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Keywords: [TSA|p], authority, security, request, response
Network Working Group C. Adams
Request for Comments: 3161 Entrust
Category: Standards Track P. Cain
BBN
D. Pinkas
Integris
R. Zuccherato
Entrust
August 2001
Internet X.509 Public Key Infrastructure
Time-Stamp Protocol (TSP)
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.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes the format of a request sent to a Time
Stamping Authority (TSA) and of the response that is returned. It
also establishes several security-relevant requirements for TSA
operation, with regards to processing requests to generate responses.
1. Introduction
A time-stamping service supports assertions of proof that a datum
existed before a particular time. A TSA may be operated as a Trusted
Third Party (TTP) service, though other operational models may be
appropriate, e.g., an organization might require a TSA for internal
time-stamping purposes.
Non-repudiation services [ISONR] require the ability to establish the
existence of data before specified times. This protocol may be used
as a building block to support such services. An example of how to
prove that a digital signature was generated during the validity
period of a public key certificate is given in an annex.
Adams, et al. Standards Track [Page 1]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"SHALL", "RECOMMENDED", "MAY", and "OPTIONAL" in this document (in
uppercase, as shown) are to be interpreted as described in [RFC2119].
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 "proof-of-existence" for this particular datum at an
instant in time.
The TSA's role is to time-stamp a datum to establish evidence
indicating that a datum existed before a particular time. 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applied to a message before the corresponding certificate was revoked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.
This standard does not establish overall security requirements for
TSA operation, just like other PKIX standards do not establish such
requirements for CA operation. Rather, it is anticipated that a TSA
will make known to prospective clients the policies it implements to
ensure accurate time-stamp generation, and clients will make use of
the services of a TSA only if they are satisfied that these policies
meet their needs.
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 "valid request" 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 use a trustworthy source of time.
2. to include a trustworthy time value for each time-stamp token.
3. to include a unique integer for each newly generated time-stamp
token.
Adams, et al. Standards Track [Page 2]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
4. to produce a time-stamp token upon receiving a valid request
from the requester, when it is possible.
5. to include within each time-stamp token an identifier to
uniquely indicate the security policy under which the token was
created.
6. to only time-stamp a hash representation of the datum, i.e., a
data imprint associated with a one-way collision resistant
hash-function uniquely identified by an OID.
7. to examine the OID of the one-way collision resistant hash-
function and to verify that the hash value length is consistent
with the hash algorithm.
8. not to examine the imprint being time-stamped in any way (other
than to check its length, as specified in the previous bullet).
9. not to include any identification of the requesting entity in
the time-stamp tokens.
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 TimeStampResp,
as defined below) to the requesting entity.
Upon receiving the response (which is or includes a TimeStampResp
that normally contains a TimeStampToken (TST), as defined below), the
requesting entity SHALL verify the status error returned in the
response and if no error is present it SHALL verify the various
fields contained in the TimeStampToken and the validity of the
digital signature of the TimeStampToken. In particular, it SHALL
verify 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
Adams, et al. Standards Track [Page 3]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
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, 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. For more details about replay attack detection, see
the security considerations section (item 6). If any of the
verifications above fails, the TimeStampToken SHALL be rejected.
Then, since the TSA's 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.
Then, 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.
2.3. Identification of the TSA
The TSA MUST sign each time-stamp message with a key reserved
specifically for that purpose. A TSA MAY have distinct private keys,
e.g., to accommodate different policies, different algorithms,
different private key sizes or to increase the performance. 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.
The following object identifier identifies the KeyPurposeID having
value id-kp-timeStamping.
id-kp-timeStamping OBJECT IDENTIFIER ::= {iso(1)
identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
kp (3) timestamping (8)}
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
Adams, et al. Standards Track [Page 4]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
--time-stamped
reqPolicy TSAPolicyId OPTIONAL,
nonce INTEGER OPTIONAL,
certReq BOOLEAN DEFAULT FALSE,
extensions [0] IMPLICIT Extensions OPTIONAL }
The version field (currently v1) describes the version of the Time-
Stamp request.
