PKIX Working Group                        C. Adams(Entrust Technologies)
Internet Draft                                             P. Cain (BBN)
expires in six months                                   D. Pinkas (Bull)
                                     R. Zuccherato(Entrust Technologies)
                                                           July 29, 1997


                     Internet Public Key Infrastructure

                       Part V:  Time Stamp Protocols

                    <draft-ietf-pkix-ipki5tsp-00.txt>


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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]).  We also give an example of how to place a signature at a
particular point in time, from which the appropriate CRLs may be
checked.

.  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 key was put on a CRL, 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.

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. Requirements of the TSA

The TSA is required to:
     1. include a monotonically incrementing value of the time of day
        into its time stamp token.
     2. produce a time stamp token upon receiving a valid request from
        the requester.
     3. include within each time stamp token an identifier to
        uniquely determine the trust and validation policy used for this
        signature.
     4. support time stamping of a hash representation of the message
        from the requester.

.  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 appropriate ARL should be checked to verify that the
certificate is still valid.  The token can now be used to establish a
trusted time reference.

. Request and Token Formats

A time stamping request is as follows.

TimeStampReq ::= SEQUENCE  {
     requester                [0] GeneralName OPTIONAL,
     reqPolicy                [1] PolicyInformation OPTIONAL,
     tsa                          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 PKIX Part 1 [PKIX1].

The tsa field identifies the name of the TSA.  GeneralName is defined
in Section 4.2.1.7 of PKIX Part 1 [PKIX1].

MessageImprint ::= SEQUENCE  {
     hashAlgorithm                AlgorithmIdentifier,
     hashedMessage                OCTET STRING  }

The hash algorithm indicated in the hashAlgorithm field must be a strong

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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
}

TSTInfo ::= SEQUENCE  {
     policy                       PolicyInformation,
     status                       PKIStatusInfo,
     requester               [0]  GeneralName OPTIONAL,
       --must be present if the requester field is present in
       --TimeStampReq
     tsa                          GeneralName,
     signatureAlgorithm           AlgorithmIdentifier,
     certId                       CertId,
       --must refer to the TSA’s public verification certificate
     certs                        SEQUENCE OF Certificate OPTIONAL,
     genTime                      GeneralizedTime,
     messageImprint               MessageImprint
       --this field must have the same value as the similar field
       --in TimeStampReq
}

PKIStatusInfo is defined in Section 3.2.3 of PKIX Part 3 [PKIX3].  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.

CertId is defined in Section 3.2.4 of PKIX Part 3 [PKIX3].

. Time Stamp Protocol Using E-mail

This section specifies a means for conveying ASN.1-encoded messages
for the protocol exchanges described in Section 4 via Internet mail.

A simple MIME object is specified as follows.

   Content-Type: application/x-pkix5
   Content-Transfer-Encoding: base64

   <<the ASN.1 DER-encoded PKIX-5 message, base64-encoded>>

This MIME object can be sent and received using common MIME processing

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engines and provides a simple Internet mail transport for PKIX-5
messages.

. 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. The TSA private key is compromised and the corresponding
        certificate is revoked.  In this case, any token signed by the
        TSA 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
        may provide a means to discriminate between genuine and false
        tokens.

     2. 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 at a later date to renew the trust
        that exists in the TSA’s signature.

. 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, Part I:  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, Part
III:  Certificate Management Protocols,” draft-ietf-pkix-ipki3cmp-
0X.txt, 1997 (work in progress).

. Authors’ Addresses

Carlisle Adams                        Pat Cain
Entrust Technologies                  BBN
 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
 Les Clayes sous Bois            K1V 1A7
FRANCE                                CANADA
D.Pinkas@frcl.bull.fr                 robertz@entrust.com


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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 4 of this document).  A time stamp token attribute
can have a single attribute value.

The object identifier timeStampToken identifies the time stamp token
attribute type.

timeStampToken ::= { pkcs-9 n <<To be supplied>> }

[PKCS7] RSA Laboratories, “The Public-Key Cryptography Standards
(PKCS)”, RSA Data Security Inc., Redwood City, California, November
 Release.

[PKCS9] RSA Laboratories, “The Public-Key Cryptography Standards
(PKCS)”, RSA Data Security Inc., Redwood City, California, November
 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 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.

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     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 may be examined to determine the validity of the
signature at that time.







































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