Internet Engineering Task Force (IETF) A. Jerman Blazic
Request for Comments: 6283 S. Saljic
Category: Standards Track SETCCE
ISSN: 2070-1721 T. Gondrom
July 2011
Extensible Markup Language Evidence Record Syntax (XMLERS)
Abstract
In many scenarios, users must be able to demonstrate the (time of)
existence, integrity, and validity of data including signed data for
long or undetermined periods of time. This document specifies XML
syntax and processing rules for creating evidence for long-term non-
repudiation of existence and integrity of data. The Extensible
Markup Language Evidence Record Syntax XMLERS provides alternative
syntax and processing rules to the ASN.1 (Abstract Syntax Notation
One) ERS (Evidence Record Syntax) (RFC 4998) syntax by using XML.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6283.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(
http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 6283 XMLERS July 2011
Table of Contents
1. Introduction ....................................................3
1.1. Motivation .................................................3
1.2. General Overview and Requirements ..........................4
1.3. Terminology ................................................6
1.4. Conventions Used in This Document ..........................7
2. Evidence Record .................................................7
2.1. Structure ..................................................8
2.2. Generation ................................................12
2.3. Verification ..............................................13
3. Archive Time-Stamp .............................................13
3.1. Structure .................................................13
3.1.1. Hash Tree ..........................................13
3.1.2. Time-Stamp .........................................14
3.1.3. Cryptographic Information List .....................15
3.2. Generation ................................................16
3.2.1. Generation of Hash Tree ............................17
3.2.2. Reduction of Hash Tree .............................19
3.3. Verification ..............................................21
4. Archive Time-Stamp Sequence and Archive Time-Stamp Chain .......22
4.1. Structure .................................................23
4.1.1. Digest Method ......................................23
4.1.2. Canonicalization Method ............................24
4.2. Generation ................................................25
4.2.1. Time-Stamp Renewal .................................25
4.2.2. Hash Tree Renewal ..................................26
4.3. Verification ..............................................27
5. Encryption .....................................................28
6. Version ........................................................29
7. Storage of Policies ............................................30
8. XSD Schema for the Evidence Record .............................30
9. Security Considerations ........................................34
9.1. Secure Algorithms .........................................34
9.2. Redundancy ................................................34
9.3. Secure Time-Stamps ........................................35
9.4. Time-Stamp Verification ...................................35
10. IANA Considerations ...........................................36
11. References ....................................................37
11.1. Normative References .....................................37
11.2. Informative References ...................................39
Appendix A. Detailed Verification Process of an Evidence Record ...41
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1. Introduction
The purpose of the document is to define XML schema and processing
rules for Evidence Record Syntax in XML (Extensible Markup Language)
format. The document is related to initial ASN.1 (Abstract Syntax
Notation One) syntax for Evidence Record Syntax as defined in
[RFC4998].
1.1. Motivation
The evolution of electronic commerce and electronic data exchange in
general requires introduction of non-repudiable proof of data
existence as well as data integrity and authenticity. Such data and
non-repudiable proof of existence must endure for long periods of
time, even when the initial information to prove its existence and
integrity weakens or ceases to exist. Mechanisms such as digital
signatures defined in [RFC5652], for example, do not provide absolute
reliability on a long-term basis. Algorithms and cryptographic
material used to create a signature can become weak in the course of
time, and information needed to validate digital signatures may
become compromised or simply cease to exist, for example, due to the
disbanding of a certificate service provider. Providing a stable
environment for electronic data on a long-term basis requires the
introduction of additional means to continually provide an
appropriate level of trust in evidence on data existence, integrity,
and authenticity.
All integrity and authenticity protecting techniques used today
suffer from the problem of degrading reliability over time, including
techniques for Time-Stamping, which are generally recognized as data
existence and integrity proof mechanisms. Over long periods of time
cryptographic algorithms used may become weak or encryption keys
compromised. Some of the problems might not even be of technical
nature like a Time-Stamping Authority going out of business and
ceasing its service. To create a stable environment where proof of
existence and integrity can endure well into the future a new
technical approach must be used.
