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Versions: 00                                                            
PKIX Working Group                        C. Adams(Entrust Technologies)
Internet Draft                       R. Zuccherato(Entrust Technologies)
expires in six months                                      July 29, 1997

                     Internet Public Key Infrastructure

                          Part VI: Notary Protocols


Status of this Memo

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This document describes a general notary service and the protocols to
be used when communicating with it.  The Notary Authority is a Trusted
Third Party (TTP) that can be used as one component in building reliable
non-repudiation services (see [ISONR]).  We also give an example of how
to use the notary to extend the lifetime of a signature beyond key
expiry or revocation.

1. Introduction

A Notary Authority (NA) is a Trusted Third Party that verifies
correctness of specific data submitted to it.  The Notary Authority
provides the notary service in order that non-repudiation evidence may
be constructed relating to the validity and correctness of an entity's
claim to possess data and/or the validity and correctness of various
types of data at a particular instant in time.  When notarizing
possession of data, the NA verifies the mathematical correctness of the
actual signature value contained in the request and also checks the full
certification path from the signing entity to a trusted point (e.g., the
NA's CA, or the root CA in a hierarchy) along with all relevant CRLs and
ARLs (Authority Revocation Lists).  It then includes a trusted time and
creates a notary token.

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When notarizing data, the NA verifies the correctness of the data and
creates a notary token.  In this case, however, data "correctness" is
not as focused in scope as signature correctness; the particular
definition to be applied is therefore necessarily policy- and datatype-
dependent.  For example, the data may itself contain one or more
signatures (where "correctness" relates to the validity of these
signatures), or it may contain assertions (where "correctness" relates
to the truth value of these statements), or it may contain a contract
(where "correctness" relates to the legal validity of the document).

In all cases, the trust that PKI entities have in the Notary Authority
is transferred to the contents of the notary token (just as trust in a
CA is transferred to the certificates that it issues).  As a particular
example, a notary token pertaining to a signature may be useful for
extending the life of that signature beyond the expiry or subsequent
revocation of its corresponding verification certificate.

2. Requirements of the Notary Authority

The Notary Authority is required to:

     1. verify the correctness of the enclosed digital signature using
        all available and appropriate CRLs, ARLs, and public key
        certificates and produce a signed notary token attesting to the
        validity of the signature, if asked by the requester.
     2. verify the correctness of the enclosed data with respect to
        explicitly stated policies using all available and appropriate
        resources and produce a signed notary token attesting to the
        validity of the data, if asked by the requester.
     3. include a monotonically incrementing value of the time of day
        or a time stamp token into its notary token.
     4. include within each signed notary token an identifier to
        uniquely determine the trust and validation policy used for its
     5. indicate in the token whether or not the signature or data
        verified, and if not, the reason the verification failed.
     6. provide a signed receipt (i.e. in the form of an appropriately
        defined notary token) to the requester, where appropriate, as
        defined by policy.

3. Notary Transactions

As the first transaction of this mechanism, the requesting entity
requests a notarization by sending a request (which is or includes a
TimeStampReq, as defined below) including the data for which validity
and/or possession is to be notarized to the Notary Authority.  Upon
receiving the request, the Notary Authority reviews and checks the
validity of the request.  If the request is valid, the Notary Authority
performs the notarization and sends a response (which is or includes a
TimeStampToken, as defined below) to the requesting entity.  Otherwise,
the Notary Authority returns an error message (in the form of an
appropriately defined notary token).

Upon receiving the token, the requesting entity verifies its validity.
The requester should verify that it contains the correct time, the
correct name for the NA, the correct data imprint, a valid signature,

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and satisfactory status, service and policy fields.  The token can now
be used to authenticate the correctness or possession of the data.

4. Request and Token Formats

The ServiceType type indicates which type of Notary Service is required.

ServiceType ::= INTEGER  { npd(1), nd(2), nb(3) }

The value npd (Notarize Possession of Data) is used when only the
signature on the NotaryReq (i.e., possession of the data in the request)
is to be verified.  In this case the Notary Authority would be merely
providing evidence that the requester possessed the data in the request
and a valid signature key at the time indicated.  This is really an
extension of the Time Stamp Authority in that we are given the
additional assurance about the validity of the signature, as well as the
time before which it was applied.  The value nd (Notarize Data) is used
when only the data included in NotaryReqInfo is to be verified.  This
verification may mean verifying a digital signature contained in the
data, verifying the correctness of the data, or verifying the intent of
parties to a contract contained in the data, for example.  The exact
interpretation of this service must be clearly indicated in the NA’s
policy statement, but is implementation and policy dependent, and thus
beyond the scope of this document.  The value nb (Notarize Both) is used
when both the signature and data are to be verified.  A given NA may
support any subset of the above services.

