C. Adams(Entrust Technologies) Internet Draft P. Cain (BBN) expires in six months D. Pinkas (Bull) R. Zuccherato(Entrust Technologies) November 7, 1997 Time Stamp Protocols Status of this Memo This document is an Internet-Draft. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract This document describes the format of the data returned by a Time Stamp Authority and the protocols to be used when communicating with it. The time stamping service can be used as a Trusted Third Party (TTP) as one component in building reliable non-repudiation services (see [ISONR]). In order to reduce the amount of trust required of a TSA we introduce the optional Temporal Data Authority (TDA) whose function is to provide further corroborating evidence of the time contained in the token. We also give an example of how to place a signature at a particular point in time, from which the appropriate certificate status information (e.g. CRLs) may be checked. 1. Introduction In order to associate a message 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 message at an instant in time. A TSA may also be used when a trusted time reference is required and when the local clock available cannot be trusted by all parties. The TSA's role is to time stamp a message to establish evidence indicating the time before which the message was generated. This can then be used, for example, to verify that a digital signature was applied before the certificate was revoked thus allowing a revoked public key certificate to be used for verifying signatures created prior to the time of revocation.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 Document Expiration: May 7, 1998 Page 1 infrastructure operation. The TSA can also be used to indicate the time of submission when a deadline is critical, or to indicate the time of transaction for entries in a log. An exhaustive list of possible uses of a TSA is beyond the scope of this document. 2. The TSA The TSA is a TTP that creates time stamp tokens in order to indicate that a message existed at a particular point in time. 2.1. Requirements of the TSA The TSA is required to: 1. verify only the time. The TSA does not examine or verify the data being time stamped or the requesting entities in any way. 2. include a monotonically incrementing value of the time of day into its time stamp token. 3. produce a time stamp token upon receiving a valid request from the requester. 4. include within each time stamp token an identifier to uniquely determine the trust and validation policy used for this signature. 5. only time stamp a hash representation of the message. 6. 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. 7. include supplementary temporal information in the time stamp token (from TDA's) if asked by the requester. 8. provide a signed receipt (i.e. in the form of an appropriately defined time stamp token) to the requester, where appropriate, as defined by policy. 2.2. TSA Transactions As the first message of this mechanism, the requesting entity requests a time stamping service by sending a request (which is or includes a TimeStampReq, as defined below) to the Time Stamping Authority. As the second message, the Time Stamping Authority responds by sending a response (which is or includes a TimeStampToken, as defined below) to the requesting entity. Upon receiving the token, the requesting entity verifies its validity by verifying the signature in the TimeStampToken and by verifying that what was time stamped corresponds to what was requested to be time stamped. The requester should verify that the TimeStampToken contains the correct time, the correct TSA name, the correct data imprint and the correct hash algorithm OID. Since the TSA's certificate may have been revoked, the status of the certificate should be checked (e.g. by checking the appropriate ARL) to verify that the certificate is still valid. If TemporalDataToken's are included in the TimeStampToken, then these should also be verified as was the TimeStampToken. The token can now be used to establish a trusted time reference. Document Expiration: May 7, 1998 Page 2 2.3. Identification of the TSA The TSA must sign all time stamp messages with a key reserved specifically for that purpose. The corresponding certificate must contain the extended key usage field extension as defined in [PKIX1] Section 4.2.1.14 with KeyPurposeID having value id-kp-timeStamping. This extension must be critical. 2.4. Request and Token Formats A time stamping request is as follows. TimeStampReq ::= SEQUENCE { requester [0] GeneralName OPTIONAL, reqPolicy [1] PolicyInformation OPTIONAL, tsa GeneralName, tdas SEQUENCE OF GeneralName, messageImprint MessageImprint --a hash of the data to be time stamped } The reqPolicy field, if included, indicates the policy under which the TimeStampToken should be provided. PolicyInformation is defined in Section 4.2.1.5 of [PKIX1]. The tsa field identifies the name of the TSA. GeneralName is defined in Section 4.2.1.7 of [PKIX1]. The tdas field identifies those TDAs which are requested to provide supplementary temporal evidence in the time stamp token. MessageImprint ::= SEQUENCE { hashAlgorithm AlgorithmIdentifier, hashedMessage OCTET STRING } The hash algorithm indicated in the hashAlgorithm field must be a strong hash algorithm. That means that it must be one-way and collision resistant. It is up to the Time Stamp Authority to decide whether or not the given hash algorithm is "sufficient" (based on the current state of knowledge in cryptanalysis and the current state of the art in computational resources, for example). The hashedMessage field should contain the hash of the message to be time stamped. The