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 © 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.

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be used, for example, to verify that a digital signature was applied to a message before the corresponding certificate was revoked thus allowing a revoked public key certificate to be used for verifying signatures created prior to the time of revocation.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).

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