A notary request is as follows.

NotaryReq ::= SEQUENCE  {
     notaryReqData                NotaryReqData,
     signature                    BIT STRING OPTIONAL
       --over the ASN.1 DER encoding of NotaryReqInfo, must be present
       --if the service field of NotaryReqInfo is npd or nb

The data and information that will be notarized is contained in the
notaryReqData field.

NotaryReqData ::= SEQUENCE  {
     notaryReqInfo                NotaryReqInfo,
     data                         Data
       --the data to be notarized
       --this field must be of type Message if the service type is nd
       --or nb

The notaryReqInfo field contains information pertaining to the notary

NotaryReqInfo ::= SEQUENCE  {
     service                      ServiceType,
     requester                    GeneralName OPTIONAL,
      --must be present if the service field is npd or nb

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     signatureAlgorithm           AlgorithmIdentifier,
      --must be present if the service field of NotaryReqInfo is
      --npd or nb
     certs                        SEQUENCE OF Certificate OPTIONAL,
     reqPolicy                    PolicyInformation OPTIONAL,
     notary                       GeneralName,
     reqTime                      TimeStampToken OPTIONAL   }

In situations where the Notary Authority will verify the identity of the
requester (i.e., when the service field is npd or nb), the notary
request must be signed by the requester using the signature field.

Similarly, in situations where the Notary Authority will certify the
time included in the request (i.e., when stipulated by the policy of the
Notary Authority), the notary request must include the reqTime field in
NotaryReqInfo. TimeStampToken is defined in Section 4 of PKIX Part 5

PolicyInformation is defined in Section of PKIX Part 1 [PKIX1].
The reqPolicy field should indicate the policy under which the
notarization is requested.  This field must be checked by the NA to
verify agreement with its own policy.

The Data type is defined to be either the message itself or a hash of
the message.  This allows a signature indicating possession of private
data to be notarized.

Data ::= CHOICE  {
     message                     Message,
     messageimprint              MessageImprint  }

In order to specify the format (i.e. the type) of the message so that
it may be parsed and understood by the NA or any verifying entity, we
define the Message data type.

Message ::= SEQUENCE  {
     format                       MESSAGECLASS.&id,   --objid
     rawdata                      MESSAGECLASS.&Type  --open type

     &id                          OBJECT IDENTIFIER UNIQUE,
     &Type                                                    }

If the requester prefers to send a hash of the message instead, the
MessageImprint data type should be used.

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

The hash algorithm indicated in the hashAlgorithm field should be a
“strong” hash algorithm (that is, it should be one-way and collision
resistant).  It is up to the Notary Authority to decide whether or not
the given hash algorithm is sufficiently “strong”.

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The hashedMessage field should contain the hash of the DER encoding of
the message expressed as a Message data type.  The hash is represented

A notary token is as follows.

NotaryToken ::= SEQUENCE  {
     notaryInfo                   NotaryInfo,
     signature                    BIT STRING,
      --over the ASN.1 DER encoding of NotaryInfo

NotaryInfo ::= SEQUENCE  {
     notaryReqInfo                NotaryReqInfo,
       --must be the same value as the notaryReqInfo field in
     messageImprint               MessageImprint,
       --if the data field in NotaryReqData is MessageImprint, this
       --must contain that same value, otherwise it contains a hash of
       --the data field in NotaryReqData using the hash algorithm
       --specified in the signatureAlgorithm parameter
     signature                    BIT STRING OPTIONAL,
       --must be present if service field of notaryReqInfo is npd or nb
       --must be the same value as the signature field in NotaryReq
     policy                       PolicyInformation,
     status                       PKIStatusInfo,
     time                         NotaryTime,
     signatureAlgorithm           AlgorithmIdentifier,
     certId                       CertId,
       --must refer to the NA’s public verification certificate
     certs                        SEQUENCE OF Certificate OPTIONAL,
       --if present, must indicate the chain of trust used to verify the
     crls                         SEQUENCE OF CertificateList OPTIONAL

NotaryTime ::= CHOICE  {
     genTime                      GeneralizedTime,
     timeStampToken               TimeStampToken   }

PKIStatusInfo is defined in Section 3.2.3 of PKIX Part 3 [PKIX3].  If
the PKIStatus field has value ‘waiting’ (3), then this token is a
receipt, as defined in Section 2.  Otherwise, the status field is
present to indicate whether or not the notary request was fulfilled and,
if not, the reason it was rejected.  A valid NotaryToken will have a
PKIStatus field with value ‘granted’ (0).

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

The crls field (if present) should contain a sequence of certificate and
authority revocation lists that is sufficient to verify the chain of
trust indicated in the certs field.

