reliable non-repudiation services (see
[ISONR]). This document describes the format of a request sent to a
Time Stamping Authority (TSA) and of the response that is returned.
An example on how to prove that a digital signature was generated
during the validity period of the corresponding public key
certificate is given in an annex.
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document (in uppercase,
as shown) are to be interpreted as described in [RFC2119].
1. Introduction
In order to associate a datum with a particular point in time, a
Time Stamp Authority (TSA) may need to be used. This Trusted Third
Party provides a ½proof-of-existence© for this particular datum at an
instant in time.
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The TSAÆs role is to time stamp a datum to establish evidence
indicating the time at which the datum existed. This can then be
used, for example, to verify that a digital signature was applied to
a message before the corresponding certificate was revoked thus
allowing a revoked public key certificate to be used for verifying
signatures created prior to the time of revocation. This is an
important public key infrastructure operation. The TSA can also be
used to indicate the time of submission when a deadline is critical,
or to indicate the time of transaction for entries in a log. An
exhaustive list of possible uses of a TSA is beyond the scope of
this document.
2. The TSA
The TSA is a TTP that creates time stamp tokens in order to indicate
that a datum existed at a particular point in time.
For the remainder of this document a ævalid requestÆ shall mean one
that can be decoded correctly, is of the form specified in Section 2.4,
and is from a supported TSA subscriber.
2.1. Requirements of the TSA
The TSA is REQUIRED:
1. to provide a trustworthy source of time.
2. not to include any identification of the requesting entity in
the time stamp tokens.
3. to include a monotonically incrementing value of the time
for each newly generated time stamp token.
4. to include a monotonically incrementing integer for each
newly generated time stamp token.
5. to produce a time stamp token upon receiving a valid request
from the requester, when it is possible.
6. to include within each time stamp token an identifier to
uniquely indicate the security policy under which the token
was created.
7. to only time stamp a hash representation of the datum, i.e.
a data imprint associated with a one-way collision resistant
hash-function OID.
8. to examine the OID of the one-way collision resistant hash-
function and to verify that the hashvalue length is
consistent with the hash algorithm.
9. not to examine the imprint being time stamped in any way.
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10. to sign each time stamp token using a key generated
exclusively for this purpose and have this property of the
key indicated on the corresponding certificate.
11. to include additional information in the time stamp token,
if asked by the requester using the extensions field, only
for the extensions that are supported by the TSA. If this is
not possible, the TSA shall respond with an error message.
2.2. TSA Transactions
As the first message of this mechanism, the requesting entity requests
a time stamp token by sending a request (which is or includes a
TimeStampReq, as defined below) to the Time Stamping Authority. As
the second message, the Time Stamping Authority responds by sending a
response (which is or includes a TimeStampToken, as defined below) to
the requesting entity.
Upon receiving the response (which is or includes a TimeStampResp,
as defined below), the requesting entity verifies the status error
returned in the response and if no error is present verifies the
various fields contained in the TimeStampToken and the validity of the
digital signature of the TimeStampToken. In particular, it verifies
that what was time stamped corresponds to what was requested to be
time stamped. The requester SHALL verify that the TimeStampToken
contains the correct certificate identifier of the TSA, the correct
data imprint and the correct hash algorithm OID. It SHALL then verify
the timeliness of the response by verifying either the time included
in the response against a local trusted time reference, if one is
available, and/or the value of the nonce (large random number with a
high probability that it is generated by the client only once)
included in the response against the value included in the request.
Since the TSAÆs certificate may have been revoked, the status of the
certificate SHOULD be checked (e.g. by checking the appropriate CRL)
to verify that the certificate is still valid.
The client application SHOULD check the policy field to determine
whether or not the policy under which the token was issued is
acceptable for the application. The client MAY ignore this field
if that is acceptable for the intended application.
