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Keywords: tokens, public key certificates
Network Working Group D. Pinkas
Request for Comments: 3628 Bull
Category: Informational N. Pope
J. Ross
Security & Standards
November 2003
Policy Requirements for Time-Stamping Authorities (TSAs)
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document defines requirements for a baseline time-stamp policy
for Time-Stamping Authorities (TSAs) issuing time-stamp tokens,
supported by public key certificates, with an accuracy of one second
or better. A TSA may define its own policy which enhances the policy
defined in this document. Such a policy shall incorporate or further
constrain the requirements identified in this document.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions and Abbreviations . . . . . . . . . . . . . . . . 5
3.1. Definitions. . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Abbreviations. . . . . . . . . . . . . . . . . . . . . . 6
4. General Concepts. . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Time-Stamping Services . . . . . . . . . . . . . . . . . 6
4.2. Time-Stamping Authority. . . . . . . . . . . . . . . . . 7
4.3. Subscriber . . . . . . . . . . . . . . . . . . . . . . . 7
4.4. Time-Stamp Policy and TSA Practice Statement . . . . . . 8
4.4.1. Purpose. . . . . . . . . . . . . . . . . . . . . 8
4.4.2. Level of Specificity . . . . . . . . . . . . . . 8
4.4.3. Approach . . . . . . . . . . . . . . . . . . . . 8
5. Time-Stamp Policies . . . . . . . . . . . . . . . . . . . . . 9
5.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Identification . . . . . . . . . . . . . . . . . . . . . 9
5.3. User Community and Applicability . . . . . . . . . . . . 10
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5.4. Conformance. . . . . . . . . . . . . . . . . . . . . . . 10
6. Obligations and Liability . . . . . . . . . . . . . . . . . . 10
6.1. TSA Obligations. . . . . . . . . . . . . . . . . . . . . 10
6.1.1. General. . . . . . . . . . . . . . . . . . . . . 10
6.1.2. TSA Obligations Towards Subscribers. . . . . . . 11
6.2. Subscriber Obligations . . . . . . . . . . . . . . . . . 11
6.3. Relying Party Obligations. . . . . . . . . . . . . . . . 11
6.4. Liability. . . . . . . . . . . . . . . . . . . . . . . . 11
7. Requirements on TSA Practices . . . . . . . . . . . . . . . . 12
7.1. Practice and Disclosure Statements . . . . . . . . . . . 12
7.1.1. TSA Practice Statement . . . . . . . . . . . . . 12
7.1.2. TSA Disclosure Statement . . . . . . . . . . . . 13
7.2. Key Management Life Cycle. . . . . . . . . . . . . . . . 15
7.2.1. TSU Key Generation . . . . . . . . . . . . . . . 15
7.2.2. TSU Private Key Protection . . . . . . . . . . . 15
7.2.3. TSU Public Key Distribution. . . . . . . . . . . 16
7.2.4. Rekeying TSU's Key . . . . . . . . . . . . . . . 17
7.2.5. End of TSU Key Life Cycle. . . . . . . . . . . . 17
7.2.6. Life Cycle Management of the Cryptographic Module
used to Sign Time-Stamps . . . . . . . . . . . . 17
7.3. Time-Stamping. . . . . . . . . . . . . . . . . . . . . . 18
7.3.1. Time-Stamp Token . . . . . . . . . . . . . . . . 18
7.3.2. Clock Synchronization with UTC . . . . . . . . . 19
7.4. TSA Management and Operation . . . . . . . . . . . . . . 20
7.4.1. Security Management. . . . . . . . . . . . . . . 20
7.4.2. Asset Classification and Management. . . . . . . 21
7.4.3. Personnel Security . . . . . . . . . . . . . . . 22
7.4.4. Physical and Environmental Security. . . . . . . 23
7.4.5. Operations Management. . . . . . . . . . . . . . 25
7.4.6. System Access Management . . . . . . . . . . . . 26
7.4.7. Trustworthy Systems Deployment and Maintenance . 27
7.4.8. Compromise of TSA Services . . . . . . . . . . . 28
7.4.9. TSA Termination. . . . . . . . . . . . . . . . . 29
7.4.10. Compliance with Legal Requirements . . . . . . . 29
7.4.11. Recording of Information Concerning Operation
of Time-Stamping Services. . . . . . . . . . . . 30
7.5. Organizational . . . . . . . . . . . . . . . . . . . . . 31
8. Security Considerations . . . . . . . . . . . . . . . . . . . 32
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
10. References. . . . . . . . . . . . . . . . . . . . . . . . . . 33
10.1. Normative References. . . . . . . . . . . . . . . . . . 33
10.2. Informative References. . . . . . . . . . . . . . . . . 34
Annex A (informative): Coordinated Universal Time . . . . . . . . 35
Annex B (informative): Possible for Implementation Architectures
and Time-Stamping Services . . . . . . . . 36
Annex C (informative): Long Term Verification of Time-Stamp
Tokens . . . . . . . . . . . . . . . . . . 38
Annex D (informative): Model TSA Disclosure Statement . . . . . . 39
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Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . . 42
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 43
1. Introduction
The contents of this Informational RFC is technically equivalent to
ETSI TS 102 023 V 1.2.1 (2002-06) [TS 102023]. The ETSI TS is under
the ETSI Copyright (C). Individual copies of this ETSI deliverable
can be downloaded from http://www.etsi.org
In creating reliable and manageable digital evidence it is necessary
to have an agreed upon method of associating time data to transaction
so that they might be compared to each other at a later time. The
quality of this evidence is based on creating and managing the data
structure that represent the events and the quality of the parametric
data points that anchor them to the real world. In this instance
this being the time data and how it was applied.
A typical transaction is a digitally signed document, where it is
necessary to prove that the digital signature from the signer was
applied when the signer's certificate was valid.
A timestamp or a time mark (which is an audit record kept in a secure
audit trail from a trusted third party) applied to a digital
signature value proves that the digital signature was created before
the date included in the time-stamp or time mark.
To prove the digital signature was generated while the signer's
certificate was valid, the digital signature must be verified and the
following conditions satisfied:
1. the time-stamp (or time mark) was applied before the end of the
validity period of the signer's certificate,
2. the time-stamp (or time mark) was applied either while the
signer's certificate was not revoked or before the revocation
date of the certificate.
Thus a time-stamp (or time mark) applied in this manner proves that
the digital signature was created while the signer's certificate was
valid. This concept proves the validity of a digital signature over
the whole of any certificate chain.
Policy requirements to cover that case is the primary reason of this
document. However, it should be observed that these policy
requirements can be used to address other needs.
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The electronic time stamp is gaining interest from the business
sector as an important component of electronic signatures. It is
also featured by the ETSI Electronic Signature Format standard [TS
101733] or Electronic Signature Formats for long term electronic
signatures [RFC 3126], built upon the Time-Stamp Protocol [RFC 3161].
Agreed minimum security and quality requirements are necessary in
order to ensure trustworthy validation of long-term electronic
signatures.
The European Directive 1999/93/EC [Dir 99/93/EC] defines
certification service provider as "an entity or a legal or natural
person who issues certificates or provides other services related to
electronic signatures". One example of a certification-service-
provider is a Time-Stamping Authority.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119
[RFC 2119].
2. Overview
These policy requirements are aimed at time-stamping services used in
support of qualified electronic signatures (i.e., in line with
article 5.1 of the European Directive on a community framework for
electronic signatures) but may be applied to any application
requiring to prove that a datum existed before a particular time.
These policy requirements are based on the use of public key
cryptography, public key certificates and reliable time sources. The
present document may be used by independent bodies as the basis for
confirming that a TSA may be trusted for providing time-stamping
services.
This document addresses requirements for synchronizing TSAs issuing
time-stamp tokens with Coordinated universal time (UTC) and digitally
signed by TSUs.
Subscriber and relying parties should consult the TSA's practice
statement to obtain further details of precisely how this time-stamp
policy is implemented by the particular TSA (e.g., protocols used in
providing this service).
