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rfc:rfc3161

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

  1. -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

  1. - this message contains a warning that a revocation is
  2. - imminent

revocationNotification (5)

  1. - 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)} ;
  1. - 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   }
  1. - When the TimeStampToken is not present
  2. - failInfo indicates the reason why the
  3. - time-stamp request was rejected and
  4. - 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

  1. - contentType is id-signedData as defined in [CMS]
  2. - content is SignedData as defined in([CMS])
  3. - eContentType within SignedData is id-ct-TSTInfo
  4. - 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
 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
 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.
 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.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Adams, et al. Standards Track [Page 26]

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