2.3. Identification of the TSA
The TSA MUST sign all time stamp messages with one or more keys
reserved specifically for that purpose. The corresponding certificate
MUST contain only one instance of the extended key usage field
extension as defined in [RFC2459] Section 4.2.1.13 with KeyPurposeID
having value id-kp-timeStamping. This extension MUST be critical .
A TSAÆs certificate MAY contain an Authority Information Access
extension [RFC2459] in order to convey the method of contacting the
TSA. The accessMethod field in this extension MUST contain the OID
id-ad-timestamping:
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id-ad OBJECT IDENTIFIER ::= { id-pkix 48 }
id-ad-timestamping OBJECT IDENTIFIER ::= { id-ad X }
The value of the accessLocation field defines the transport
(e.g. HTTP) used to access the TSA and may contain other transport
dependent information(e.g. a URL).
2.4. Request and Response Formats
2.4.1. Request Format
A time stamping request is as follows:
TimeStampReq ::= SEQUENCE {
version Integer { v1(1) },
messageImprint MessageImprint,
--a hash algorithm OID and the hash value of the data to be
--time stamped
reqPolicy [0] PolicyInformation OPTIONAL,
nonce [1] Integer OPTIONAL,
extensions [2] EXPLICIT Extensions OPTIONAL
}
The version field (currently v1) describes the version of the
TimeStamp request.
The messageImprint field SHALL contain the hash of the datum to be
time stamped. The hash is represented as an OCTET STRING. Its length
MUST match the length of the hash value for that algorithm (e.g.
20 bytes for SHA-1 or 16 bytes for MD5).
MessageImprint ::= SEQUENCE {
hashAlgorithm AlgorithmIdentifier,
hashedMessage OCTET STRING }
The hash algorithm indicated in the hashAlgorithm field MUST be a known
hash algorithm (one-way and collision resistant).
The reqPolicy field, if included, indicates the policy under which the
TimeStampToken should be provided. PolicyInformation is defined in
Section 4.2.1.5 of [RFC2459].
The nonce, if included, allows to verify the timeliness of the
response when no local clock is available. The nonce is a large random
number with a high probability that it is generated by the client only
once (e.g. a 64 bits integer). In such a case the same nonce value
shall be included in the response, otherwise the response shall be
rejected.
The extensions field is a generic way to add additional information to
the request in the future. EXTENSIONS is defined in [RFC 2459]. If an
extension, whether it is marked critical or not critical, is used
by a requester but is not recognized by a time stamping server, the
server SHALL not issue a token and return a failure (badRequest).
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The time stamp request does not identify the requester, as this
information is not validated by the TSA (See Section 2.1).
In situations where the TSA requires the identity of the requesting
entity, alternate identification /authentication means have to be used
(e.g. CMS encapsulation [CMS] or TLS authentication [RFC2246]).
2.4.2. Response Format
A time stamping response is as follows:
TimeStampResp ::= SEQUENCE {
status PKIStatusInfo,
timeStampToken TimeStampToken OPTIONAL
}
The status is using the same error codes that are defined in
section 3.2.3 of [RFC2510] but adds two new one.
When the PKIStatusInfo contains the value zero a Time Stamp Token is
present. Otherwise, the status indicates the reason why the time stamp
request was rejected.
PKIFailureInfo ::= BITSTRING {
badAlg (0),
-- unrecognized or unsupported Algorithm Identifier
badRequest (2),
-- transaction not permitted or supported
badDataFormat (5),
-- the data submitted has the wrong format
timeNotAvailable (14),
-- the TSAs time source is not available
addInfoNotAvailable (15)
-- the additional information requested could not be understood
or is not available
}
These are the only values of PKIFailureInfo that are supported.
Compliant servers MUST NOT produce any other values.
Compliant clients MAY ignore any other values.
The statusString field of PKIStatusInfo MAY be used to include reason
text such as ½messageImprint field is not correctly formatted©.
If the error code returned is different from zero, then the
TimeStampToken is not returned.
A TimeStampToken is as follows. It is encapsulated as a SignedData
construct [CMS] in the EncapsulatedContentInfo field. The signed-data
content type from [CMS] shall have ASN.1 type SignedData.