This document does not specify:
- protocols used to access the TSUs;
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NOTE 1: A time-stamping protocol is defined in RFC 3161 [RFC 3161]
and profiled in TS 101 861 [TS 101861].
- how the requirements identified herein may be assessed by an
independent body;
- requirements for information to be made available to such
independent bodies;
- requirements on such independent bodies.
NOTE 2: See CEN Workshop Agreement 14172 "EESSI Conformity Assessment
Guidance" [CWA 14172].
3. Definitions and Abbreviations
3.1. Definitions
For the purposes of the present document, the following terms and
definitions apply:
NOTE: Where a definition is copied from a referenced document this is
indicated by inclusion of the reference identifier number at the end
of the definition.
relying party: recipient of a time-stamp token who relies on that
time-stamp token.
subscriber: entity requiring the services provided by a TSA and which
has explicitly or implicitly agreed to its terms and
conditions.
time-stamp token: data object that binds a representation of a datum
to a particular time, thus establishing evidence that the datum
existed before that time.
time-stamping authority: authority which issues time-stamp tokens.
TSA Disclosure statement: set of statements about the policies and
practices of a TSA that particularly require emphasis or
disclosure to subscribers and relying parties, for example to
meet regulatory requirements.
TSA practice statement: statement of the practices that a TSA employs
in issuing time-stamp tokens.
TSA system: composition of IT products and components organized to
support the provision of time-stamping services.
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time-stamp policy: named set of rules that indicates the
applicability of a time-stamp token to a particular community
and/or class of application with common security requirements.
time-stamping unit: set of hardware and software which is managed as
a unit and has a single time-stamp token signing key active at
a time.
Coordinated Universal Time (UTC): Time scale based on the second as
defined in ITU-R Recommendation TF.460-5 [TF.460-5].
NOTE: For most practical purposes UTC is equivalent to mean
solar time at the prime meridian. More specifically, UTC is a
compromise between the highly stable atomic time (Temps
Atomique International
- TAI) and solar time derived from the irregular Earth
rotation (related to the Greenwich mean sidereal time (GMST) by
a conventional relationship). (See annex A for more details).
UTC(k): Time-scale realized by the laboratory "k" and kept in close
agreement with UTC, with the goal to reach plus or minus 100
ns. (See ITU-R Recommendation TF.536-1 [TF.536-1]).
NOTE: A list of UTC(k) laboratories is given in section 1 of
Circular T disseminated by BIPM and available from the BIPM
website (http://www.bipm.org/).
3.2. Abbreviations
For the purposes of the present document, the following abbreviations
apply:
TSA Time-Stamping Authority
TSU Time-Stamping Unit
TST Time-Stamp Token
UTC Coordinated Universal Time
4. General Concepts
4.1. Time-Stamping Services
The provision of time-stamping services is broken down into the
following component services for the purposes of classifying
requirements:
- Time-stamping provision: This service component generates
time-stamp tokens.
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- Time-stamping management: The service component that monitors and
controls the operation of the time-stamping services to ensure
that the service is provided as specified by the TSA. This
service component is responsibile for the installation and
de-installation of the time-stamping provision service. For
example, time-stamping management ensures that the clock used for
time-stamping is correctly synchronized with UTC.
This subdivision of services is only for the purposes of clarifying
the requirements specified in the current document and places no
restrictions on any subdivision of an implementation of time-stamping
services.
4.2. Time-Stamping Authority
The authority to issue time-stamp tokens, trusted by the users of the
time-stamping services, i.e., subscribers and relying parties, is
called the Time-Stamping Authority (TSA). TSA has overall
responsibility for time-stamping services identified in clause 4.1.
The TSA has responsibility for the operation of one or more TSU's
which creates and signs on behalf of the TSA. The TSA responsible
for issuing a time-stamp token is identifiable (see 7.3.1 h).
The TSA may use other parties to provide parts of the Time-Stamping
Services. However, the TSA always maintains overall responsibility
and ensures that the policy requirements identified in the present
document are met. For example, a TSA may sub-contract all the
component services, including the services which generate time-stamp
tokens using the TSU's keys. However, the private key or keys used
to generate the time-stamp tokens belong to the TSA which maintains
overall responsibility for meeting the requirements in this document.
A TSA may operate several identifiable time-stamping units. Each
unit has a different key. See Annex B for possible implementations.
A TSA is a certification-service-provider, as defined in the EU
Directive on Electronic Signatures (see article 2(11)), which issues
time-stamp tokens.
4.3. Subscriber
The subscriber may be an organization comprising several end-users or
an individual end-user.
When the subscriber is an organization, some of the obligations that
apply to that organization will have to apply as well to the end-
users. In any case the organization will be held responsible if the
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obligations from the end-users are not correctly fulfilled and
therefore the organization is expected to suitably inform its end
users.
When the subscriber is an end-user, the end-user will be held
directly responsible if its obligations are not correctly fulfilled.
4.4. Time-Stamp Policy and TSA Practice Statement
This section explains the relative roles of Time-stamp policy and TSA
practice statement. It places no restriction on the form of a time-
stamp policy or practice statement specification.
4.4.1. Purpose
In general, the time-stamp policy states "what is to be adhered to,"
while a TSA practice statement states "how it is adhered to", i.e.,
the processes it will use in creating time-stamps and maintaining the
accuracy of its clock. The relationship between the time-stamp
policy and TSA practice statement is similar in nature to the
relationship of other business policies which state the requirements
of the business, while operational units define the practices and
procedures of how these policies are to be carried out.
The present document specifies a time-stamp policy to meet general
requirements for trusted time-stamping services. TSAs specify in TSA
practice statements how these requirements are met.
4.4.2. Level of Specificity
The TSA practice statement is more specific than a time-stamp policy.
A TSA practice statement is a more detailed description of the terms
and conditions as well as business and operational practices of a TSA
in issuing and otherwise managing time-stamping services. The TSA
practice statement of a TSA enforces the rules established by a
time-stamp policy. A TSA practice statement defines how a specific
TSA meets the technical, organizational and procedural requirements
identified in a time-stamp policy.
NOTE: Even lower-level internal documentation may be appropriate for
a TSA detailing the specific procedures necessary to complete the
practices identified in the TSA practice statement.
4.4.3. Approach
The approach of a time-stamp policy is significantly different from a
TSA practice statement. A time-stamp policy is defined independently
of the specific details of the specific operating environment of a
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TSA, whereas a TSA practice statement is tailored to the
organizational structure, operating procedures, facilities, and
computing environment of a TSA. A time-stamp policy may be defined
by the user of times-stamp services, whereas the TSA practice
statement is always defined by the provider.
5. Time-Stamp Policies
5.1. Overview
A time-stamp policy is a "named set of rules that indicates the
applicability of a time-stamp token to a particular community and/or
class of application with common security requirements" (see clauses
3.1 and 4.4).
The present document defines requirements for a baseline time-stamp
policy for TSAs issuing time-stamp tokens, supported by public key
certificates, with an accuracy of 1 second or better.
NOTE 1: Without additional measures the relying party may not be able
to ensure the validity of a time-stamp token beyond the end of the
validity period of the supporting certificate. See Annex C on
verification of the validity of a time-stamp token beyond the
validity period of the TSU's certificate.
A TSA may define its own policy which enhances the policy defined in
this document. Such a policy shall incorporate or further constrain
the requirements identified in this document.
If an accuracy of better than 1 second is provided by a TSA and if
all the TSUs have that same characteristics, then the accuracy shall
be indicated in the TSA's disclosure statement (see section 7.1.2)
that each time-stamp token is issued with an accuracy of better than
1 second.
NOTE 2: It is required that a time-stamp token includes an identifier
for the applicable policy (see section 7.3.1).