Long-term non-repudiation of data existence and demonstration of data
integrity techniques have been already introduced, for example, by
long-term signature syntaxes like those defined in [RFC5126]. Long-
term signature syntaxes and processing rules address only the long-
term endurance of the digital signatures themselves, while Evidence
Record Syntax broadens this approach for data of any type or format
including digital signatures.
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XMLERS (Extensible Markup Language Evidence Record Syntax) is based
on Evidence Record Syntax as defined in [RFC4998] and is addressing
the same problem of long-term non-repudiable proof of data existence
and demonstration of data integrity on a long-term basis. XMLERS
does not supplement the [RFC4998] specification. Following
extensible markup language standards and [RFC3470] guidelines it
introduces the same approach but in a different format and with
adapted processing rules.
The use of Extensible Markup Language (XML) format is already
recognized by a wide range of applications and services and is being
selected as the de facto standard for many applications based on data
exchange. The introduction of Evidence Record Syntax in XML format
broadens the horizon of XML use and presents a harmonized syntax with
a growing community of XML-based standards including those related to
security services such as [XMLDSig] or [XAdES].
Due to the differences in XML processing rules and other
characteristics of XML, XMLERS does not present a direct
transformation of ERS in ASN.1 syntax. XMLERS is based on different
processing rules as defined in [RFC4998] and it does not support, for
example, the import of ASN.1 values in XML tags. Creating Evidence
Records in XML syntax must follow the steps as defined in this
document. XMLERS is a standalone document and is based on [RFC4998]
conceptually only. The content of this document provides enough
information for implementation of Evidence Record Syntax (represented
in XML format). References to [RFC4998] are for informative purposes
only.
Evidence Record Syntax in XML format is based on long-term archive
service requirements as defined in [RFC4810]. XMLERS delivers the
same (level of) non-repudiable proof of data existence as ASN.1 ERS
[RFC4998]. The XML syntax supports archive data grouping (and de-
grouping) together with simple or complex Time-Stamp renewal
processes. Evidence Records can be embedded in the data itself or
stored separately as a standalone XML file.
1.2. General Overview and Requirements
XMLERS specifies the XML syntax and processing rules for creating
evidence for the long-term non-repudiation of existence and integrity
of data in a unit called the "Evidence Record". XMLERS is defined to
meet the requirements for data structures as set out in [RFC4810].
This document also refers to the ASN.1 ERS specification as defined
in [RFC4998].
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An Evidence Record may be generated and maintained for a single data
object or a group of data objects that form an archive object. A
data object (binary chunk or a file) may represent any kind of
document or part of it. Dependencies among data objects, their
validation, or any other relationship than "a data object is a part
of particular archived object" are outside the scope of this
document.
Evidence Record is closely related to Time-Stamping techniques.
However, Time-Stamps as defined in [RFC3161] can cover only a single
unit of data and do not provide processing rules for maintaining a
long-term stability of Time-Stamps applied over a data object.
Evidence for an archive object is created by acquiring a Time-Stamp
from a trustworthy authority for a specific value that is
unambiguously related to a single or more data objects. Relationship
between several data objects and a single Time-Stamped value is
addressed using a hash tree, a technique first described by Merkle
[MER1980] and later in [RFC4998], with data structures and procedures
as specified in this document. The Evidence Record Syntax enables
processing of several archive objects within a single processing pass
using a hash tree technique and acquiring only one Time-Stamp to
protect all archive objects. The leaves of the hash tree are hash
values of the data objects in a group. A Time-Stamp is requested
only for the root hash of the hash tree. The deletion of a data
object in the tree does not influence the provability of others. For
any particular data object, the hash tree can be reduced to a few
sets of hash values, which are sufficient to prove the existence of a
single data object. Similarly, the hash tree can be reduced to prove
existence of a data group, provided all members of the data group
have the same parent node in the hash tree. Archive Time-Stamps are
comprised of an optional reduced hash tree and a Time-Stamp.
