Internet Draft C. Adams(Entrust Technologies)
PKIX Working Group P. Cain (BBN)
expires in six months D. Pinkas (Bull)
R. Zuccherato(Entrust Technologies)
September 23, 1998
Internet X.509 Public Key Infrastructure
Time Stamp Protocols
<draft-ietf-pkix-time-stamp-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]). In order to reduce the amount of trust required of a TSA we
introduce (in Appendix C) 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was applied before the certificate was revoked thus allowing a revoked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: March 23, 1999 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.
The Temporal Data Authority (TDA) is a TTP that creates a temporal data
token. (See Appendix C.) 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
more difficult. For a list of possible types of temporal data, see
Appendix D.
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.
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 provide a trusted source of time.
2. not to include any identification of the requesting entity in
the time stamp token.
3. not to examine the imprint being time stamped in any way.
4. to include a monotonically incrementing value of the time of day
into its time stamp token.
5. to produce a time stamp token upon receiving a valid request
from the requester.
6. to include within each time stamp token an identifier to
uniquely indicate the trust and validation policy under which
the token was created.
7. to only time stamp a hash representation of the message, i.e. a
data imprint associated with a one-way collision resistant hash-
function OID.
8. to examine the OID of the one-way collision resistant hash-
function and to verify that this function is "sufficient" (see
Section 2.4).
9. 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.
10. to include supplementary temporal information in the time stamp
token (from TDA's) if asked by the requester. If this is not
possible, the TSA shall respond with an error message.
11. to 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.
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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 TimeStampToken, as defined below) to
the requesting entity.
Upon receiving the token, the requesting entity verifies its validity
by verifying the digital 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 TSA name, the correct data imprint and the correct
hash algorithm OID. It should 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, and/or the value of
the nonce included in the response against the value included in the
request. 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. If
TemporalDataToken's are included in the TimeStampToken, then these
should also be verified as was the TimeStampToken (see Appendix C). The
token can now be used to establish a trusted time reference.
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 [CCP]
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 {
version Integer { v1(0) },
reqPolicy PolicyInformation OPTIONAL,
tdas SEQUENCE OF GeneralName OPTIONAL,
nonce Integer,
messageImprint MessageImprint OPTIONAL
--a hash algorithm OID and the hash value 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 [CCP].
The tdas field identifies those TDAs which are requested to provide
supplementary temporal evidence in the time stamp token. (See Appendix
C.)
Document Expiration: March 23, 1999 Page 3
The messageImprint field may be absent in situations where the requester
requires a verifiable source of time that is independent of any
particular message.
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.
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,
it is suggested that alternate identification means be used (e.g. CMS
encapsulation [CMS] or TLS authentication [TLS]).
A TimeStampToken is as follows. It is encapsulated as a SignedData
construct [CMS]. The content is of type TSTInfo.
TimeStampToken ContentInfo ::= SEQUENCE {
contentType OBJECT IDENTIFIER,
--{pkcs-7 2}, SignedData
content [0] SEQUENCE {
version INTEGER,
digestAlgorithms AlgorithmIdentifier,
contentInfo SEQUENCE {
contentType OBJECT IDENTIFIER,
--OID signalling TSTInfo
content TSTInfo }
certificate [0] Certificate,
signerInfos SignerInfos } }
The time stamp token must contain only the signature of the TSA. In
some environments, the CA might not perform a proof-of-possession of
the private key when issuing certificates. In these instances, either
the certificate of the TSA, or the certificate issuer and serial number
shall be included as a signed attribute.
TSTInfo ::= SEQUENCE {
version Integer { v1(0) },
policy PolicyInformation,
status PKIStatusInfo,
tsa GeneralName,
tstTime TSTTime,
tdaTokens SEQUENCE OF TemporalDataToken
OPTIONAL,
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nonce Integer OPTIONAL,
--must be present if the similar field in TimeStampReq is
--present
--this field must have the same value as the similar field
--in TimeStampReq
messageImprint MessageImprint OPTIONAL,
--must be present if the similar field in TimeStampReq is
--present
--must have the same value as the similar field in
--TimeStampReq
serialNumber Integer OPTIONAL,
tsaFreeData OCTET STRING OPTIONAL
--contains supplementary information from the TSA
}
PKIStatusInfo is defined in Section 3.2.3 of [CMP]. 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. Since not all
environments will require the use of receipts, support for the value
`waiting' is optional.