5.2. Identification
The object-identifier [X.208] of the baseline time-stamp policy is:
itu-t(0) identified-organization(4) etsi(0) time-stamp-policy(2023)
policy-identifiers(1) baseline-ts-policy (1)
In the TSA disclosure statement made available to subscribers and
relying parties, a TSA shall also include the identifier for the
time-stamp policy to indicate its conformance.
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5.3. User Community and Applicability
This policy is aimed at meeting the requirements of time-stamping
qualified electronic signatures (see European Directive on Electronic
Signatures) for long term validity (e.g., as defined in TS 101 733
[TS 101733]), but is generally applicable to any requirement for an
equivalent quality.
This policy may be used for public time-stamping services or time-
stamping services used within a closed community.
5.4. Conformance
The TSA shall use the identifier for the time-stamp policy in time-
stamp tokens as given in section 5.2, or define its own time-stamp
policy that incorporates or further constrains the requirements
identified in the present document:
a) if the TSA claims conformance to the identified time-stamp policy
and makes available to subscribers and relying parties on request
the evidence to support the claim of conformance; or
b) if the TSA has been assessed to conform to the identified time-
stamp policy by an independent party.
A conformant TSA must demonstrate that:
a) it meets its obligations as defined in section 6.1;
b) it has implemented controls which meet the requirements specified
in section 7.
6. Obligations and Liability
6.1. TSA Obligations
6.1.1. General
The TSA shall ensure that all requirements on TSA, as detailed in
section 7, are implemented as applicable to the selected trusted
time-stamp policy.
The TSA shall ensure conformance with the procedures prescribed in
this policy, even when the TSA functionality is undertaken by sub-
contractors.
The TSA shall also ensure adherence to any additional obligations
indicated in the time-stamp either directly or incorporated by
reference.
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The TSA shall provide all its time-stamping services consistent with
its practice statement.
6.1.2. TSA Obligations Towards Subscribers
The TSA shall meet its claims as given in its terms and conditions
including the availability and accuracy of its service.
6.2. Subscriber Obligations
The current document places no specific obligations on the subscriber
beyond any TSA specific requirements stated in the TSA's terms and
condition.
NOTE: It is advisable that, when obtaining a time-stamp token, the
subscriber verifies that the time-stamp token has been correctly
signed and that the private key used to sign the time-stamp token has
not been compromised.
6.3. Relying Party Obligations
The terms and conditions made available to relying parties (see
section 7.1.2) shall include an obligation on the relying party that,
when relying on a time-stamp token, it shall:
a) verify that the time-stamp token has been correctly signed and
that the private key used to sign the time-stamp has not been
compromised until the time of the verification;
NOTE: During the TSU's certificate validity period, the validity
of the signing key can be checked using current revocation status
for the TSU's certificate. If the time of verification exceeds
the end of the validity period of the corresponding certificate,
see annex C for guidance.
b) take into account any limitations on the usage of the time-stamp
indicated by the time-stamp policy;
c) take into account any other precautions prescribed in agreements
or elsewhere.
6.4. Liability
The present document does not specify any requirement on liability.
In particular, it should be noticed that a TSA may disclaim or limit
any liability unless otherwise stipulated by the applicable law.
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7. Requirements on TSA Practices
The TSA shall implement the controls that meet the following
requirements.
These policy requirements are not meant to imply any restrictions on
charging for TSA services.
The requirements are indicated in terms of the security objectives,
followed by more specific requirements for controls to meet those
objectives where it is necessary to provide confidence that those
objective will be met.
NOTE: The details of controls required to meet an objective is a
balance between achieving the necessary confidence whilst
minimizing the restrictions on the techniques that a TSA may
employ in issuing time-stamp tokens. In the case of section 7.4
(TSA management and operation), a reference is made to a source of
more detailed control requirements. Due to these factors the
specificity of the requirements given under a given topic may
vary.
The provision of a time-stamp token in response to a request is at
the discretion of the TSA depending on any service level agreements
with the subscriber.
7.1. Practice and Disclosure Statements
7.1.1. TSA Practice Statement
The TSA shall ensure that it demonstrates the reliability necessary
for providing time-stamping services.
In particular:
a) The TSA shall have a risk assessment carried out in order to
evaluate business assets and threats to those assets in order to
determine the necessary security controls and operational
procedures.
b) The TSA shall have a statement of the practices and procedures
used to address all the requirements identified in this time-stamp
policy.
NOTE 1: This policy makes no requirement as to the structure of
the TSA practice statement.
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c) The TSA's practice statement shall identify the obligations of all
external organizations supporting the TSA services including the
applicable policies and practices.
d) The TSA shall make available to subscribers and relying parties
its practice statement, and other relevant documentation, as
necessary, to assess conformance to the time-stamp policy.
NOTE 2: The TSA is not generally required to make all the details
of its practices public.
e) The TSA shall disclose to all subscribers and potential relying
parties the terms and conditions regarding use of its time-
stamping services as specified in section 7.1.2.
f) The TSA shall have a high level management body with final
authority for approving the TSA practice statement.
g) The senior management of the TSA shall ensure that the practices
are properly implemented.
h) The TSA shall define a review process for the practices including
responsibilities for maintaining the TSA practice statement.
i) The TSA shall give due notice of changes it intends to make in its
practice statement and shall, following approval as in (f) above,
make the revised TSA practice statement immediately available as
required under (d) above.
7.1.2. TSA Disclosure Statement
The TSA shall disclose to all subscribers and potential relying
parties the terms and conditions regarding use of its time-stamping
services. This statement shall at least specify for each time-stamp
policy supported by the TSA:
a) The TSA contact information.
b) The time-stamp policy being applied.
c) At least one hashing algorithm which may be used to represent the
datum being time-stamped. (No hash algorithm is mandated).
d) The expected life-time of the signature used to sign the time-
stamp token (depends on the hashing algorithm being used, the
signature algorithm being used and the private key length).
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e) The accuracy of the time in the time-stamp tokens with respect to
UTC.
f) Any limitations on the use of the time-stamping service.
g) The subscriber's obligations as defined in section 6.2, if any.
h) The relying party's obligations as defined in section 6.3.
i) Information on how to verify the time-stamp token such that the
relying party is considered to "reasonably rely" on the time-stamp
token (see section 6.3) and any possible limitations on the
validity period.
j) The period of time during which TSA event logs (see section
7.4.10) are retained.
k) The applicable legal system, including any claim to meet the
requirements on time-stamping services under national law.
l) Limitations of liability.
m) Procedures for complaints and dispute settlement.
n) If the TSA has been assessed to be conformant with the identified
time-stamp policy, and if so by which independent body.
NOTE 1: It is also recommended that the TSA includes in its
time-stamping disclosure statement availability of its service,
for example the expected mean time between failure of the time-
stamping service, the mean time to recovery following a failure,
and provisions made for disaster recovery including back-up
services;
This information shall be available through a durable means of
communication. This information shall be available in a readily
understandable language. It may be transmitted electronically.
NOTE 2: A model TSA disclosure statement which may be used as the
basis of such a communication is given in annex D. Alternatively
this may be provided as part of a subscriber / relying party
agreement. These TSA disclosure statements may be included in a
TSA practice statement provided that they are conspicuous to the
reader.
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7.2. Key Management Life Cycle
7.2.1. TSA Key Generation
The TSA shall ensure that any cryptographic keys are generated in
under controlled circumstances.
In particular:
a) The generation of the TSU's signing key(s) shall be undertaken in
a physically secured environment (see section 7.4.4) by personnel
in trusted roles (see section 7.4.3) under, at least, dual
control. The personnel authorized to carry out this function
shall be limited to those requiring to do so under the TSA's
practices.
b) The generation of the TSU's signing key(s) shall be carried out
within a cryptographic module(s) which either:
- meets the requirements identified in FIPS 140-1 [FIPS 140-1]
level 3 or higher, or
- meets the requirements identified in CEN Workshop Agreement
14167-2 [CWA 14167-2], or
- is a trustworthy system which is assured to EAL 4 or higher in
accordance to ISO 15408 [ISO 15408], or equivalent security
criteria. This shall be to a security target or protection
profile which meets the requirements of the current document,
based on a risk analysis and taking into account physical and
other non-technical security measures.
c) The TSU key generation algorithm, the resulting signing key length
and signature algorithm used for signing time-stamp tokens key
shall be recognized by any national supervisory body, or in
accordance with existing current state of art, as being fit for
the purposes of time-stamp tokens as issued by the TSA.
