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

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.

   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.

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

   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

     --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.

   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),

       -- 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])

   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.

   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.

   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.

   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.

   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.

   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.

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.

   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.

   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

   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.,

# 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.

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

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:

   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

     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  }

-- 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

     -- 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.

   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).

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.