Besides a Time-Stamp other artifacts are also preserved in Evidence
Record: data necessary to verify the relationship between a time-
stamped value and a specific data object, packed into a structure
called a "hash tree", and long-term proofs for the formal
verification of the included Time-Stamp(s).
Because digest algorithms or cryptographic methods used may become
weak or certificates used within a Time-Stamp (and signed data) may
be revoked or expire, the collected evidence data must be monitored
and renewed before such events occur. This document introduces XML-
based syntax and processing rules for the creation and continuous
renewal of evidence data.
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1.3. Terminology
Archive Data Object: An archive data object is a data unit that is
archived and has to be preserved for a long time by the long-term
archive service.
Archive Data Object Group: An archive data object group is a set of
archive data objects that, for some reason, (logically) belong
together; e.g., a group of document files or a document file and a
signature file could represent an archive data object group.
Archive Object (AO): An AO is an archive data object or an archive
data object group.
Archive Time-Stamp (ATS): An ATS contains a Time-Stamp Token, useful
data for validation, and optionally a set of ordered lists of hash
values (a hash tree). An Archive Time-Stamp relates to a data object
if the hash value of this data object is part of the first hash value
list of the Archive Time-Stamp or its hash value matches the Time-
Stamped value. An Archive Time-Stamp relates to a data object group
if it relates to every data object of the group and no other data
object (i.e., the hash values of all but no other data objects of the
group are part of the first hash value list of the Archive Time-
Stamp) (see Section 3).
Archive Time-Stamp Chain (ATSC): An ATSC holds a sequence of Archive
Time-Stamps generated during the preservation period.
Archive Time-Stamp Sequence (ATSSeq): AN ATSSeq is a sequence of
Archive Time-Stamp Chains.
Canonicalization: Canonicalization refers to processing rules for
transforming an XML document into its canonical form. Two XML
documents may have different physical representations, but they may
have the same canonical form. For example, a sort order of
attributes does not change the meaning of the document as defined in
[XMLC14N].
Cryptographic Information: Cryptographic information is data or part
of data related to the validation process of signed data, e.g.,
digital certificates, digital certificate chains, and Certificate
Revocation Lists.
Digest Method: Digest method is a digest algorithm, which is a strong
one-way function, for which it is computationally infeasible to find
an input that corresponds to a given output or to find two different
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input values that correspond to the same output. A digest algorithm
transforms input data into a short value of fixed length. The output
is called digest value, hash value, or data fingerprint.
Evidence: Evidence is information that may be used to resolve a
dispute about various aspects of authenticity, validity, and
existence of archived data objects.
Evidence Record: An Evidence Record is a collection of evidence
compiled for a given archive object over time. An Evidence Record
includes ordered collection of ATSs, which are grouped into ATSCs and
ATSSeqs.
Long-Term Archive (LTA): An LTA is a service responsible for
generation, collection, and maintenance (renewal) of evidence data.
An LTA may also preserve data for long periods of time, e.g. storage
of archive data and associated evidences.
Hash Tree: A hash tree is a collection of hash values of protected
objects (input data objects and generated evidence within archival
period) that are unambiguously related to the Time-Stamped value
within an Archive Time-Stamp.
Time-Stamp Token (TS): A TS is a cryptographically secure
confirmation generated by a Time-Stamping Authority (TSA), e.g.,
[RFC3161], which specifies a structure for Time-Stamps and a protocol
for communicating with a Time-Stamp Authority. Besides this, other
data structures and protocols may also be appropriate, such as
defined in [ISO-18014-1.2002], [ISO-18014-2.2002],
[ISO-18014-3.2004], and [ANSI.X9-95.2005].
1.4. Conventions Used in This Document
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 [RFC2119].