The messageImprint field SHOULD 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 SHOULD be a
known hash algorithm (one-way and collision resistant). That means
that it SHOULD be one-way and collision resistant. The Time Stamp
Authority SHOULD check whether or not the given hash algorithm is
known to be "sufficient" (based on the current state of knowledge in
cryptanalysis and the current state of the art in computational
resources, for example). If the TSA does not recognize the hash
algorithm or knows that the hash algorithm is weak (a decision left
to the discretion of each individual TSA), then the TSA SHOULD refuse
to provide the time-stamp token by returning a pkiStatusInfo of
'bad_alg'.
The reqPolicy field, if included, indicates the TSA policy under
which the TimeStampToken SHOULD be provided. TSAPolicyId is defined
as follows:
TSAPolicyId ::= OBJECT IDENTIFIER
The nonce, if included, allows the client 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 the client generates it
only once (e.g., a 64 bit integer). In such a case the same nonce
value MUST be included in the response, otherwise the response shall
be rejected.
If the certReq field is present and set to true, the TSA's public key
certificate that is referenced by the ESSCertID identifier inside a
SigningCertificate attribute in the response MUST be provided by the
TSA in the certificates field from the SignedData structure in that
response. That field may also contain other certificates.
Adams, et al. Standards Track [Page 5]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
If the certReq field is missing or if the certReq field is present
and set to false then the certificates field from the SignedData
structure MUST not be present in the response.
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 SHALL return a failure
(unacceptedExtension).
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 based on the definition of status in section 3.2.3
of [RFC2510] as follows:
PKIStatusInfo ::= SEQUENCE {
status PKIStatus,
statusString PKIFreeText OPTIONAL,
failInfo PKIFailureInfo OPTIONAL }
When the status contains the value zero or one, a TimeStampToken MUST
be present. When status contains a value other than zero or one, a
TimeStampToken MUST NOT be present. One of the following values MUST
be contained in status:
PKIStatus ::= INTEGER {
granted (0),
-- when the PKIStatus contains the value zero a TimeStampToken, as
requested, is present.
grantedWithMods (1),
-- when the PKIStatus contains the value one a TimeStampToken,
with modifications, is present.
rejection (2),
waiting (3),
revocationWarning (4),
Adams, et al. Standards Track [Page 6]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
-- this message contains a warning that a revocation is
-- imminent
revocationNotification (5)
-- notification that a revocation has occurred }
Compliant servers SHOULD NOT produce any other values. Compliant
clients MUST generate an error if values it does not understand are
present.
When the TimeStampToken is not present, the failInfo indicates the
reason why the time-stamp request was rejected and may be one of the
following values.
PKIFailureInfo ::= BIT STRING {
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 TSA's time source is not available
unacceptedPolicy (15),
-- the requested TSA policy is not supported by the TSA
unacceptedExtension (16),
-- the requested extension is not supported by the TSA
addInfoNotAvailable (17)
-- the additional information requested could not be understood
-- or is not available
systemFailure (25)
-- the request cannot be handled due to system failure }
These are the only values of PKIFailureInfo that SHALL be supported.
Compliant servers SHOULD NOT produce any other values. Compliant
clients MUST generate an error if values it does not understand are
present.
The statusString field of PKIStatusInfo MAY be used to include reason
text such as "messageImprint field is not correctly formatted".
A TimeStampToken is as follows. It is defined as a ContentInfo
([CMS]) and SHALL encapsulate a signed data content type.
TimeStampToken ::= ContentInfo
-- contentType is id-signedData ([CMS])
-- content is SignedData ([CMS])
Adams, et al. Standards Track [Page 7]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
The fields of type EncapsulatedContentInfo of the SignedData
construct have the following meanings:
eContentType is an object identifier that uniquely specifies the
content type. For a time-stamp token it is defined as:
id-ct-TSTInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 4}
eContent is the content itself, carried as an octet string.
The eContent SHALL be the DER-encoded value of TSTInfo.
The time-stamp token MUST NOT contain any signatures other than the
signature of the TSA. The certificate identifier (ESSCertID) of the
TSA certificate MUST be included as a signerInfo attribute inside a
SigningCertificate attribute.