SignedData ::= SEQUENCE {
version CMSVersion,
digestAlgorithms DigestAlgorithmIdentifiers,
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encapContentInfo EncapsulatedContentInfo,
certificates [0] IMPLICIT CertificateSet OPTIONAL,
crls [1] IMPLICIT
CertificateRevocationLists OPTIONAL,
signerInfos SignerInfos }
SignerInfos ::= SET OF SignerInfo
EncapsulatedContentInfo ::= SEQUENCE {
eContentType ContentType,
eContent [0] EXPLICIT OCTET STRING OPTIONAL }
ContentType ::= OBJECT IDENTIFIER
The fields of type EncapsulatedContentInfo have the following meanings:
eContentType is an object identifier that uniquely specifies the content
type. For a time stamping token it is defined as:
id-ct-TSTInfo OBJECT IDENTIFIER ::= {id-ct 4}
with:
id-ct OBJECT IDENTIFIER ::= { id-smime 1 }
id-smime OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 16 }
eContent is the content itself, carried as an octet string. The eContent
content type shall have ASN.1 type TSTInfo.
The time stamp token MUST NOT contain any signatures other than the
signature of the TSA. The certificate identifier of the TSA
certificate shall be included as a signed attribute.
TSTInfo ::= SEQUENCE {
version Integer { v1(1) },
policy PolicyInformation,
messageImprint MessageImprint,
-- MUST have the same value as the similar field in
-- TimeStampReq
serialNumber Integer,
genTime GeneralizedTime,
accuracy [0] Accuracy OPTIONAL,
nonce [1] Integer OPTIONAL,
-- MUST be present if the similar field was present
-- in TimeStampReq. In that case it must have the same value.
tsa [2] GeneralName OPTIONAL,
extensions [3] EXPLICIT Extensions OPTIONAL
}
The version field (currently v1) describes the version of the
Timestamp token.
Conforming timestamping servers MUST be able to provide version 1
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Timestamp tokens. Among the optional fileds, only the nonce field
needs to be supported, if the similar field is present in
TimeStampReq.
Conforming timestamping requesters MUST be able to recognize
version 1 Timestamp tokens with all the optional fields present,
but are not mandated to understand the semantics of any extension,
if present.
The policy field MUST indicate the TSAs policy under which the response was
produced. If a similar field was present in the TimeStampReq, then it MUST
have the same value, otherwise an error (badRequest) MUST be returned. This
policy MAY include the following types of information (although this list
is certainly not exhaustive):
* The conditions under which the time stamp may be used.
* The availability of a time-stamp log, to allow later verification
that a time-stamp token is authentic.
The messageImprint must have the same value as the similar field in
TimeStampReq, provided that the size of the hash value matches the expected
size of the hash algoritm identified in hashAlgorithm.
The serialNumber field shall include a strictly monotonically
increasing integer from one TimeStampToken to the next (e.g. 45, 236,
245, 1023, ...). This guarantees that each token is unique and allows
to compare the ordering of two time stamps from the same TSA. This is
useful in particular when two times stamps from the same TSA bear the
same time. This field also provides the way to build a unique
identifier to reference the token. It should be noticed that the
monotonic property must remain valid even after a possible
interruption (e.g. crash) of the service.
genTime is the time at which the timestamp has been created by the
TSA. The ASN.1 GeneralizedTime syntax can include fraction-of-second
details. Such syntax, without the restrictions from [RFC 2459 ]
Section 4.1.2.5.2., where GeneralizedTime is limited to represent
time with one second, may to be used here. However, when there is no
need to have a precison better than the second, then GeneralizedTime
with a precision limited to one second should be used (as in
[RFC 2459 ]).
The syntax is: YYYYMMDDhhmmss[.s...]Z
Example: 19990609001326.34352Z
X.690 | ISO/IEC 8825-1 provides the restrictions for a DER-encoding.