2. Evidence Record
An Evidence Record is a unit of data that is to be used to prove the
existence of an archive object (a single archive data object or a
archive data object group) at a certain time. Through the lifetime
of an archive object, an Evidence Record also demonstrates the data
objects' integrity and non-repudiability. To achieve this,
cryptographic means are used, i.e., the LTA obtains Time-Stamp Tokens
from the Time-Stamping Authority (TSA). It is possible to store the
Evidence Record separately from the archive object or to integrate it
into the data itself.
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As cryptographic means are used to support Evidence Records, such
records may lose their value over time. Time-Stamps obtained from
Time-Stamping Authorities may become invalid for a number of reasons,
usually due to time constraints of Time-Stamp validity or when
cryptographic algorithms lose their security properties. Before the
used Time-Stamp Tokens become unreliable, the Evidence Record has to
be renewed. This may result in a series of Time-Stamp Tokens, which
are linked between themselves according to the cryptographic methods
and algorithms used.
Evidence Records can be supported with additional information, which
can be used to ease the processes of Evidence Record validation and
renewal. Information such as digital certificates and Certificate
Revocation Lists as defined in [RFC5280] or other cryptographic
material can be collected, enclosed, and processed together with
archive object data (i.e., Time-Stamped).
2.1. Structure
The Evidence Record contains one or several Archive Time-Stamps
(ATSs). An ATS contains a Time-Stamp Token and optionally other
useful data for Time-Stamp validation, e.g., certificates, CRLs
(Certificate Revocation Lists), or OCSP (Online Certificate Status
Protocol) responses and also specific attributes such as service
policies.
Initially, an ATS is acquired and later, before it expires or becomes
invalid, a new ATS is acquired, which prolongs the validity of the
archived object (its data objects together with all previously
generated Archive Time-Stamps). This process MUST continue during
the desired archiving period of the archive data object(s). A series
of successive Archive Time-Stamps is collected in Archive Time-Stamp
Chains and a series of chains in Archive Time-Stamp Sequence.
In XML syntax the Evidence Record is represented by the
<EvidenceRecord> root element, which has the following structure
described in Pseudo-XML with the full XML schema defined in Section 8
(where "?" denotes zero or one occurrences, "+" denotes one or more
occurrences, and "*" denotes zero or more occurrences):
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<EvidenceRecord Version>
<EncryptionInformation>
<EncryptionInformationType>
<EncryptionInformationValue>
</EncryptionInformation> ?
<SupportingInformationList>
<SupportingInformation Type /> +
</SupportingInformationList> ?
<ArchiveTimeStampSequence>
<ArchiveTimeStampChain Order>
<DigestMethod Algorithm />
<CanonicalizationMethod Algorithm />
<ArchiveTimeStamp Order>
<HashTree /> ?
<TimeStamp>
<TimeStampToken Type />
<CryptographicInformationList>
<CryptographicInformation Order Type /> +
</CryptographicInformationList> ?
</TimeStamp>
<Attributes>
<Attribute Order Type /> +
</Attributes> ?
</ArchiveTimeStamp> +
</ArchiveTimeStampChain> +
</ArchiveTimeStampSequence>
</EvidenceRecord>
The syntax of an Evidence Record is defined as an XML schema
[XMLSchema], see Section 8. The schema uses the following XML
namespace [XMLName] urn:ietf:params:xml:ns:ers as default namespace
with a detailed xml schema header listed in Section 8.
The XML elements and attributes have the following meanings:
The "Version" attribute MUST be included and indicates the syntax
version, for compatibility with future revisions of this
specification and to distinguish it from earlier non-conformant or
proprietary versions of XMLERS. Current version of XMLERS is 1.0.
The used versioning scheme is described in detail in Section 6.
<EncryptionInformation> element is OPTIONAL and holds information
on cryptographic algorithms and cryptographic material used to
encrypt archive data (in case archive data is encrypted, e.g., for
privacy purposes). This optional information is needed to
unambiguously re-encrypt data objects when processing Evidence
Records. When omitted, data objects are not encrypted or
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non-repudiation proof is not needed for the unencrypted data.