TSTInfo ::= SEQUENCE {
version INTEGER { v1(1) },
policy TSAPolicyId,
messageImprint MessageImprint,
-- MUST have the same value as the similar field in
-- TimeStampReq
serialNumber INTEGER,
-- Time-Stamping users MUST be ready to accommodate integers
-- up to 160 bits.
genTime GeneralizedTime,
accuracy Accuracy OPTIONAL,
ordering BOOLEAN DEFAULT FALSE,
nonce INTEGER OPTIONAL,
-- MUST be present if the similar field was present
-- in TimeStampReq. In that case it MUST have the same value.
tsa [0] GeneralName OPTIONAL,
extensions [1] IMPLICIT Extensions OPTIONAL }
The version field (currently v1) describes the version of the time-
stamp token.
Conforming time-stamping servers MUST be able to provide version 1
time-stamp tokens.
Among the optional fields, only the nonce field MUST be supported.
Conforming time-stamping requesters MUST be able to recognize version
1 time-stamp tokens with all the optional fields present, but are not
mandated to understand the semantics of any extension, if present.
Adams, et al. Standards Track [Page 8]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
The policy field MUST indicate the TSA's 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
(unacceptedPolicy) 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 token may be used.
* The availability of a time-stamp token 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 algorithm identified in hashAlgorithm.
The serialNumber field is an integer assigned by the TSA to each
TimeStampToken. It MUST be unique for each TimeStampToken issued by
a given TSA (i.e., the TSA name and serial number identify a unique
TimeStampToken). It should be noticed that the property MUST be
preserved even after a possible interruption (e.g., crash) of the
service.
genTime is the time at which the time-stamp token has been created by
the TSA. It is expressed as UTC time (Coordinated Universal Time) to
reduce confusion with the local time zone use. UTC is a time scale,
based on the second (SI), as defined and recommended by the CCIR, and
maintained by the Bureau International des Poids et Mesures (BIPM). A
synonym is "Zulu" time which is used by the civil aviation and
represented by the letter "Z" (phonetically "Zulu").
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 the
time with a granularity of one second, may be used here.
GeneralizedTime values MUST include seconds. However, when there is
no need to have a precision 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 following restrictions for a
DER-encoding.
Adams, et al. Standards Track [Page 9]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
The encoding MUST terminate with a "Z" (which means "Zulu" time). The
decimal point element, if present, MUST be the point option ".". The
fractional-seconds elements, if present, MUST omit all trailing 0's;
if the elements correspond to 0, they MUST be wholly omitted, and the
decimal point element also MUST be omitted.
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 ::= SEQUENCE {
seconds INTEGER OPTIONAL,
millis [0] INTEGER (1..999) OPTIONAL,
micros [1] INTEGER (1..999) OPTIONAL }
If either seconds, millis or micros is missing, then a value of zero
MUST be taken for the missing field.
By adding the accuracy value to the GeneralizedTime, an upper limit
of the time at which the time-stamp token has been created by the TSA
can be obtained. In the same way, by subtracting the accuracy to the
GeneralizedTime, a lower limit of the time at which the time-stamp
token has been created by the TSA can be obtained.
accuracy can be decomposed in seconds, milliseconds (between 1-999)
and microseconds (1-999), all expressed as integer.
When the accuracy optional field is not present, then the accuracy
may be available through other means, e.g., the TSAPolicyId.
If the ordering field is missing, or if the ordering field is present
and set to false, then the genTime field only indicates the time at
which the time-stamp token has been created by the TSA. In such a
case, the ordering of time-stamp tokens issued by the same TSA or
different TSAs is only possible when the difference between the
genTime of the first time-stamp token and the genTime of the second
time-stamp token is greater than the sum of the accuracies of the
genTime for each time-stamp token.
Adams, et al. Standards Track [Page 10]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
If the ordering field is present and set to true, every time-stamp
token from the same TSA can always be ordered based on the genTime
field, regardless of the genTime accuracy.
The nonce field MUST be present if it was present in the
TimeStampReq. In such a case it MUST equal the value provided in the
TimeStampReq structure.
The purpose of the tsa field is to give a 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
that signed the response will always occur through the use of the
certificate identifier (ESSCertID Attribute) inside a
SigningCertificate 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].
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. The mechanisms described below are optional; additional
optional mechanisms may be defined in the future.
3.1. 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.