7.2.2. TSU Private Key Protection
The TSA shall ensure that TSU private keys remain confidential and
maintain their integrity.
In particular:
a) The TSU private signing key shall be held and used within a
cryptographic module which:
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- meets the requirements identified in FIPS 140-1 [FIPS 140-1]
level 3 or higher; or
- meets the requirements identified in CEN Workshop Agreement
14167-2 [CWA 14167-2]; or
- is a trustworthy system which is assured to EAL 4 or higher in
accordance to ISO 15408 [ISO 15408], or equivalent security
criteria. This shall be a security target or protection profile
which meets the requirements of the current document, based on
a risk analysis and taking into account physical and other
non-technical security measures.
NOTE: Backup of TSU private keys is deprecated in order to
minimize risk of key compromise.
b) If TSU private keys are backed up, they shall be copied, stored
and recovered only by personnel in trusted roles using, at least,
dual control in a physically secured environment. (see section
7.4.4). The personnel authorized to carry out this function shall
be limited to those requiring to do so under the TSA's practices.
c) Any backup copies of the TSU private signing keys shall be
protected to ensure its confidentiality by the cryptographic
module before being stored outside that device.
7.2.3. TSU Public Key Distribution
The TSA shall ensure that the integrity and authenticity of the TSU
signature verification (public) keys and any associated parameters
are maintained during its distribution to relying parties.
In particular:
a) TSU signature verification (public) keys shall be made available
to relying parties in a public key certificate.
NOTE: For example, TSU's certificates may be issued by a
certification authority operated by the same organization as the
TSA, or issued by another authority.
b) The TSU's signature verification (public) key certificate shall be
issued by a certification authority operating under a certificate
policy which provides a level of security equivalent to, or higher
than, this time-stamping policy.
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7.2.4. Rekeying TSU's Key
The life-time of TSU's certificate shall be not longer than the
period of time that the chosen algorithm and key length is recognized
as being fit for purpose (see section 7.2.1c)).
NOTE 1: The following additional considerations apply when limiting
that lifetime:
- Section 7.4.10 requires that records concerning time-stamping
services shall be held for a period of time,as appropriate, for at
least 1 year after the expiration of the validity of the TSU's
signing keys. The longer the validity period of the TSU
certificates will be, the longer the size of the records to be
kept will be.
- Should a TSU private key be compromised, then the longer the
life-time, the more affected time-stamp tokens there will be.
NOTE 2: TSU key compromise does not only depend on the
characteristics of the cryptographic module being used but also on
the procedures being used at system initialization and key export
(when that function is supported).
7.2.5. End of TSU Key Life Cycle
The TSA shall ensure that TSU private signing keys are not used
beyond the end of their life cycle.
In particular:
a) Operational or technical procedures shall be in place to ensure
that a new key is put in place when a TSU's key expires.
b) The TSU private signing keys, or any key part, including any
copies shall be destroyed such that the private keys cannot be
retrieved.
c) The TST generation system SHALL reject any attempt to issue TSTs
if the signing private key has expired.
7.2.6. Life Cycle Management of the Cryptographic Module used to Sign
Time-Stamps
The TSA shall ensure the security of cryptographic hardware
throughout its lifecycle.
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In particular the TSA shall ensure that:
a) Time-stamp token signing cryptographic hardware is not tampered
with during shipment;
b) Time-stamp token signing cryptographic hardware is not tampered
with while stored;
c) Installation, activation and duplication of TSU's signing keys in
cryptographic hardware shall be done only by personnel in trusted
roles using, at least, dual control in a physically secured
environment. (see section 7.4.4);
d) Time-stamp token signing cryptographic hardware is functioning
correctly; and
e) TSU private signing keys stored on TSU cryptographic module are
erased upon device retirement.
7.3. Time-Stamping
7.3.1. Time-Stamp Token
The TSA shall ensure that time-stamp tokens are issued securely and
include the correct time.
In particular:
a) The time-stamp token shall include an identifier for the time-
stamp policy;
b) Each time-stamp token shall have a unique identifier;
c) The time values the TSU uses in the time-stamp token shall be
traceable to at least one of the real time values distributed by a
UTC(k) laboratory.
NOTE 1: The Bureau International des Poids et Mesures (BIPM)
computes UTC on the basis of its local representations UTC(k) from
a large ensemble of atomic clocks in national metrology institutes
and national astronomical observatories round the world. The BIPM
disseminates UTC through its monthly Circular T [list 1]. This is
available on the BIPM website (www.bipm.org) and it officially
identifies all those institutes having recognized UTC(k) time
scales.
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d) The time included in the time-stamp token shall be synchronized
with UTC within the accuracy defined in this policy and, if
present, within the accuracy defined in the time-stamp token
itself;
e) If the time-stamp provider's clock is detected (see section
7.3.2c)) as being out of the stated accuracy (see section 7.1.2e))
then time-stamp tokens shall not be issued.
f) The time-stamp token shall include a representation (e.g., hash
value) of the datum being time-stamped as provided by the
requestor;
g) The time-stamp token shall be signed using a key generated
exclusively for this purpose.
NOTE 2: A protocol for a time-stamp token is defined in RFC 3631
and profiled in TS 101 861 [TS 101861].
NOTE 3: In the case of a number of requests at approximately the
same time, the ordering of the time within the accuracy of the TSU
clock is not mandated.
h) The time-stamp token shall include:
- where applicable, an identifier for the country in which the
TSA is established;
- an identifier for the TSA;
- an identifier for the unit which issues the time-stamps.
7.3.2. Clock Synchronization with UTC
The TSA shall ensure that its clock is synchronized with UTC within
the declared accuracy.
In particular:
a) The calibration of the TSU clocks shall be maintained such that
the clocks shall not be expected to drift outside the declared
accuracy.
b) The TSU clocks shall be protected against threats which could
result in an undetected change to the clock that takes it outside
its calibration.
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NOTE 1: Threats may include tampering by unauthorized personnel,
radio or electrical shocks.
c) The TSA shall ensure that, if the time that would be indicated in
a time-stamp token drifts or jumps out of synchronization with
UTC, this will be detected (see also 7.3.1e)).
NOTE 2: Relying parties are required to be informed of such events
(see section 7.4.8).
d) The TSA shall ensure that clock synchronization is maintained when
a leap second occurs as notified by the appropriate body. The
change to take account of the leap second shall occur during the
last minute of the day when the leap second is scheduled to occur.
A record shall be maintained of the exact time (within the
declared accuracy) when this change occurred. See annex A for
more details.
NOTE 3: A leap second is an adjustment to UTC by skipping or
adding an extra second on the last second of a UTC month. First
preference is given to the end of December and June, and second
preference is given to the end of March and September.
7.4. TSA Management and Operation
7.4.1. Security Management
The TSA shall ensure that the administrative and management
procedures applied are adequate and correspond to recognized best
practice.
In particular:
TSA General
a) The TSA shall retain responsibility for all aspects of the
provision of time-stamping services within the scope of this
time-stamp policy, whether or not functions are outsourced to
subcontractors. Responsibilities of third parties shall be
clearly defined by the TSA and appropriate arrangements made to
ensure that third parties are bound to implement any controls
required by the TSA. The TSA shall retain responsibility for the
disclosure of relevant practices of all parties.
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b) The TSA management shall provide direction on information security
through a suitable high level steering forum that is responsible
for defining the TSA's information security policy. The TSA shall
ensure publication and communication of this policy to all
employees who are impacted by it.
c) The information security infrastructure necessary to manage the
security within the TSA shall be maintained at all times. Any
changes that will impact on the level of security provided shall
be approved by the TSA management forum.