hash is represented as an OCTET STRING. A time stamp token is as follows. The signature is computed over tstInfo (encoded using the ASN.1 distinguished encoding rules (DER)). TimeStampToken ::= SEQUENCE { tstInfo TSTInfo, signature BIT STRING, --over the ASN.1 DER encoding of tstInfo } Document Expiration: May 7, 1998 Page 3 TSTInfo ::= SEQUENCE { policy PolicyInformation OPTIONAL, status PKIStatusInfo, requester [0] GeneralName OPTIONAL, --must be present if the requester field is present in --TimeStampReq, and must be identical to that field tsa GeneralName, signatureAlgorithm AlgorithmIdentifier, certId CertId, --must refer to the TSA's public verification certificate certs SEQUENCE OF Certificate OPTIONAL, --additional certificates that may be needed by end entities --to verify the TimeStampToken genTime GeneralizedTime, tdaTokens SEQUENCE OF TemporalDataToken, messageImprint MessageImprint --this field must have the same value as the similar field --in TimeStampReq } PKIStatusInfo is defined in Section 3.2.3 of [PKIX3]. If the PKIStatus field has value 'waiting' (3), then this token is a receipt, as defined in Section 2.1. Otherwise, the status field is present to indicate whether or not the time stamping request was fulfilled and, if not, the reason it was rejected. A valid time stamp token will always have value 0 (granted) in the PKIStatus field of PKIStatusInfo. TemporalDataToken is defined in Section 3 of this document. The tdaTokens field contains the supplementary evidence requested in the TimeStampReq. CertId is defined in Section 3.2.4 of [PKIX3]. 3. The TDA The Temporal Data Authority is a TTP that creates a temporal data token. This temporal data token associates a message with a particular event and provides supplementary evidence for the time included in the time stamp token. For example, a TDA could associate the message with the most recent closing value of the Dow Jones Average. The temporal data with which the message is associated should be unpredictable in order to prevent forward dating of tokens. Authentic values of this data should also be available from a large number of trustworthy sources in order to make collusion or corruption of data impossible. For a list of possible types of temporal data, see Appendix C. 3.1. Requirements of the TDA The TDA is required to: 1. verify only the temporal data. The TDA does not examine or verify the data being time stamped or requesting entities in any way. 2. include the current data associated with a specific unpredictable event in each temporal data token. Document Expiration: May 7, 1998 Page 4 3. produce a temporal data token upon receiving a valid request from the TSA. 4. include within each temporal data token an identifier to uniquely determine the trust and validation policy used for this signature. 5. only produce a temporal data token on a hash representation of the message. 6. sign each temporal data token using a key generated exclusively for this purpose and have this property of the key indicated on the corresponding certificate. 3.2. TDA Transactions As the first message of this mechanism, the TSA requests a temporal data token by sending a request (which is or includes a TemporalDataReq, as defined below) to the TDA. As the second message, the TDA responds by sending a response (which is or includes a TemporalDataToken, as defined below) to the TSA. 3.3. Identification of the TDA The TDA must sign all temporal data tokens with a key reserved specifically for that purpose. The corresponding certificate must contain the extended key usage field extension as defined in [PKIX1] Section 4.2.1.14 with KeyPurposeID having value id-kp-temporalData. This extension must be critical. id-kp-temporalData OBJECT IDENTIFIER ::= {id-kp ??} -- Notarizing the validity of certain information. Key usage bits -- that may be consistent: digitalSignature, nonRepudiation 3.4. Request and Token Formats A temporal data request is as follows. TemporalDataReq ::= SEQUENCE { tda GeneralName, messageImprint MessageImprint --a hash of the data to be time stamped, must be the same --value as the corresponding field in TimeStampReq } A temporal data token is as follows. The signature is computed over tdtInfo (encoded using the ASN.1 distinguished encoding rules (DER)). TemporalDataToken ::= SEQUENCE { tdtInfo TDTInfo, signature BIT STRING, --over the ASN.1 DER encoding of tstInfo } Document Expiration: May 7, 1998 Page 5 TDTInfo ::= SEQUENCE { tda GeneralName, signatureAlgorithm AlgorithmIdentifier, certId CertId, --must refer to the TDA's public verification certificate temporalData TemporalData, messageImprint MessageImprint --this field must have the same value as the similar field --in TimeStampReq } The temporalData field contains the actual temporal data that will be used as substantiating evidence in the time stamp token. TemporalData ::= SEQUENCE { format TEMPORALDATACLASS.&id, --objid rawdata TEMPORALDATACLASS.&Type --open type } TEMPORALDATACLASS ::= CLASS { &id OBJECT IDENTIFIER UNIQUE, &Type } WITH SYNTAX { &Type IDENTIFIED BY &id } 4. Transports 4.1. File Based Protocol A file containing a time stamp message must contain only the DER encoding of one PKI 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. 4.2. Socket Based Protocol The socket based protocol for time stamp messages is identical to that used in [PKIX3] Section 5.2 except that port 309 must be used. 4.3. Time Stamp Protocol Using E-mail This section specifies a means for conveying ASN.1-encoded messages for the protocol exchanges described in Sections 2 and 3 via Internet mail. A simple MIME object is specified as follows. Content-Type: application/timestamp Content-Transfer-Encoding: base64 <> This MIME object can be sent and received using common MIME processing Document Expiration: May 7, 1998 Page 6 engines and provides a simple Internet mail transport for Time Stamp messages. 