Two MIME objects are specified as follows:
Content-Type: application/timestamp-query
Content-Transfer-Encoding: base64
<<the ASN.1 DER-encoded Time-Stamp message, base64-encoded>>
Content-Type: application/timestamp-reply
Content-Transfer-Encoding: base64
<<the ASN.1 DER-encoded Time-Stamp message, base64-encoded>>
These MIME objects can be respectively sent and received using common
MIME processing engines and provides a simple Internet mail transport
for Time-Stamp messages.
Adams, et al. Standards Track [Page 11]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
For the application/timestamp-query and application/timestamp-reply
MIME types, implementations SHOULD include the optional "name" and
"filename" parameters. Including a file name helps preserve type
information when time-stamp queries and replies are saved as files.
When these parameters are included, a file name with the appropriate
extension SHOULD be selected:
MIME Type File Extension
application/timestamp-query .TSQ
application/timestamp-reply .TSR
In addition, the file name SHOULD be limited to eight characters
followed by a three letter extension. The eight character filename
base can be any distinct name.
3.2. File Based Protocol
A file containing a time-stamp message MUST contain only the DER
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.
A Time-Stamp Request SHOULD be contained in a file with file
extension .tsq (like Time-Stamp Query). A Time-Stamp Response
SHOULD be contained in a file with file extension .tsr (like
Time-Stamp Reply).
3.3. 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 that 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.
Adams, et al. Standards Track [Page 12]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
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
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 that 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 an unsigned 32-bit integer. It
is the time in seconds indicating the minimum interval after which
the client SHOULD check the status again.
It provides an estimate of the time that the end entity should send
its next pollReq.
Adams, et al. Standards Track [Page 13]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
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.
Two MIME objects are specified as follows.
Content-Type: application/timestamp-query
<<the ASN.1 DER-encoded Time-Stamp Request message>>
Content-Type: application/timestamp-reply
<<the ASN.1 DER-encoded Time-Stamp Response message>>
These MIME objects 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-response or
with an HTTP error.
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 "trust" in the
time-stamp token.
1. When a TSA shall not be used anymore, but the TSA private key has
not been compromised, the authority's certificate SHALL be
revoked. When the reasonCode extension relative to the revoked
certificate from the TSA is present in the CRL entry extensions,
it SHALL be set either to unspecified (0), affiliationChanged (3),
superseded (4) or cessationOfOperation (5). In that case, at any
future time, the tokens signed with the corresponding key will be
considered as invalid, but tokens generated before the revocation
time will remain valid. When the reasonCode extension relative to
the revoked certificate from the TSA is not present in the CRL
entry extensions, then all the tokens that have been signed with
the corresponding key SHALL be considered as invalid. For that
reason, it is recommended to use the reasonCode extension.
Adams, et al. Standards Track [Page 14]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
2. When the TSA private key has been compromised, then the
corresponding certificate SHALL be revoked. In that case, the
reasonCode extension relative to the revoked certificate from the
TSA may or may not be present in the CRL entry extensions. When
it is present then it SHALL be set to keyCompromise (1). Any
token signed by the TSA using that private key cannot be trusted
anymore. For this reason, it is imperative that the TSA's 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. Two time-stamp tokens from 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 aren't) at a later date to renew
the trust that exists in the TSA's signature. time-stamp tokens
could also be kept with an Evidence Recording Authority to
maintain this trust.
4. A client application using only a nonce and no local clock SHOULD
be concerned about the amount of time it is willing to wait for a
response. A `man-in-the-middle' attack can introduce delays.
Thus, any TimeStampResp that takes more than an acceptable period
of time SHOULD be considered suspect. Since each transport method
specified in this document has different delay characteristics,
the period of time that is considered acceptable will depend upon
the particular transport method used, as well as other environment
factors.
5. If different entities obtain time-stamp tokens on the same data
object using the same hash algorithm, or a single entity obtains
multiple time-stamp tokens on the same object, the generated
time-stamp tokens will include identical message imprints; as a
result, an observer with access to those time-stamp tokens could
infer that the time-stamps may refer to the same underlying data.