NOTE 1: See ISO/IEC 17799 [ISO 17799] for guidance on information
security management including information security infrastructure,
management information security forum and information security
policies.
d) The security controls and operating procedures for TSA facilities,
systems and information assets providing the time-stamping
services shall be documented, implemented and maintained.
NOTE 2: The present documentation (commonly called a system
security policy or manual) should identify all relevant targets,
objects and potential threats related to the services provided and
the safeguards required to avoid or limit the effects of those
threats, consistent with the Risk Assessment required under
section 7.1.1a). It should describe the rules, directives and
procedures regarding how the specified services and the associated
security assurance are granted in addition to stating policy on
incidents and disasters.
e) TSA shall ensure that the security of information is maintained
when the responsibility for TSA functions has been outsourced to
another organization or entity.
7.4.2. Asset Classification and Management
The TSA shall ensure that its information and other assets receive an
appropriate level of protection.
In particular:
- The TSA shall maintain an inventory of all assets and shall assign
a classification for the protection requirements to those assets
consistent with the risk analysis.
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7.4.3. Personnel Security
The TSA shall ensure that personnel and hiring practices enhance and
support the trustworthiness of the TSA's operations.
In particular (TSA general):
a) The TSA shall employ personnel which possess the expert knowledge,
experience and qualifications necessary for the offered services
and as appropriate to the job function.
NOTE 1: TSA personnel should be able to fulfill the requirement of
"expert knowledge, experience and qualifications" through formal
training and credentials, actual experience, or a combination of
the two.
NOTE 2: Personnel employed by a TSA include individual personnel
contractually engaged in performing functions in support of the
TSA's time-stamping services. Personnel who may be involved in
monitoring the TSA services need not be TSA personnel.
b) Security roles and responsibilities, as specified in the TSA's
security policy, shall be documented in job descriptions. Trusted
roles, on which the security of the TSA's operation is dependent,
shall be clearly identified.
c) TSA personnel (both temporary and permanent) shall have job
descriptions defined from the view point of separation of duties
and least privilege, determining position sensitivity based on the
duties and access levels, background screening and employee
training and awareness. Where appropriate, these shall
differentiate between general functions and TSA specific
functions. These should include skills and experience
requirements.
d) Personnel shall exercise administrative and management procedures
and processes that are in line with the TSA's information security
management procedures (see section 7.4.1).
NOTE 3: See ISO/IEC 17799 [ISO 17799] for guidance.
The following additional controls shall be applied to time-
stamping management:
e) Managerial personnel shall be employed who possess:
- knowledge of time-stamping technology; and
- knowledge of digital signature technology; and
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- knowledge of mechanisms for calibration or synchronization the
TSU clocks with UTC; and
- familiarity with security procedures for personnel with security
responsibilities; and
- experience with information security and risk assessment.
f) All TSA personnel in trusted roles shall be free from conflict of
interest that might prejudice the impartiality of the TSA
operations.
g) Trusted roles include roles that involve the following
responsibilities:
- Security Officers: Overall responsibility for administering the
implementation of the security practices.
- System Administrators: Authorized to install, configure and
maintain the TSA trustworthy systems for time-stamping
management.
- System Operators: Responsible for operating the TSA trustworthy
systems on a day-to-day basis. Authorized to perform system
backup and recovery.
- System Auditors: Authorized to view archives and audit logs of
the TSA trustworthy systems.
h) TSA personnel shall be formally appointed to trusted roles by
senior management responsible for security.
i) The TSA shall not appoint to trusted roles or management any
person who is known to have a conviction for a serious crime or
other offense which affects his/her suitability for the position.
Personnel shall not have access to the trusted functions until any
necessary checks are completed.
NOTE 4: In some countries it may not be possible for TSA to obtain
information on past convictions without the collaboration of the
candidate employee.
7.4.4. Physical and Environmental Security
The TSA shall ensure that physical access to critical services is
controlled and physical risks to its assets minimized.
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In particular (general):
a) For both the time-stamping provision and the time-stamping
management:
- physical access to facilities concerned with time-stamping
services shall be limited to properly authorized individuals;
- controls shall be implemented to avoid loss, damage or
compromise of assets and interruption to business activities;
and
- controls shall be implemented to avoid compromise or theft of
information and information processing facilities.
b) Access controls shall be applied to the cryptographic module to
meet the requirements of security of cryptographic modules as
identified in clauses 7.2.1 and 7.2.2.
c) The following additional controls shall be applied to time-
stamping management:
- The time-stamping management facilities shall be operated in an
environment which physically protects the services from
compromise through unauthorized access to systems or data.
- Physical protection shall be achieved through the creation of
clearly defined security perimeters (i.e., physical barriers)
around the time-stamping management. Any parts of the premises
shared with other organizations shall be outside this
perimeter.
- Physical and environmental security controls shall be
implemented to protect the facility that houses system
resources, the system resources themselves, and the facilities
used to support their operation. The TSA's physical and
environmental security policy for systems concerned with time-
stamping management shall address as a minimum the physical
access control, natural disaster protection, fire safety
factors, failure of supporting utilities (e.g., power,
telecommunications), structure collapse, plumbing leaks,
protection against theft, breaking and entering, and disaster
recovery.
- Controls shall be implemented to protect against equipment,
information, media and software relating to the time-stamping
services being taken off-site without authorization.
NOTE 1: See ISO/IEC 17799 [ISO 17799] for guidance on physical and
environmental security.
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NOTE 2: Other functions may be supported within the same secured
area provided that the access is limited to authorized personnel.
7.4.5. Operations Management
The TSA shall ensure that the TSA system components are secure and
correctly operated, with minimal risk of failure:
In particular (general):
a) The integrity of TSA system components and information shall be
protected against viruses, malicious and unauthorized software.
b) Incident reporting and response procedures shall be employed in
such a way that damage from security incidents and malfunctions
shall be minimized.
c) Media used within the TSA trustworthy systems shall be securely
handled to protect media from damage, theft, unauthorized access
and obsolescence.
NOTE 1: Every member of personnel with management responsibilities
is responsible for planning and effectively implementing the
time-stamp policy and associated practices as documented in the
TSA practice statement.
d) Procedures shall be established and implemented for all trusted
and administrative roles that impact on the provision of time-
stamping services.
Media handling and security
e) All media shall be handled securely in accordance with
requirements of the information classification scheme (see section
7.4.2). Media containing sensitive data shall be securely
disposed of when no longer required.
System Planning
f) Capacity demands shall be monitored and projections of future
capacity requirements made to ensure that adequate processing
power and storage are available.
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Incident reporting and response
g) The TSA shall act in a timely and coordinated manner in order to
respond quickly to incidents and to limit the impact of breaches
of security. All incidents shall be reported as soon as possible
after the incident.
The following additional controls shall be applied to time-stamping
management:
Operations procedures and responsibilities
h) TSA security operations shall be separated from other operations.
NOTE 2: TSA security operations' responsibilities include:
- operational procedures and responsibilities;
- secure systems planning and acceptance;
- protection from malicious software;
- housekeeping;
- network management;
- active monitoring of audit journals, event analysis and
follow-up;
- media handling and security;
- data and software exchange.
These operations shall be managed by TSA trusted personnel, but, may
actually be performed by, non-specialist, operational personnel
(under supervision), as defined within the appropriate security
policy, and, roles and responsibility documents.
7.4.6. System Access Management
The TSA shall ensure that TSA system access is limited to properly
authorized individuals.
In particular (general):
a) Controls (e.g., firewalls) shall be implemented to protect the
TSA's internal network domains from unauthorized access including
access by subscribers and third parties.
NOTE 1: Firewalls should also be configured to prevent all
protocols and accesses not required for the operation of the TSA.