PKIFailureInfo ::= BITSTRING {
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 TSAs time source is not available
tdaNotAvailable {15},
-- at least one of the TDAs that were requested isn't available
}
The statusString field of PKIStatusInfo can be used to include reason
text such as "messageImprint field is not correctly formatted".
The purpose of the tsa field is to identify the name of the TSA. It
must correspond to one of the subject names included in the certificate
that is to be used to verify the token.
TemporalDataToken is defined in Appendix C of this document. The
tdaTokens field contains the supplementary evidence requested in the
TimeStampReq.
TSTTime ::= SEQUENCE {
genTime GeneralizedTime,
milliseconds INTEGER (0..999) OPTIONAL }
GeneralizedTime shall be used as described in [CCP] Section 4.1.2.5.2.
Since GeneralizedTime only represents time with one second granularity,
the milliseconds field, if present, shall be used to indicate the
number of milliseconds, and thus provide a finer time resolution.
Document Expiration: March 23, 1999 Page 5
The serialNumber field, if present, shall include a strictly
monotonically increasing Integer from one TimeStampToken to the next.
This will guarantee that each token is unique and provides for
additional auditing capabilities.
There may be situations where the TSA may wish to include supplementary
non-time stamp related information in the time stamp token (e.g. billing
information, usage statistics, etc.). The format of this information is
TSA dependant and the value can be placed in the tsafreedata field as an
OCTET STRING. Conformant clients are not required to process this
field, if present.
3. Transports
There is no mandatory transport mechanism in this document. All
mechanisms are optional.
3.1. 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.
3.2. Socket Based Protocol
The socket based protocol for time stamp messages is identical to that
used in [CMP] Section 5.2 except that port 309 must be used.
3.3. 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 C via
Internet mail.
A simple MIME object is specified as follows.
Content-Type: application/timestamp
Content-Transfer-Encoding: base64
<<the ASN.1 DER-encoded Time Stamp message, base64-encoded>>
This MIME object can be sent and received using common MIME processing
engines and provides a simple Internet mail transport for Time Stamp
messages.
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 C via the
HyperText Transfer Protocol.
A simple MIME object is specified as follows.
Document Expiration: March 23, 1999 Page 6
Content-Type: application/timestamp
<<the ASN.1 DER-encoded Time Stamp message>>
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.
4. Security Considerations
This entire document concerns 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. When there is a reason to believe that the TSA can no longer
be trusted, the authority's certificate must be revoked and
placed on the appropriate CRL. Thus, at any future time the
tokens signed with the corresponding key will not be valid.
2. 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.
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. An application using the TSA service 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
TimeStampToken that takes more than an acceptable period of time
should be considered suspect.
5. In certain circumstances, a TSA may not be able to
produce a valid response to a request (for example, if it is
unable to compute signatures for a period of time). In these
situations the TSA must wait until it is again able to produce a
valid response before responding, if this is possible. If this
is not possible, it must ignore the requests and not respond.
Under no circumstances shall a TSA produce an unsigned response
to a request.
6. A CA shall normally conduct a test for proof of possession for
each user's signing private key (including the TSA signing
private key). However, in some environments, the CA might not
perform a proof-of-possession of the private key when issuing
certificates. In these instances, in order to prevent certain
Document Expiration: March 23, 1999 Page 7
attacks and to properly check the validity of the binding
between an end entity and a key pair, a certificate identifier
of the TSA shall be included as a signed attribute.
5. Patent Information
The following 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. Implementers of this protocol SHOULD perform their
own patent search and determine whether or not any encumberences exist
on their implementation.
# 4,309,569 Method of Providing Digital Signatures
(issued) January 5, 1982
(inventor) Ralph C. Merkle
(assignee) The Board of Trustees of the Leland Stanford Junior
University
# 5,001,752 Public/Key Date-Time Notary Facility
(issued) March 19, 1991
(inventor) Addison M. Fischer
# 5,022,080 Electronic Notary
(issued) June 4, 1991
(inventors) Robert T. Durst, Kevin D. Hunter
# 5,136,643 Public/Key Date-Time Notary Facility
(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
(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
(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
(issued) December 13, 1994
(inventors) Stuart A. Haber, Wakefield S. Stornetta Jr.