Details on how to process encrypted archive data and generate
Evidence Record(s) are described in Section 5.
<SupportingInformationList> element is OPTIONAL and can hold
information to support processing of Evidence Records. An example
of this supporting information may be a processing policy, like a
cryptographic policy (e.g., [RFC5698]) or archiving policies,
which can provide input about preservation and evidence
validation. Each data object is put into a separate child element
<SupportingInformation>, with an OPTIONAL Type attribute to
indicate its type for processing directions. As outlined, Types
to be used must be defined in the specification of the information
structure to be stored or in this standard. As outlined in
Section 9.4, cryptographic information may also be stored in the
SupportingInformation element, in which case its Section 3.1.3
defined type MUST be used. Or as defined in Section 7
cryptographic policies [RFC5698] MAY be stored, in which case the
used type is defined in the relevant RFC. Note that if supporting
information and policies are relevant for and already available at
or before the time of individual renewal steps (e.g., to indicate
the DSSC crypto policy [RFC5698]) that was used at the time of the
individual renewal) they SHOULD be stored in the <Attributes>
element of the individual Archive Time-Stamp (see below) as this
is integrity protected by the Archive Time-Stamps. Supporting
information that is relevant for the whole Evidence Record (like
the LTA's current Cryptographic Algorithms Security Suitability
policy (DSSC, [RFC5698]) or that was not available at the time of
renewal (and therefore could not later be stored in the protected
<Attributes> element) can be stored in this
<SupportingInformation> element.
<ArchiveTimeStampSequence> is REQUIRED and contains a sequence of
one or more <ArchiveTimeStampChain>.
<ArchiveTimeStampChain> is a REQUIRED element that holds a
sequence of Archive Time-Stamps generated during the preservation
period. Details on Archive Time-Stamp Chains and Archive Time-
Stamp Sequences are described in Section 4. The sequences of
Archive Time-Stamp Chains and Archive Time-Stamps MUST be ordered
and the order MUST be indicated with "Order" attribute of the
<ArchiveTimeStampChain> and <ArchiveTimeStamp> elements.
<DigestMethod> is a REQUIRED element and contains an attribute
"Algorithm" that identifies the digest algorithm used within one
Archive Time-Stamp Chain to calculate digest values from the
archive data object(s), previous Archive Time-Stamp Sequence,
Time-Stamps, and within a Time-Stamp Token.
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<CanonicalizationMethod> is a REQUIRED element that specifies
which canonicalization algorithm is applied to the archive data
for XML data objects or <ArchiveTimeStampSequence> or <TimeStamp>
elements prior to performing digest value calculations.
<HashTree> is an OPTIONAL element that holds a structure as
described in Section 3.1.1.
<TimeStamp> is REQUIRED and holds a <TimeStampToken> element with
a Time-Stamp Token (as defined in Section 3.1.2) provided by the
Time-Stamping Authority and an OPTIONAL element
<CryptographicInformationList>.
<CryptographicInformationList> is an OPTIONAL element that allows
the storage of data needed in the process of Time-Stamp Token
validation in case when such data is not provided by the Time-
Stamp Token itself. This could include possible trust anchors,
certificates, revocation information, or the current definition of
the suitability of cryptographic algorithms, past and present.
Each data object is put into a separate child element
<CryptographicInformation>, with a REQUIRED Order attribute to
indicate the order within its parent element. These items may be
added based on the policy used. This data is protected by
successive Time-Stamps in the sequence of the Archive Time-Stamps.
<Attributes> element is OPTIONAL and contains additional
information that may be provided by an LTA used to support
processing of Evidence Records. An example of this supporting
information may be a processing policy, like a renewal, a
cryptographic (e.g., [RFC5698]), or an archiving policy. Such
policies can provide inputs, which are relevant for preservation
of the data object(s) and evidence validation at a later stage.