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b) The TSA shall ensure effective administration of user (this
includes operators, administrators and auditors) access to
maintain system security, including user account management,
auditing and timely modification or removal of access.
c) The TSA shall ensure that access to information and application
system functions is restricted in accordance with the access
control policy and that the TSA system provides sufficient
computer security controls for the separation of trusted roles
identified in TSA's practices, including the separation of
security administrator and operation functions. Particularly, use
of system utility programs is restricted and tightly controlled.
d) TSA personnel shall be properly identified and authenticated
before using critical applications related to time-stamping.
e) TSA personnel shall be accountable for their activities, for
example by retaining event logs (see section 7.4.10).
The following additional controls shall be applied to time-stamping
management:
f) The TSA shall ensure that local network components (e.g., routers)
are kept in a physically secure environment and that their
configurations are periodically audited for compliance with the
requirements specified by the TSA.
g) Continuous monitoring and alarm facilities shall be provided to
enable the TSA to detect, register and react in a timely manner upon
any unauthorized and/or irregular attempts to access its resources.
NOTE 2: This may use, for example, an intrusion detection system,
access control monitoring and alarm facilities.
7.4.7. Trustworthy Systems Deployment and Maintenance
The TSA shall use trustworthy systems and products that are protected
against modification.
NOTE: The risk analysis carried out on the TSA's services (see
section 7.1.1) should identify its critical services requiring
trustworthy systems and the levels of assurance required.
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In particular:
a) An analysis of security requirements shall be carried out at the
design and requirements specification stage of any systems
development project undertaken by the TSA or on behalf of the TSA
to ensure that security is built into IT systems.
b) Change control procedures shall be applied for releases,
modifications and emergency software fixes of any operational
software.
7.4.8. Compromise of TSA Services
The TSA shall ensure in the case of events which affect the security
of the TSA's services, including compromise of TSU's private signing
keys or detected loss of calibration, that relevant information is
made available to subscribers and relying parties.
In particular:
a) The TSA's disaster recovery plan shall address the compromise or
suspected compromise of TSU's private signing keys or loss of
calibration of a TSU clock, which may have affected time-stamp
tokens which have been issued.
b) In the case of a compromise, or suspected compromise or loss of
calibration the TSA shall make available to all subscribers and
relying parties a description of compromise that occurred.
c) In the case of compromise to a TSU's operation (e.g., TSU key
compromise), suspected compromise or loss of calibration the TSU
shall not issue time-stamp tokens until steps are taken to recover
from the compromise
d) In case of major compromise of the TSA's operation or loss of
calibration, wherever possible, the TSA shall make available to
all subscribers and relying parties information which may be used
to identify the time-stamp tokens which may have been affected,
unless this breaches the privacy of the TSAs users or the security
of the TSA services.
NOTE: 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 backdated tokens. Two
time-stamp tokens from two different TSAs may be another way to
address this issue.
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7.4.9. TSA Termination
The TSA shall ensure that potential disruptions to subscribers and
relying parties are minimized as a result of the cessation of the
TSA's time-stamping services, and in particular ensure continued
maintenance of information required to verify the correctness of
time-stamp tokens.
In particular:
a) Before the TSA terminates its time-stamping services the following
procedures shall be executed as a minimum:
- the TSA shall make available to all subscribers and relying
parties information concerning its termination;
- TSA shall terminate authorization of all subcontractors to act
on behalf of the TSA in carrying out any functions relating to
the process of issuing time-stamp tokens;
- the TSA shall transfer obligations to a reliable party for
maintaining event log and audit archives (see section 7.4.10)
necessary to demonstrate the correct operation of the TSA for a
reasonable period;
- the TSA shall maintain or transfer to a reliable party its
obligations to make available its public key or its
certificates to relying parties for a reasonable period;
- TSU private keys, including backup copies, shall be destroyed
in a manner such that the private keys cannot be retrieved.
b) The TSA shall have an arrangement to cover the costs to fulfill
these minimum requirements in case the TSA becomes bankrupt or for
other reasons is unable to cover the costs by itself.
c) The TSA shall state in its practices the provisions made for
termination of service. This shall include:
- notification of affected entities;
- transferring the TSA obligations to other parties.
d) The TSA shall take steps to have the TSU's certificates revoked.
7.4.10. Compliance with Legal Requirements
The TSA shall ensure compliance with legal requirements.
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In particular:
a) The TSA shall ensure that the requirements of the European data
protection Directive [Dir 95/46/EC], as implemented through
national legislation, are met.
b) Appropriate technical and organizational measures shall be taken
against unauthorized or unlawful processing of personal data and
against accidental loss or destruction of, or damage to, personal
data.
c) The information contributed by users to the TSA shall be
completely protected from disclosure unless with their agreement
or by court order or other legal requirement.
7.4.11. Recording of Information Concerning Operation of Time-Stamping
Services
The TSA shall ensure that all relevant information concerning the
operation of time-stamping services is recorded for a defined period
of time, in particular for the purpose of providing evidence for the
purposes of legal proceedings.
In particular:
General
a) The specific events and data to be logged shall be documented by
the TSA.
b) The confidentiality and integrity of current and archived records
concerning operation of time-stamping services shall be
maintained.
c) Records concerning the operation of time-stamping services shall
be completely and confidentially archived in accordance with
disclosed business practices.
d) Records concerning the operation of time-stamping services shall
be made available if required for the purposes of providing
evidence of the correct operation of the time-stamping services
for the purpose of legal proceedings.
e) The precise time of significant TSA environmental, key management
and clock synchronization events shall be recorded.
f) Records concerning time-stamping services shall be held for a
period of time after the expiration of the validity of the TSU's
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signing keys as appropriate for providing necessary legal evidence
and as notified in the TSA disclosure statement (see section
7.1.2).
g) The events shall be logged in a way that they cannot be easily
deleted or destroyed (except if reliably transferred to long-term
media) within the period of time that they are required to be
held.
NOTE: This may be achieved, for example, through the use of
write-only media, a record of each removable media used and the
use of off-site backup.
h) Any information recorded about subscribers shall be kept
confidential except as where agreement is obtained from the
subscriber for its wider publication.
TSU key management
i) Records concerning all events relating to the life-cycle of TSU
keys shall be logged.
j) Records concerning all events relating to the life-cycle of TSU
certificates (if appropriate) shall be logged.
Clock Synchronization
k) Records concerning all events relating to synchronization of a
TSU's clock to UTC shall be logged. This shall include
information concerning normal re-calibration or synchronization of
clocks use in time-stamping.
l) Records concerning all events relating to detection of loss of
synchronization shall be logged.
7.5. Organizational
The TSA shall ensure that its organization is reliable.
In particular that:
a) Policies and procedures under which the TSA operates shall be
non-discriminatory.
b) The TSA shall make its services accessible to all applicants whose
activities fall within its declared field of operation and that
agree to abide by their obligations as specified in the TSA
disclosure statement.
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c) The TSA is a legal entity according to national law.
d) The TSA has a system or systems for quality and information
security management appropriate for the time-stamping services it
is providing.
e) The TSA has adequate arrangements to cover liabilities arising
from its operations and/or activities.
f) It has the financial stability and resources required to operate
in conformity with this policy.
NOTE 1: This includes requirements for TSA termination identified
in section 7.4.9.
g) It employs a sufficient number of personnel having the necessary
education, training, technical knowledge and experience relating
to the type, range and volume of work necessary to provide time-
stamping services.
NOTE 2: Personnel employed by a TSA include individual personnel
contractually engaged in performing functions in support of the
TSA's time-stamping services. Personnel who may be involved only
in monitoring the TSA services need not be TSA personnel.
h) It has policies and procedures for the resolution of complaints
and disputes received from customers or other parties about the
provisioning of the time-stamping services or any other related
matters.
i) It has a properly documented agreement and contractual
relationship in place where the provisioning of services involves
subcontracting, outsourcing or other third party arrangements.
8. Security Considerations
When verifying time-stamp tokens it is necessary for the verifier to
ensure that the TSU certificate is trusted and not revoked. This
means that the security is dependent upon the security of the CA that
has issued the TSU certificate for both issuing the certificate and
providing accurate revocation status information for that
certificate.