4.4. Time Stamp Protocol via HTTP This subsection specifies a means for conveying ASN.1-encoded messages for the protocol exchanges described in Sections 2 and 3 via the HyperText Transfer Protocol. A simple MIME object is specified as follows. Content-Type: application/timestamp <> This MIME object can be sent and received using common HTTP processing engines over WWW links and provides a simple browser-server transport for Time Stamp messages. 5. Security Considerations When designing a TSA/TDA service, the following considerations have been identified that have an impact upon the validity or "trust" in the time stamp token. 1. The TSA/TDA private key is compromised and the corresponding certificate is revoked. In this case, any token signed by the TSA/TDA using that private key cannot be trusted. 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/TDA may provide a means to discriminate between genuine and false tokens. 2. The TSA/TDA 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/TDA 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/TDA's signature. Time stamp tokens could also be kept with an Evidence Recording Authority to maintain this trust. 3. When there is a reason to believe that the TSA/TDA can no longer be trusted, the authority's certificate must be revoked and placed on the appropriate ARL. Thus, at any future time the tokens signed with the corresponding key will not be valid. 4. Since the TSA does not verify message data or the identity of the entities, the requester field in TimeStampReq and TimeStampToken should be considered untrusted. If authentication of this field is needed, it is recommended that the Notary Authority be used, as described in [NOTARY]. Document Expiration: May 7, 1998 Page 7 8. References [ISONR] ISO/IEC 10181-5: Security Frameworks in Open Systems. Non-Repudiation Framework. [PKIX1] R. Housley, W. Ford, W. Polk, D. Solo, "Internet Public Key Infrastructure, X.509 Certificate and CRL Profile," draft- ietf-pkix-ipki-part1-0X.txt, 1997 (work in progress). [PKIX3] C. Adams, S. Farrell, "Internet Public Key Infrastructure, Certificate Management Protocols," draft-ietf-pkix-ipki3cmp- 0X.txt, 1997 (work in progress). [NOTARY] C. Adams, R. Zuccherato, "Notary Protocols," draft-adams- notary-0X.txt, 1997 (work in progress). 9. Authors' Addresses Carlisle Adams Pat Cain Entrust Technologies BBN 750 Heron Road, Suite 800 70 Fawcett Street Ottawa, Ontario Cambridge, MA 02138 K1V 1A7 U.S.A. CANADA pcain@bbn.com cadams@entrust.com Denis Pinkas Robert Zuccherato Bull S.A. Entrust Technologies Rue Jean Jaures 750 Heron Road, Suite 800 B.P. 68 Ottawa, Ontario 78340 Les Clayes sous Bois K1V 1A7 FRANCE CANADA D.Pinkas@frcl.bull.fr robert.zuccherato@entrust.com Document Expiration: May 7, 1998 Page 8 APPENDIX A - Storage of Data and Token A time stamp token is meaningless without its associated data. Thus, a method is required to allow users to store the data and token together securely. They may be stored as a PKCS #7 SignedData object as described in [PKCS7]. That is, the contentType is signedData and contentInfo is Data, which contains the message associated with the time stamp token. The SignedData object is signed by the person storing the data and token. For this purpose, we define a PKCS #9 [PKCS9] time stamp token attribute type. This attribute type specifies the time stamp token, which must be included as an authenticated attribute of the SignedData object. The time stamp token attribute type has ASN.1 type TimeStampToken (as defined in Section 2.4 of this document). A time stamp token attribute must have a single attribute value. The object identifier timeStampToken identifies the time stamp token attribute type. timeStampToken ::= { pkcs-9 n <> } [PKCS7] RSA Laboratories, "The Public-Key Cryptography Standards (PKCS)", RSA Data Security Inc., Redwood City, California, November 1993 Release. [PKCS9] RSA Laboratories, "The Public-Key Cryptography Standards (PKCS)", RSA Data Security Inc., Redwood City, California, November 1993 Release. 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 signature that incorporates an untrusted time, the following basic technique may be used: A) Time stamping information needs to be obtained by the signer or a verifier. 1) The signature is presented to the Time Stamping Authority (TSA). The TSA then returns a TimeStampToken (TST) upon that signature. Document Expiration: May 7, 1998 Page 9 2) The invoker of the service must then verify that the TimeStampToken is correct. B) The validity of the evidence must be verified : 1) The date/time indicated by the signer in the signature shall be compared with the date/time in the TST. If they are not close enough (e.g., less than a few hours) the evidence is considered to be invalid. 2) The certificate included in the signed message should be verified to be valid at the time of the signature. It must first be verified and then the appropriate CRL must be checked. The signature has now been placed at a particular point in time. The appropriate CRLs or other certificate status information mechanism may be examined to determine the validity of the signature at that time. APPENDIX C - Possible Types of Temporal Data 1) Stock market information 2) Sports results 3) Official weather data for a specific location 4) Lottery results 5) Birth or death announcements in specific newspapers 6) Headlines in specific newspapers Document Expiration: May 7, 1998 Page 10