The encoding shall terminate with a "Z". The decimal point element,
if present, shall be the point option ".". The fractional-seconds
elements, if present, shall omit all trailing 0's; if the elements
correspond to 0, they shall be wholly omitted, and the decimal point
element also shall be omitted.
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Midnight (GMT) shall be represented in the form: "YYYYMMDD000000Z"
where "YYYYMMDD" represents the day following the midnight in question.
Here are a few examples of valid representations:
"19920521000000Z"
"19920622123421Z"
"19920722132100.3Z"
accuracy represents the time deviation around the UTC time contained in
GeneralizedTime.
Accuracy ::= CHOICE {
seconds [1] INTEGER,
millis [2] INTEGER (1..999),
micros [3] INTEGER (1..999)
}
By adding the accuracy value to the GeneralizedTime, an upper limit
of the time at which the timestamp has been created by the TSA can
be obtained. In the same way, by substracting the accuracy to the
GeneralizedTime, a lower limit of the time at which the timestamp
has been created by the TSA can be obtained.
accuracy is expressed as an integer, either in seconds, milliseconds
(between 1-999) or microseconds (1-999).
When the accuracy field, which is optional, is missing, then, by
default, an accuracy of one second is meant.
The nonce field MUST be present if it was present in the TimeStampReq.
The purpose of the tsa field is to give an hint in identifying the
name of the TSA. If present, it MUST correspond to one of the subject
names included in the certificate that is to be used to verify the
token. However, the actual identification of the entity which signed
the response will always occur through the use of the certificate
identifier (ESSCertID Attribute) which is part of the signerInfo
(See Section 5 of [ESS]).
extensions is a generic way to add additional information in the
future. EXTENSIONS is defined in [RFC 2459]. However, the version 1
only supports non-critical extensions. This means that conforming
requesters are not mandated to understand the semantics of any
extension. Particular extension field types may be specified in
standards or may be defined and registered by any organization or
community.
3. Transports
There is no mandatory transport mechanism for TSA messages in this
document. All mechanisms described below are optional.
3.1. File Based Protocol
A file containing a time stamp message MUST contain only the DER
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encoding of one TSA message, i.e. there MUST be no extraneous header or
trailer information in the file. Such files can be used to transport
time stamp messages using for example, FTP.
3.2. Socket Based Protocol
The following simple TCP-based protocol is to be used for transport
of TSA messages. This protocol is suitable for cases where an
entity initiates a transaction and can poll to pick up the results.
The protocol basically assumes a listener process on a TSA which
can accept TSA messages on a well-defined port (IP port number 318).
Typically an initiator binds to this port and submits the initial
TSA message. The responder replies with a TSA message and/or with
a reference number to be used later when polling for the actual TSA
message response.
If a number of TSA response messages are to be produced for a given
request (say if a receipt must be sent before the actual token can be
produced) then a new polling reference is also returned.
When the final TSA response message has been picked up by the
initiator then no new polling reference is supplied.
The initiator of a transaction sends a "direct TCP-based TSA message"
to the recipient. The recipient responds with a similar message.
A "direct TCP-based TSA message" consists of:
length (32-bits), flag (8-bits), value (defined below)
The length field contains the number of octets of the remainder of
the message (i.e., number of octets of "value" plus one). All 32-bit
values in this protocol are specified to be in network byte order.
Message name flag value
tsaMsg '00'H DER-encoded TSA message
-- TSA message
pollRep '01'H polling reference (32 bits),
time-to-check-back (32 bits)
-- poll response where no TSA message response ready; use polling
-- reference value (and estimated time value) for later polling
pollReq '02'H polling reference (32 bits)
-- request for a TSA message response to initial message
negPollRep '03'H '00'H
-- no further polling responses (i.e., transaction complete)
partialMsgRep '04'H next polling reference (32 bits),
time-to-check-back (32 bits),
DER-encoded TSA message
-- partial response (receipt) to initial message plus new polling
-- reference (and estimated time value) to use to get next part of
-- response
finalMsgRep '05'H DER-encoded TSA message
-- final (and possibly sole) response to initial message
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errorMsgRep '06'H human readable error message
-- produced when an error is detected (e.g., a polling reference
-- is received which doesn't exist or is finished with)
The sequence of messages which can occur is:
a) entity sends tsaMsg and receives one of pollRep, negPollRep,
partialMsgRep or finalMsgRep in response.
b) end entity sends pollReq message and receives one of negPollRep,
partialMsgRep,finalMsgRep or errorMsgRep in response.