When a time-stamp is verified as valid at a given point of time, this
does not mean that it will necessarily remain valid later on. Every
time, a time-stamp token is verified during the validity period of
the TSU certificate, it must be verified again against the current
revocation status information, since in case of compromise of a TSU
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private key, all the time-stamp tokens generated by that TSU become
invalid. Annex C provides guidance about the long term verification
of time-stamp tokens.
In applying time-stamping to applications, consideration also needs
to be given to the security of the application. In particular, when
applying time-stamps it is necessary to ensure that the integrity of
data is maintained before the time-stamp is applied. The requester
ought to really make sure that the hash value included in the time-
stamp token matches with the hash of the data.
9. Acknowledgments
The development of this document was supported by ETSI and the
European Commission. Special thanks are due to Franco Ruggieri for
his valuable inputs.
10. References
10.1. Normative References
[RFC 2119] Bradner, S. "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[TF.460-5] ITU-R Recommendation TF.460-5 (1997): Standard-
frequency and time-signal emissions.
[TF.536-1] ITU-R Recommendation TF.536-1 (1998): Time-scale
notations.
[CWA 14167-2] CEN Workshop Agreement 14167-2: Cryptographic Module
for CSP Signing Operations - Protection Profile
(MCSO-PP).
[FIPS 140-1] FIPS PUB 140-1 (1994): Security Requirements for
Cryptographic Modules.
[ISO 15408] ISO/IEC 15408 (1999) (parts 1 to 3): Information
technology - Security techniques and Evaluation
criteria for IT security.
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10.2. Informative References
[CWA 14172] CEN Workshop Agreement 14172: EESSI Conformity
Assessment Guidance.
[Dir 95/46/EC] Directive 95/46/EC of the European Parliament and of
the Council of 24 October 1995 on the protection of
individuals with regard to the processing of personal
data and on the free movement of such data.
[Dir 99/93/EC] Directive 1999/93/EC of the European Parliament and of
the Council of 13 December 1999 on a Community
framework for electronic signatures.
[ISO 17799] ISO/IEC 17799: Information technology Code of practice
for information security management
[RFC 3126] Pinkas, D., Ross, J. and N. Pope, "Electronic
Signature Formats for long term electronic
signatures", RFC 3126, September 2001.
[RFC 3161] Adams, C., Cain, P., Pinkas, D. and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
[TS 101733] ETSI Technical Specification TS 101 733 V.1.2.2
(2000-12) Electronic Signature Formats. Note: copies
of ETSI TS 101 733 can be freely downloaded from the
ETSI web site www.etsi.org.
[TS 101861] ETSI Technical Specification TS 101 861 V1.2.1.
(2001-11). Time stamping profile. Note: copies of
ETSI TS 101 861 can be freely downloaded from the ETSI
web site www.etsi.org.
[TS 102023] ETSI Technical Specification TS 102 023. Policy
requirements for Time-Stamping Authorities. Note:
copies of ETSI TS 102 023 can be freely downloaded
from the ETSI web site www.etsi.org.
[X.208] CCITT Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1), 1988.
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Annex A (informative): Coordinated Universal Time
Coordinated Universal Time (UTC) is the international time standard
that became effective on January 1, 1972. UTC has superseded
Greenwich Mean Time (GMT), but in practice they are never more than 1
second different. Hence many people continue to refer to GMT when in
fact they operate to UTC.
Zero (0) hours UTC is midnight in Greenwich, England, which lies on
the zero longitudinal meridian. Universal time is based on a 24 hour
clock, therefore, afternoon hours such as 4 pm UTC are expressed as
16:00 UTC (sixteen hours, zero minutes).
International Atomic Time (TAI) is calculated by the Bureau
International des Poids et Mesures (BIPM) from the readings of more
than 200 atomic clocks located in metrology institutes and
observatories in more than 30 countries around the world.
Information on TAI is made available every month in the BIPM Circular
T (ftp://62.161.69.5/pub/tai/publication). It is that TAI does not
lose or gain with respect to an imaginary perfect clock by more than
about one tenth of a microsecond (0.0000001 second) per year.
Coordinated Universal Time (UTC): Time scale, based on the second, as
defined and recommended by the International Telecommunications Radio
Committee (ITU-R), and maintained by the Bureau International des
Poids et Mesures (BIPM). The maintenance by BIPM includes
cooperation among various national laboratories around the world.
The full definition of UTC is contained in ITU-R Recommendation
TF.460-4.
Atomic Time, with the unit of duration the Systeme International (SI)
second defined as the duration of 9 192 631 770 cycles of radiation,
corresponds to the transition between two hyperfine levels of the
ground state of caesium 133. TAI is the International Atomic Time
scale, a statistical timescale based on a large number of atomic
clocks.
Universal Time (UT) is counted from 0 hours at midnight, with unit of
duration the mean solar day, defined to be as uniform as possible
despite variations in the rotation of the Earth.
- UT0 is the rotational time of a particular place of
observation. It is observed as the diurnal motion of stars or
extraterrestrial radio sources.
- UT1 is computed by correcting UT0 for the effect of polar
motion on the longitude of the observing site. It varies from
uniformity because of the irregularities in the Earth's
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rotation. UT1, is based on the somewhat irregular rotation of
the Earth. Rotational irregularities usually result in a net
decrease in the Earth's average rotational velocity, and
ensuing lags of UT1 with respect to UTC.
Coordinated Universal Time (UTC) is the basis for international
time-keeping and follows TAI exactly except for an integral number of
seconds, 32 in year 2001. These leap seconds are inserted on the
advice of the International Earth Rotation Service (IERS)
(http://hpiers.obspm.fr/) to ensure that, having taken into account
irregularities, the Sun is overhead within 0,9 seconds of 12:00:00
UTC on the meridian of Greenwich. UTC is thus the modern successor
of Greenwich Mean Time, GMT, which was used when the unit of time was
the mean solar day.
Adjustments to the atomic, i.e., UTC, time scale consist of an
occasional addition or deletion of one full second, which is called a
leap second. Twice yearly, during the last minute of the day of June
30 and December 31, Universal Time, adjustments may be made to ensure
that the accumulated difference between UTC and UT1 will not exceed
0,9 s before the next scheduled adjustment. Historically,
adjustments, when necessary, have usually consisted of adding an
extra second to the UTC time scale in order to allow the rotation of
the Earth to "catch up". Therefore, the last minute of the UTC time
scale, on the day when an adjustment is made, will have 61 seconds.
Adjustments dates are typically announced several months in advance
in IERS Bulletin C:
ftp://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat.
Coordinated Universal Time (UTC) differs thus from TAI by an integral
number of seconds. UTC is kept within 0,9 s of UT1 by the
introduction of one-second steps to UTC, the "leap second". To date
these steps have always been positive.
Annex B (informative): Possible for Implementation Architectures
and Time-Stamping Services
B.1. Managed Time-Stamping Service
Some organizations may be willing to host one or more Time-Stamping
Units in order to take advantage of both the proximity and the
quality of the Time-Stamping Service, without being responsible for
the installation, operation and management of these Time-Stamping
Units.
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This can be achieved by using units that are installed in the
premises from the hosting organization and then remotely managed by a
Time-Stamping Authority that takes the overall responsibility of the
quality of the service delivered to the hosting organization.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ +
+ Time-Stamping Authority +
+_____________ _____________ _____________+
|+ __________ | | | | __________ +|
|+| | | | Time - | | | |+|
|+| Time - |<-------------| Stamping |------------->| Time - |+|
|+| Stamping | | Install. | Management | Install. | | Stamping |+|
|+| Unit | | Management | | Management | | Unit |+|
|+|__________| | |_____________| | |__________|+|
|+ | | +|
|+ | | +|
|+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++|
| Hosting | | Hosting |
| Organization | | Organization |
|______________| |______________|
Figure B.1: Managed Time-stamping Service
The requirements for time-stamping services described in the current
document includes requirements on both the time-stamping management
and for the operation of the unit which issues the time-stamp tokens.
