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
<draft-ietf-pkix-ipki6np-00.txt>
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Abstract
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.
Document Expiration: Jan. 29, 1998 Page 1
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
signature.
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,
Document Expiration: Jan. 29, 1998 Page 2
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 NAs
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
request.
NotaryReqInfo ::= SEQUENCE {
service ServiceType,
requester GeneralName OPTIONAL,
--must be present if the service field is npd or nb
Document Expiration: Jan. 29, 1998 Page 3
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
[PKIX5].
PolicyInformation is defined in Section 4.2.1.5 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
}
MESSAGECLASS ::= CLASS {
&id OBJECT IDENTIFIER UNIQUE,
&Type }
WITH SYNTAX { &Type IDENTIFIED BY &id }
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.
Document Expiration: Jan. 29, 1998 Page 4
The hashedMessage field should contain the hash of the DER encoding of
the message expressed as a Message data type. The hash is represented
as an OCTET STRING.
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
--NotaryReqData
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 NAs public verification certificate
certs SEQUENCE OF Certificate OPTIONAL,
--if present, must indicate the chain of trust used to verify the
--signature
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
Document Expiration: Jan. 29, 1998 Page 5
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 requesters 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 requesters 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 notarys 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 notarys 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
tokens.
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 NAs signature.
Document Expiration: Jan. 29, 1998 Page 6
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
CANADA
cadams@entrust.com
Robert Zuccherato
Entrust Technologies
750 Heron Road, Suite 800
Ottawa, Ontario
K1V 1A7
CANADA
robert.zuccherato@entrust.com
Document Expiration: Jan. 29, 1998 Page 7
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
Authority.
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
policy.
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 Notarys valid verification key.
In this situation the signers signing key (and therefore, its
signature) is only valid until some specified time T1. The NAs
signing key (and therefore, its signature) is valid until some specified
time T2 that is (usually) after time T1. Without notarization, the
signers signature would only be valid until time T1. With
notarization, the signers signature remains valid until time T2,
Document Expiration: Jan. 29, 1998 Page 8
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 NAs) instead of
two signatures (the Time Stamp Authoritys and the original), but at the
cost of putting more trust in the Trusted Third Party.
Document Expiration: Jan. 29, 1998 Page 9
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