(assignee) Bell Communications Research, Inc.,
# 5,422,95 Personal Date/Time Notary Device
(issued) June 6, 1995
(inventor) Addison M. Fischer
Document Expiration: March 23, 1999 Page 8
6. References
[CMP] C. Adams, S. Farrell, "Internet X.509 Public Key Infrastructure,
Certificate Management Protocols," draft-ietf-pkix-ipki3cmp-
0X.txt, 1998 (work in progress).
[TLS] T. Dierks, C. Allen, "The TLS Protocol, Version 1.0," draft-ietf-
tls-protocol-0X.txt, 1998 (work in progress).
[CMS] R. Housley "Cryptographic Message Syntax", draft-ietf-smime-cms-
0X.txt, 1998 (work in progress).
[CCP] R. Housley, W. Ford, W. Polk, D. Solo, "Internet X.509 Public Key
Infrastructure, Certificate and CRL Profile," draft-ietf-pkix-ipki-
part1-0X.txt, 1998 (work in progress).
[ISONR] ISO/IEC 10181-5: Security Frameworks in Open Systems.
Non-Repudiation Framework.
7. Authors' Addresses
Carlisle Adams Pat Cain
Entrust Technologies BBN
750 Heron Road 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
B.P. 68 Ottawa, Ontario
78340 Les Clayes sous Bois K1V 1A7
FRANCE CANADA
Denis.Pinkas@bull.net robert.zuccherato@entrust.com
Document Expiration: March 23, 1999 Page 9
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 [CMS]. 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. This signature is to be used only for storage and for
verifying the integrity of the token and data. Anyone using the token
and data at some future time must verify the data and token at that
time. This is just a method for keeping the two pieces of information
together, with some integrity.
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 a signed 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 <<To be supplied>> }
[CMS] R. Housley "Cryptographic Message Syntax", draft-ietf-smime-cms-
0X.txt, 1998 (work in progress).
[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.
Document Expiration: March 23, 1999 Page 10
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 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 - 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 more difficult. For a
list of possible types of temporal data, see Appendix D.
C.1. Requirements of the TDA
The TDA is required:
1. to only provide a trusted source of temporal data.
2. not to examine the imprint being time stamped.
3. to include the current data associated with a specific
unpredictable event in each temporal data token.
4. to produce a temporal data token upon receiving a valid request
from the TSA.
5. to only produce a temporal data token on a hash representation
of the message.
6. to 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.
C.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.
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C.3. Verifying a TemporalDataToken
The TSA is required to verify the structure of the TemporalDataToken.
It must verify the digital signature in the TemporalDataToken and also
verify that what was signed corresponds to what was requested to be
signed. The requester should verify that the TemporalDataToken contains
the correct TDA name, the correct data imprint and the correct hash
algorithm OID. It should also verify the timeliness of the response by
verifying the value of the nonce included in the response against the
value included in the request (exact match needed). Since the TDA'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.
In order to verify the TemporalData inside a TemporalDataToken, it is
necessary to know the form of the temporal data that the TDA has
included in the token.
The TSA is not required to verify the TemporalData. However, either the
entity requesting a Time Stamping Token or an entity verifying a Time
Stamping Token containing temporal information may be interested in such
a verification.
In the first case, it is unlikely that the temporal information will be
available ahead of time and thus the entity requesting a Time Stamping
Token may need to enter into an online protocol with the TDA, or some
other entity, to obtain it. A secure link with that trusted source may
be necessary, i.e. the communication channel or the information itself
must be authenticated and integrity protected. Such a protocol is TDA
dependent and is outside the scope of this document.
In the second case, if the verification occurs some time after the Time
Stamping Token has been produced, then it is possible to rely on an
authentic source (e.g. a newspaper or a CD-ROM) to verify it against.
The exact method of verification is TDA dependent and is thus outside
the scope of this document.
C.4. 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 [CCP]
Section 4.2.1.14 with KeyPurposeID having value id-kp-temporalData.
This extension must be critical.
id-kp-temporalData OBJECT IDENTIFIER ::= {id-kp ??}
-- Providing temporal data in support of time stamping services. Key
-- usage bits that may be consistent: digitalSignature,
-- nonRepudiation
C.5. Request and Token Formats
A temporal data request from a TSA is as follows.