The TSA, as identified in the time-stamp token, has the
responsibility to ensure that these requirements are met (for example
through contractual obligations).
It should be clear that the hosting organization will generally want
to be able to monitor the use of the service and, at a minimum, know
whether the service is working or not and even be able to measure the
performances of the service, e.g., the number of time-stamps
generated during some period of time. Such monitoring can be
considered to be outside of TSA's time-stamping authority.
Therefore the description of the management operation described in
the main body of the document is not limitative. Monitoring
operations, if performed directly on the unit, may be permitted by
the Time-Stamping service provider.
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B.2. Selective Alternative Quality
Some relying parties may be willing to take advantage of particular
characteristics from a time-stamp token such as a specific signature
algorithm and/or key length or a specific accuracy for the time
contained in the time stamp token. These parameters can be
considered as specifying a "quality" for the time stamp token.
Time stamp tokens with various qualities may be issued by different
time-stamping units operated by the same or different TSAs.
A particular time-stamping unit will only provide one combination of
algorithm and key length (since a time-stamping unit is a set of
hardware and software which is managed as a unit and has a single
time-stamp token signing key). In order to obtain different
combinations of algorithm and key length, different time-stamping
units shall be used.
A particular time-stamping unit may provide a fixed accuracy for the
time contained in the time stamp token or different accuracy if
instructed to do so either by using a specific mode of access (e.g.,
e-mail or http) or by using specific parameters in the request.
Annex C (informative): Long Term Verification of Time-Stamp Tokens
Usually, a time-stamp token becomes unverifiable beyond the end of
the validity period of the certificate from the TSU, because the CA
that has issued the certificate does not warrant any more that it
will publish revocation data, including data about revocations due to
key compromises. However, verification of a time-stamp token might
still be performed beyond the end of the validity period of the
certificate from the TSU, if, at the time of verification, it can be
known that:
- the TSU private key has not been compromised at any time up to
the time that a relying part verifies a time-stamp token;
- the hash algorithms used in the time-stamp token exhibits no
collisions at the time of verification;
- the signature algorithm and signature key size under which the
time-stamp token has been signed is still beyond the reach of
cryptographic attacks at the time of verification.
If these conditions cannot be met, then the validity may be
maintained by applying an additional time-stamp to protect the
integrity of the previous one.
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The present document does not specify the details of how such
protection may be obtained. For the time being, and until some
enhancements are defined to support these features, the information
may be obtained using-out-of bands means or alternatively in the
context of closed environments. As an example, should a CA guaranty
to maintain the revocation status of TSU certificates after the end
of its validity period, this would fulfill the first requirement.
NOTE 1: An alternative to Time-Stamping is for a Trusted Service
Provider to record a representation of a datum bound to a particular
time in an audit trail, thus establishing evidence that the datum
existed before that time. This technique, which is called Time-
Marking, can be a valuable alternative for checking the long term
validity of signatures.
NOTE 2: The TSA or other trusted third party service provider may
support the verification of time-stamp tokens.
Annex D (informative): Model TSA Disclosure Statement Structure.
The TSA disclosure statement contains a section for each defined
statement type. Each section of a TSA disclosure statement contains
a descriptive statement, which MAY include hyperlinks to the relevant
certificate policy/certification practice statement sections.
D.1. STATEMENT TYPE: Entire agreement
STATEMENT DESCRIPTION: A statement indicating that the
disclosure statement is not the entire agreement, but only a
part of it.
D.2. STATEMENT TYPE: TSA contact info
STATEMENT DESCRIPTION: The name, location and relevant contact
information for the TSA.
D.3. STATEMENT TYPE: time-stamp token types and usage
STATEMENT DESCRIPTION: A description of each class/type of
time-stamp tokens issued by the TSA (in accordance with each
time-stamp policy) and any restrictions on time-stamp usage.
SPECIFIC REQUIREMENT: Indication of the policy being applied,
including the contexts for which the time-stamp token can be
used (e.g., only for use with electronic signatures), the
hashing algorithms, the expected life time of the time-stamp
token signature, any limitations on the use of the time-stamp
token and information on how to verify the time-stamp token.
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D.4. STATEMENT TYPE: Reliance limits.
STATEMENT DESCRIPTION: reliance limits, if any.
SPECIFIC REQUIREMENT: Indication of the accuracy of the time in
the time-stamp token, and the period of time for which TSA
event logs (see section 7.4.10) are maintained (and hence are
available to provide supporting evidence).
D.5. STATEMENT TYPE: Obligations of subscribers.
STATEMENT DESCRIPTION: The description of, or reference to, the
critical subscriber obligations.
SPECIFIC REQUIREMENT: No specific requirements identified in
the current document. Where applicable the TSA may specify
additional obligations.
D.6. STATEMENT TYPE: TSU public key status checking obligations of
relying parties.
STATEMENT DESCRIPTION: The extent to which relying parties are
obligated to check the TSU public key status, and references to
further explanation.
SPECIFIC REQUIREMENT: Information on how to validate the TSU
public key status, including requirements to check the
revocation status of TSU public key, such that the relying
party is considered to "reasonably rely" on the time-stamp
token (see section 6.3).
D.7. STATEMENT TYPE: Limited warranty and disclaimer/Limitation of
liability.
STATEMENT DESCRIPTION: Summary of the warranty, disclaimers,
limitations of liability and any applicable warranty or
insurance programs
SPECIFIC REQUIREMENT: Limitations of liability (see section
6.4).
D.8. STATEMENT TYPE: Applicable agreements and practice statement.
STATEMENT DESCRIPTION: Identification and references to
applicable agreements, practice statement, time-stamp policy
and other relevant documents.
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D.9. STATEMENT TYPE: Privacy policy.
STATEMENT DESCRIPTION: A description of and reference to the
applicable privacy policy.
SPECIFIC REQUIREMENT: Note: TSA's under this policy are
required to comply with the requirements of Data Protection
Legislation.
D.10. STATEMENT TYPE: Refund policy
STATEMENT DESCRIPTION: A description of and reference to the
applicable refund policy.
D.11. STATEMENT TYPE: Applicable law, complaints and dispute
resolution mechanisms.
STATEMENT DESCRIPTION: Statement of the choice of law,
complaints procedure and dispute resolution mechanisms.
SPECIFIC REQUIREMENT: The procedures for complaints and dispute
settlements. The applicable legal system.
D.12. STATEMENT TYPE: TSA and repository licenses, trust marks, and
audit.
STATEMENT DESCRIPTION: Summary of any governmental licenses,
seal programs; and a description of the audit process and if
applicable the audit firm.
SPECIFIC REQUIREMENT: If the TSA has been assessed to be
conformant with the identified time-stamp policy, and if so
through which independent party.
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Authors' Addresses
Denis Pinkas
Bull
Rue Jean Jaures,
78340 Les Clayes CEDEX
FRANCE
EMail: Denis.Pinkas@bull.net
Nick Pope
Security & Standards
192 Moulsham Street
Chelmsford, Essex
CM2 0LG
United Kingdom
EMail: pope@secstan.com
John Ross
Security & Standards
192 Moulsham Street
Chelmsford, Essex
CM2 0LG
United Kingdom
EMail: ross@secstan.com
This Informational RFC has been produced in ETSI ESI.
ETSI
F-06921 Sophia Antipolis, Cedex - FRANCE
650 Route des Lucioles - Sophia Antipolis
Valbonne - France
Tel: +33 4 92 94 42 00 Fax: +33 4 93 65 47 16
secretariat@etsi.fr
http://www.etsi.org
Contact Point
Claire d'Esclercs
ETSI
650 Route des Lucioles
F-06921 Sophia Antipolis, Cedex
FRANCE
EMail: claire.desclercs@etsi.org
Pinkas, et al. Informational [Page 42]
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Full Copyright Statement
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Acknowledgement
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