The "time-to-check-back" parameter is a 32-bit integer, defined to be
the number of seconds which have elapsed since midnight, January 1,
1970, co-ordinated universal time.
It provides an estimate of the time that the end entity should send
its next pollReq.
3.3. Time Stamp Protocol Using E-mail
This section specifies a means for conveying ASN.1-encoded messages
for the protocol exchanges described in Section 2 and Appendix D via
Internet mail.
A simple MIME object is specified as follows:
Content-Type: application/timestamp
Content-Transfer-Encoding: base64
<<the ASN.1 DER-encoded Time Stamp message, base64-encoded>>
This MIME object can be sent and received using common MIME processing
engines and provides a simple Internet mail transport for Time Stamp
messages.
3.4. Time Stamp Protocol via HTTP
This subsection specifies a means for conveying ASN.1-encoded messages
for the protocol exchanges described in Section 2 and Appendix D via the
HyperText Transfer Protocol.
A simple MIME object is specified as follows.
Content-Type: application/timestamp
<<the ASN.1 DER-encoded Time Stamp message>>
This MIME object can be sent and received using common HTTP processing
engines over WWW links and provides a simple browser-server transport
for Time Stamp messages.
Upon receiving a valid request, the server MUST respond with either a valid
response with content type application/timestamp or with an HTTP error.
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4. Security Considerations
This entire document concerns security considerations.
When designing a TSA service, the following considerations have been
identified that have an impact upon the validity or ½trust© in the time
stamp token.
1. When there is a reason to both believe that the TSA can no
longer be trusted but the TSA private key has not been
compromised, the authorityÆs certificate SHALL be revoked.
Thus, at any future time, the tokens signed with the
corresponding key will not considered as valid.
2. When the TSA private key has been compromised, then the
corresponding certificate SHALL be revoked. In this case,
any token signed by the TSA using that private key cannot
be trusted anymore. For this reason, it is imperative that
the TSAÆs private key be guarded with proper security and
controls in order to minimize the possibility of compromise.
In case the private key does become compromised, an audit
trail of all tokens generated by the TSA MAY provide a means
to discriminate between genuine and false backdated tokens.
A double time stamp for two different TSAs is another way to
address this issue.
3. The TSA signing key MUST be of a sufficient length to allow
for a sufficiently long lifetime. Even if this is done, the key
will have a finite lifetime. Thus, any token signed by the
TSA SHOULD be time stamped again (if authentic copies of old
CRLs are available) or notarized (if they arenÆt) at a later
date to renew the trust that exists in the TSAÆs signature.
Time stamp tokens could also be kept with an Evidence Recording
Authority to maintain this trust.
4. An application using the TSA service SHOULD be concerned
about the amount of time it is willing to wait for a response.
A `man-in-the-middleÆ attack can introduce delays. Thus, any
TimeStampToken that takes more than an acceptable period of time
SHOULD be considered suspect.
5. Intellectual Property Rights
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to per-
tain to the implementation or use of the technology described in this
document or the extent to which any license under such rights might
or might not be available; neither does it represent that it has made
any effort to identify any such rights. Information on the IETF's
procedures with respect to rights in standards-track and standards-
related documentation can be found in BCP-11. Copies of claims of
rights made available for publication and any assurances of licenses
to be made available, or the result of an attempt made to obtain a
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general license or permission for the use of such proprietary rights
by implementors or users of this specification can be obtained from
the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
The following eight (8) United States Patents related to time
stamping, listed in chronological order, are known by the authors
to exist at this time. This may not be an exhaustive list. Other
patents MAY exist or be issued at any time.