Adams, et al. Standards Track [Page 15]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
6. Inadvertent or deliberate replays for requests incorporating the
same hash algorithm and value may happen. Inadvertent replays
occur when more than one copy of the same request message gets
sent to the TSA because of problems in the intervening network
elements. Deliberate replays occur when a middleman is replaying
legitimate TS responses. In order to detect these situations,
several techniques may be used. Using a nonce always allows to
detect replays, and hence its use is RECOMMENDED. Another
possibility is to use both a local clock and a moving time window
during which the requester remembers all the hashes sent during
that time window. When receiving a response, the requester
ensures both that the time of the response is within the time
window and that there is only one occurrence of the hash value in
that time window. If the same hash value is present more than
once within a time window, the requester may either use a nonce,
or wait until the time window has moved to come back to the case
where the same hash value appears only once during that time
window.
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
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. This list is provided
for informational purposes; to date, the IETF has not been notified
of intellectual property rights claimed in regard to any of the
Adams, et al. Standards Track [Page 16]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
specification contained in this document. Should this situation
change, the current status may be found at the online list of claimed
rights (IETF Page of Intellectual Property Rights Notices).
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.,
# 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.,
Adams, et al. Standards Track [Page 17]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
# 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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol, Version 1.0",
RFC 2246, January 1999.
[RFC2510] Adams, C. and S. Farrell, "Internet X.509 Public Key
Infrastructure, Certificate Management Protocols", RFC
2510, March 1999.
[RFC2459] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
X.509 Public Key Infrastructure, Certificate and CRL
Profile", RFC 2459, January 1999.
[CMS] Housley, R., "Cryptographic Message Syntax", RFC 2630,
June 1999.
[DSS] Digital Signature Standard. FIPS Pub 186. National
Institute of Standards and Technology. 19 May 1994.
[ESS] Hoffman, P., "Enhanced Security Services for S/MIME", RFC
2634, June 1999.
[ISONR] ISO/IEC 10181-5: Security Frameworks in Open Systems.
Non-Repudiation Framework. April 1997.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[SHA1] Secure Hash Standard. FIPS Pub 180-1. National Institute
of Standards and Technology. 17 April 1995.
Adams, et al. Standards Track [Page 18]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
7. Authors' Addresses
Carlisle Adams
Entrust, Inc.
1000 Innovation Drive
Ottawa, Ontario
K2K 3E7
CANADA
EMail: cadams@entrust.com
Pat Cain
BBN
70 Fawcett Street
Cambridge, MA 02138
U.S.A.
EMail: pcain@bbn.com
Denis Pinkas
Integris
68 route de Versailles
B.P. 434
78430 Louveciennes
FRANCE
EMail: Denis.Pinkas@bull.net
Robert Zuccherato
Entrust, Inc.
1000 Innovation Drive
Ottawa, Ontario
K2K 3E7
CANADA
EMail: robert.zuccherato@entrust.com
Adams, et al. Standards Track [Page 19]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
APPENDIX A - Signature Time-stamp attribute using CMS
One of the major uses 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 a verifier 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 as an
unsigned attribute.
This appendix defines a Signature Time-stamp attribute that may be
used to time-stamp a digital signature.
The following object identifier identifies the Signature Time-stamp
attribute:
id-aa-timeStampToken OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) aa(2) 14 }
The Signature time-stamp 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 time-stamped.
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:
Adams, et al. Standards Track [Page 20]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
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 token 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 TimeStampToken
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 within 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.
APPENDIX C: ASN.1 Module using 1988 Syntax
PKIXTSP {iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0) id-mod-tsp(13)}
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
IMPORTS
Adams, et al. Standards Track [Page 21]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
Extensions, AlgorithmIdentifier
FROM PKIX1Explicit88 {iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7)
id-mod(0) id-pkix1-explicit-88(1)}
GeneralName FROM PKIX1Implicit88 {iso(1)
identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-pkix1-implicit-88(2)}
ContentInfo FROM CryptographicMessageSyntax {iso(1)
member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms(1)}
PKIFreeText FROM PKIXCMP {iso(1) identified-organization(3)
dod(6) internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-cmp(9)} ;
-- Locally defined OIDs --
-- eContentType for a time-stamp token
id-ct-TSTInfo OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) ct(1) 4}
-- 2.4.1
TimeStampReq ::= SEQUENCE {
version INTEGER { v1(1) },
messageImprint MessageImprint,
--a hash algorithm OID and the hash value of the data to be
--time-stamped
reqPolicy TSAPolicyId OPTIONAL,
nonce INTEGER OPTIONAL,
certReq BOOLEAN DEFAULT FALSE,
extensions [0] IMPLICIT Extensions OPTIONAL }
MessageImprint ::= SEQUENCE {
hashAlgorithm AlgorithmIdentifier,
hashedMessage OCTET STRING }
TSAPolicyId ::= OBJECT IDENTIFIER
-- 2.4.2
TimeStampResp ::= SEQUENCE {
status PKIStatusInfo,
timeStampToken TimeStampToken OPTIONAL }
Adams, et al. Standards Track [Page 22]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
-- The status is based on the definition of status
-- in section 3.2.3 of [RFC2510]
PKIStatusInfo ::= SEQUENCE {
status PKIStatus,
statusString PKIFreeText OPTIONAL,
failInfo PKIFailureInfo OPTIONAL }
PKIStatus ::= INTEGER {
granted (0),
-- when the PKIStatus contains the value zero a TimeStampToken, as
requested, is present.