The signature, certs and crls fields are included as OPTIONAL.  They
should be present, when policy dictates, for use as supplementary
evidence when resolving possible disputes.  Dispute resolution would

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most likely be handled by one or more humans, in an off-line
environment, and is beyond the scope of this document.

6. Notary Protocol Using Email

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-pkix6
   Content-Transfer-Encoding: base64

   <<the ASN.1 DER-encoded PKIX-6 message, base64-encoded>>

This MIME object can be sent and received using MIME processing engines
and provides a simple Internet mail transport for PKIX-6 messages.

7. Security Considerations

When designing a notary service, the following considerations have been
identified that have an impact upon the validity or “trust” in the
notary token.

     1. The requester’s key is compromised and the corresponding
        certificate is revoked before the notary acts upon the request.
        The notary is required to validate appropriate information
        within the request before it constructs the notary token.  It is
        therefore mandated that the NA have access to current
        information regarding certificate status.  In this situation,
        the notarization would not occur.

     2. The requester’s key is compromised and the corresponding
        certificate is revoked after the notary acts upon the request.
        This is not a concern to the NA once the notary has constructed
        the token, as long as the compromise date in the CRL is not
        before the time of notarization.  If it is, this situation
        would have to be handled by off-line, possibly human-aided,
        means specific to the situation at hand.

3.  The notary’s private key is compromised and the corresponding
        certificate is revoked.  In this case, any token signed by the
        notary cannot be trusted.  For this reason, it is imperative
        that the notary’s key be guarded with proper security and
        controls in order to minimize the possibility of compromise.
        Nevertheless, in case the private key does become compromised,
        an audit trail of all the tokens generated by the NA should be
        kept as a means to help discriminate between genuine and false

     4. The NA 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 NA
        should be time stamped again at a later date to renew the trust
        that exists in the NA’s signature.

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 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, 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).

[PKIX5] C. Adams, P. Cain, D. Pinkas, R. Zuccherato, “Internet Public
Key Infrastructure, Part V:  Time Stamp Protocols,” draft-ietf-pkix-
ipki5tsp-00.txt, 1997 (work in progress).

9. Authors’ Addresses

Carlisle Adams
Entrust Technologies
750 Heron Road, Suite 800
Ottawa, Ontario
K1V 1A7

Robert Zuccherato
Entrust Technologies
750 Heron Road, Suite 800
Ottawa, Ontario
K1V 1A7

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APPENDIX A - Storage of Data and Token

A notary token is useless without the data to which it applies.  For
this reason tokens and their related data must be securely stored
together.  The change of a single bit in either the data or the token
renders the entire notarization process for that data meaningless.
Storage of tokens and data in a secure (e.g., tamper proof) environment
is strongly recommended.

When data and notary tokens are stored together, the following ASN.1
data type may be used.

DataAndToken ::= SEQUENCE  {
     message                      Message,
     notaryToken                  NotaryToken  }

Note that this object does not need to be signed, as the notary token
already verifies the signature on the message.  Any supplementary
information whose integrity needs to be protected should be part of
the message or token.

APPENDIX B - Extending the Life of a Signature

We present an example of a possible use of this general notary service.
It produces a stand-alone token that can be used to extend the life of a
signature.  This example assumes that we have total trust in the Notary

Signature algorithms and keys have a definite lifetime.  Therefore,
signatures have a definite lifetime.  The Notary Authority can be used
to extend the lifetime of a signature.

In order to extend the lifetime of a signature in this way, the
following technique may be used.

A) The signature needs to be notarized.

     1) The signed message is presented to the Notary in the data
        field of NotaryReqInfo under service type nd and an appropriate

     2) The Notary verifies that the signature and verification key are
        valid at that time by checking expiry dates, CRLs and ARLs, and
        returns a NotaryToken.

B)  The notarized signature must be verified.

     1) The signature of the Notary in NotaryToken shall be verified
        using the Notary’s valid verification key.

In this situation the signer’s signing key (and therefore, its
signature) is only valid until some specified time T1.  The NA’s
signing key (and therefore, its signature) is valid until some specified
time T2 that is (usually) after time T1.  Without notarization, the
signer’s signature would only be valid until time T1.  With
notarization, the signer’s signature remains valid until time T2,

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regardless of subsequent revocation or expiry at time T1.

If the signature of the NA is valid, the trust we have in the NA allows
us to conclude that the original signature on the data was valid at
the time included in the notaryInfo field of the NotaryToken.  Notice
that in this example the validity of the original signature can be
confirmed from the verification of one signature (the NA’s) instead of
two signatures (the Time Stamp Authority’s and the original), but at the
cost of putting more trust in the Trusted Third Party.

Document Expiration:  Jan. 29, 1998                               Page 9