Implementers of this protocol SHOULD perform their own patent search
and determine whether or not any encumbrances exist on their
implementation.
Users of this protocol SHOULD perform their own patent search
and determine whether or not any encumbrances exist on the use of
this standard.
# 5,001,752 Public/Key Date-Time Notary Facility
Filing date: October 13, 1989
Issued: March 19, 1991
Inventor: Addison M. Fischer
# 5,022,080 Electronic Notary
Filing date: April 16, 1989
Issued: June 4, 1991
Inventors: Robert T. Durst, Kevin D. Hunter
# 5,136,643 Public/Key Date-Time Notary Facility
Filing date: December 20, 1990
Issued: August 4, 1992
Inventor: Addison M. Fischer
Note: This is a continuation of patent # 5,001,752.)
# 5,136,646 Digital Document Time-Stamping with Catenate Certificate
Filing date: August 2, 1990
Issued: August 4, 1992
Inventors: Stuart A. Haber, Wakefield S. Stornetta Jr.
(assignee) Bell Communications Research, Inc.,
# 5,136,647 Method for Secure Time-Stamping of Digital Documents
Filing date: August 2, 1990
Issued: August 4, 1992
Inventors: Stuart A. Haber, Wakefield S. Stornetta Jr.
(assignee) Bell Communications Research, Inc.,
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# 5,373,561 Method of Extending the Validity of a Cryptographic
Certificate
Filing date: December 21, 1992
Issued: December 13, 1994
Inventors: Stuart A. Haber, Wakefield S. Stornetta Jr.
(assignee) Bell Communications Research, Inc.,
# 5,422,953 Personal Date/Time Notary Device
Filing date: May 5, 1993
Issued: June 6, 1995
Inventor: Addison M. Fischer
# 5,781,629 Digital Document Authentication System
Filing date: February 21, 1997
Issued: July 14, 1998
Inventor: Stuart A. Haber, Wakefield S. Stornetta Jr.
(assignee) Surety Technologies, Inc.,
6. References
[RFC2510] C. Adams, S. Farrell, ½Internet X.509 Public Key Infrastructure,
Certificate Management Protocols,© RFC 2510, March 1999.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] T. Dierks, C. Allen, ½The TLS Protocol, Version 1.0,© RFC 2246,
January 1999.
[ESS] P. Hoffman, ½Enhanced Security Services for S/MIME©, draft-ietf-
smime-ess-0X.txt, 1999 (work in progress).
[CMS] R. Housley, ½Cryptographic Message Syntax©, RFC 2630, June 1999.
[RFC2459] R. Housley, W. Ford, W. Polk, D. Solo, ½Internet X.509 Public Key
Infrastructure, Certificate and CRL Profile,© RFC 2459, January 1999.
[PKCS9] RSA Laboratories, ½The Public-Key Cryptography Standards
(PKCS)©, RSA Data Security Inc., Redwood City, California, November
1993 Release.
[ISONR] ISO/IEC 10181-5: Security Frameworks in Open Systems.
Non-Repudiation Framework. April 1997.
7. AuthorsÆ Addresses
Carlisle Adams Pat Cain
Entrust Technologies BBN
750 Heron Road 70 Fawcett Street
Ottawa, Ontario Cambridge, MA 02138
K1V 1A7 U.S.A.
CANADA pcain@bbn.com
cadams@entrust.com
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Denis Pinkas Robert Zuccherato
Bull S.A. Entrust Technologies
Rue Jean Jaures 750 Heron Road
B.P. 68 Ottawa, Ontario
78340 Les Clayes sous Bois K1V 1A7
FRANCE CANADA
Denis.Pinkas@bull.net robert.zuccherato@entrust.com
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APPENDIX A - Signature Timestamp attribute using CMS
One of the major use of time stamping is to time stamp a digital
signature to prove that the digital signature was created before
a given time. Should the corresponding public key certificate be
revoked this allows to know whether the signature was created before
or after the revocation date.