grantedWithMods (1),
-- when the PKIStatus contains the value one a TimeStampToken,
with modifications, is present.
rejection (2),
waiting (3),
revocationWarning (4),
-- this message contains a warning that a revocation is
-- imminent
revocationNotification (5)
-- notification that a revocation has occurred }
-- When the TimeStampToken is not present
-- failInfo indicates the reason why the
-- time-stamp request was rejected and
-- may be one of the following values.
PKIFailureInfo ::= BIT STRING {
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 TSA's time source is not available
unacceptedPolicy (15),
-- the requested TSA policy is not supported by the TSA.
unacceptedExtension (16),
-- the requested extension is not supported by the TSA.
addInfoNotAvailable (17)
-- the additional information requested could not be understood
-- or is not available
systemFailure (25)
-- the request cannot be handled due to system failure }
TimeStampToken ::= ContentInfo
Adams, et al. Standards Track [Page 23]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
-- contentType is id-signedData as defined in [CMS]
-- content is SignedData as defined in([CMS])
-- eContentType within SignedData is id-ct-TSTInfo
-- eContent within SignedData is TSTInfo
TSTInfo ::= SEQUENCE {
version INTEGER { v1(1) },
policy TSAPolicyId,
messageImprint MessageImprint,
-- MUST have the same value as the similar field in
-- TimeStampReq
serialNumber INTEGER,
-- Time-Stamping users MUST be ready to accommodate integers
-- up to 160 bits.
genTime GeneralizedTime,
accuracy Accuracy OPTIONAL,
ordering BOOLEAN DEFAULT FALSE,
nonce INTEGER OPTIONAL,
-- MUST be present if the similar field was present
-- in TimeStampReq. In that case it MUST have the same value.
tsa [0] GeneralName OPTIONAL,
extensions [1] IMPLICIT Extensions OPTIONAL }
Accuracy ::= SEQUENCE {
seconds INTEGER OPTIONAL,
millis [0] INTEGER (1..999) OPTIONAL,
micros [1] INTEGER (1..999) OPTIONAL }
END
APPENDIX D: Access descriptors for Time-Stamping.
[This annex describes an extension based on the SIA extension that
will be defined in the "son-of-RFC2459". Since at the time of
publication of this document, "son-of-RFC2459" is not yet available,
its description is placed in an informative annex. The contents of
this annex will eventually become incorporated into the son-of-
RFC2459 document, at which time this annex will no longer be needed.
A future version of this document will likely omit this annex and
refer to son-of-RFC2459 directly.]
A TSA's certificate MAY contain a Subject Information Access (SIA)
extension (son of RFC2459) in order to convey the method of
contacting the TSA. The accessMethod field in this extension MUST
contain the OID id-ad-timestamping:
The following object identifier identifies the access descriptors for
time-Stamping.
Adams, et al. Standards Track [Page 24]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
id-ad-timeStamping OBJECT IDENTIFIER ::= {iso(1)
identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
ad (48) timestamping (3)}
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).
Adams, et al. Standards Track [Page 25]
RFC 3161 Time-Stamp Protocol (TSP) August 2001
Full Copyright Statement
Copyright (C) The Internet Society (2001). 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
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included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
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Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
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The limited permissions granted above are perpetual and will not be
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Adams, et al. Standards Track [Page 26]