TemporalDataReq ::= SEQUENCE {
version Integer { v1(0) },
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nonce Integer OPTIONAL,
--must be present if the similar field in TimeStampReq is
--present
--must be the same value as the corresponding field in
--TimeStampReq
messageImprint MessageImprint OPTIONAL
--must be present if the similar field in TimeStampReq is
--present
--a hash of the data to be time stamped, must be the same
--value as the corresponding field in TimeStampReq
}
A TemporalDataToken is as follows. It is encapsulated as a SignedData
construct [CMS]. The content is of type TDTInfo.
TemporalDataToken ContentInfo ::= SEQUENCE {
contentType OBJECT IDENTIFIER,
--{pkcs-7 2}, SignedData
content [0] SEQUENCE {
version INTEGER,
digestAlgorithms AlgorithmIdentifier,
contentInfo SEQUENCE {
contentType OBJECT IDENTIFIER,
--OID signaling TDTInfo
content TDTInfo }
certificate [0] Certificate,
signerInfos SignerInfos } }
The temporal data token must contain only the signature of the TDA. In
some environments, the CA might not perform a proof-of-possession of
the private key when issuing certificates. In these instances, either
the certificate of the TDA, or the certificate issuer and serial number
shall be included as a signed attribute.
TDTInfo ::= SEQUENCE {
version Integer { v1(0) },
tda GeneralName,
nonce Integer,
--must be present if the similar field in TimeStampReq is
--present
--must have the same value as the corresponding field in
--TimeStampReq
temporalData TemporalData,
messageImprint MessageImprint OPTIONAL,
--must be present if the similar field in TimeStampReq is
--present
--must have the same value as the corresponding field in
--TimeStampReq
serialNumber Integer OPTIONAL
}
The temporalData field contains the actual temporal data that will
be used as substantiating evidence in the time stamp token.
TemporalData ::= SEQUENCE {
format OBJECT IDENTIFIER,
rawdata ANY DEFINED BY format }
Document Expiration: March 23, 1999 Page 13
The serialNumber field, if present, shall include a strictly
monotonically increasing Integer from one TemporalDataToken to the next.
This will guarantee that each token is unique and provides for
additional auditing capabilities.
C.6. Security Considerations
When designing a TDA service, the following considerations have been
identified that have an impact upon the validity or "trust" in the
temporal data token.
1. When there is a reason to believe that the TDA can no longer
be trusted, the authority's certificate must be revoked and
placed on the appropriate CRL. Thus, at any future time the
tokens signed with the corresponding key will not be valid.
2. The TDA private key is compromised and the corresponding
certificate is revoked. In this case, any token signed by the
TDA using that private key cannot be trusted. For this
reason, it is imperative that the TDA'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 TDA may provide a means to discriminate between genuine
and false tokens.
3. The 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 time stamp token
containing the TDA's signature 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 TDA's signature. Time stamp tokens could also be kept with
an Evidence Recording Authority to maintain this trust.
4. In certain circumstances, a TDA may not be able to produce a
valid response to a request (for example, if it is unable to
compute signatures for a period of time). In these situations
the TDA must wait until it is again able to produce a valid
response before responding, if this is possible. If this is not
possible, it must ignore the requests and not respond. Under no
circumstances shall a TDA produce an unsigned response to a
request.
5. A CA shall normally conduct a test for proof of possession for
each user's signing private key (including the TDA signing
private key). However, in some environments, the CA might not
perform a proof-of-possession of the private key when issuing
certificates. In these instances, in order to prevent certain
attacks and to properly check the validity of the binding
between an end entity and a key pair, a certificate identifier
of the TSA shall be included as a signed attribute.
[CMS] R. Housley "Cryptographic Message Syntax", draft-ietf-smime-cms-
0X.txt, 1998 (work in progress).
Document Expiration: March 23, 1999 Page 14
APPENDIX D - Possible Types of Temporal Data
1) Stock market information
2) Sports results
3) Official weather data for a specific location
4) Lottery results
5) Headlines in specific newspapers
6) Information linking the request with previous and subsequent requests
(e.g. hash values) that can be verified against information that is
made public by the TDA.
8) A signed packet from a secure time source.
Document Expiration: March 23, 1999 Page 15
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