A sensible place to store a time stamp is in a [CMS] structure is
as an unsigned atribute.
This appendix defines a Signature Timestamp attribute that may be used to
timestamp a digital signature.
The following object identifier identifies the Signature Timestamp
attribute:
id-signatureTimeStampToken OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) id-aa(2) <TBD>}
The Signature timestamp attribute value has ASN.1 type
SignatureTimeStampToken:
SignatureTimeStampToken ::= TimeStampToken
The value of messageImprint field within TimeStampToken shall be a hash of
the value of signature field within SignerInfo for the signedData being
timestamped.
Adams, Cain, Pinkas, Zuccherato Page 15
Internet Draft Time Stamp Protocols (TSP)
APPENDIX B - Placing a Signature At a Particular Point in Time
We present an example of a possible use of this general time stamping
service. It places a signature at a particular point in time, from
which the appropriate certificate status information (e.g. CRLs) MUST be
checked. This application is intended to be used in conjunction with
evidence generated using a digital signature mechanism.
Signatures can only be verified according to a non-repudiation policy.
This policy MAY be implicit or explicit (i.e., indicated in the
evidence provided by the signer). The non-repudiation policy can
specify, among other things, the time period allowed by a signer to
declare the compromise of a signature key used for the generation of
digital signatures. Thus a signature may not be guaranteed to be valid
until the termination of this time period.
To verify a digital signature, the following basic technique may be used:
A) Time stamping information needs to be obtained soon after the signature
has been produced (e.g. within a few minutes or hours).
1) The signature is presented to the Time Stamping Authority (TSA).
The TSA then returns a TimeStampToken (TST) upon that signature.
2) The invoker of the service must then verify that the
TimeStampToken is correct.
B) The validity of the digital signature may then be verified in the
following way:
1) the Time stamp itself must be verified and it must be verified
that it applies to the signature of the signer.
2) The date/time indicated by the TSA in the Time Stamping Token
must be retrieved.
3) The certificate used by the signer must be identified
and retrieved.
4) The date/time indicated by the TSA must be inside the validity
period of the signer's certificate.
5) The revocation information about that certificate, at the
date/time of the Time Stamping operation, must be retrieved.
6) Should the certificate be revoked, then the date/time of
revocation shall be later than the date/time indicated by
the TSA
If all these conditions are successful, then the digital signature
shall be declared as valid.
Adams, Cain, Pinkas, Zuccherato Page 16
Internet Draft Time Stamp Protocols (TSP)
Appendix C û MIME Registration
To: ietf-types@iana.org
Subject: Registration of MIME media type application/timestamp
MIME media type name: application
MIME subtype name: timestamp
Required parameters: None
Optional parameters: None
Encoding considerations: binary or Base64
Security considerations: Carries a request for a timestamp and the
response. The response will be cryptographically signed.
Interoperability considerations: None
Published specification: IETF PKIX Working Group Draft on Time Stamp
Protocols
Applications which use this media type: Time Stamp clients
Additional information:
Magic number(s): None
File extension(s): .TSA
Macintosh File Type Code(s): none
Person & email address to contact for further information:
Robert Zuccherato <robert.zuccherato@entrust.com>
Intended usage: COMMON
Author/Change controller:
Robert Zuccherato <robert.zuccherato@entrust.com>
Adams, Cain, Pinkas, Zuccherato Page 17
Internet Draft Time Stamp Protocols (TSP)
Appendix D - Full Copyright Statement
Copyright (C) The Internet Society 1999. 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
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 develop-
ing Internet standards in which case the procedures for copyrights
defined in the Internet Standards process shall be followed, or as
required to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns. This
document and the information contained herein is provided on an "AS
IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK
FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL
NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY
OR FITNESS FOR A PARTICULAR PURPOSE.
Adams, Cain, Pinkas, Zuccherato Page 18
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