Each data object is put into a separate child element <Attribute>,
with a REQUIRED Order attribute to indicate the order within the
parent element and an OPTIONAL Type attribute to indicate
processing directions. The type to be used must be defined in the
specification of the information structure. For example, the type
to be used when storing a cryptographic policy [RFC5698] is
defined in Appendix A.2 of [RFC5698].
The Order attribute is REQUIRED in all cases when one or more XML
elements with the same name occur on the same level in XMLERS'
<ArchiveTimeStampSequence> structure. Although most of the XML
parsers will preserve the order of the sibling elements having the
same name, within XML structure there is no definition how to
unambiguously define such order. Preserving the correct order in
such cases is of significant importance for digest value
calculations over XML structures.
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2.2. Generation
The generation of an <EvidenceRecord> element MUST be as follows:
1. Select an archive object (a data object or a data object group) to
archive.
2. Create the initial <ArchiveTimeStamp>. This is the first ATS
within the initial <ArchiveTimeStampChain> element of the
<ArchiveTimeStampSequence> element.
3. Refresh the <ArchiveTimeStamp> when necessary by Time-Stamp
renewal or hash tree renewal (see Section 4).
The Time-Stamping service may be, for a large number of archived
objects, expensive and time-demanding, so the LTA may benefit from
acquiring one Time-Stamp Token for many archived objects, which are
not otherwise related to each other. It is possible to collect many
archive objects, build a hash tree to generate a single value to be
Time-Stamped, and respectively reduce that hash tree to small subsets
that for each archive object provide necessary binding with the Time-
Stamped hash value (see Section 3.2.1).
For performance reasons or in case of local Time-Stamp generation,
building a hash tree (<HashTree> element) can be omitted. It is also
possible to convert existing Time-Stamps into an ATS for renewal.
The case when only essential parts of documents or objects shall be
protected is out of scope for this standard, and an application that
is not defined in this document must ensure that the correct
unambiguous extraction of binary data is made for the generation of
Evidence Record.
An application may also provide evidence such as certificates,
revocation lists, etc. needed to verify and validate signed data
objects or a data object group. This evidence may be added to the
archive data object group and will be protected within the initial
(and successive) Time-Stamp(s).
Note that the <CryptographicInformationList> element of Evidence
Record is not to be used to store and protect cryptographic material
related to signed archive data. The use of this element is limited
to cryptographic material related to the Time-Stamp(s).
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2.3. Verification
The overall verification of an Evidence Record MUST be as follows:
1. Select an archive object (a data object or a data object group).
2. Re-encrypt data object or data object group, if encryption field
is used (for details, see Section 5).
3. Verify Archive Time-Stamp Sequence (details in Sections 3.3 and
4.3).
3. Archive Time-Stamp
An Archive Time-Stamp is a Time-Stamp with additional artifacts that
allow the verification of the existence of several data objects at a
certain time.
The process of construction of an ATS must support evidence on a
long-term basis and prove that the archive object existed and was
identical, at the time of the Time-Stamp, to the currently present
archive object (at the time of verification). To achieve this, an
ATS MUST be renewed before it becomes invalid (which may happen for
several reasons such as, e.g., weakening used cryptographic
algorithms, invalidation of digital certificate, or a TSA terminating
its business or ceasing its service).
3.1. Structure
An Archive Time-Stamp contains a Time-Stamp Token, with useful data
for its validation (cryptographic information), such as the
certificate chain or Certificate Revocation Lists, an optional
ordered set of ordered lists of hash values (a hash tree) that were
protected with the Time-Stamp Token and optional information
describing the renewal steps (<Attributes> element). A hash tree may
be used to store data needed to bind the Time-Stamped value with
protected objects by the Archive Time-Stamp. If a hash tree is not
present, the ATS simply refers to a single object, either input data
object or a previous TS.
3.1.1. Hash Tree
Hash tree structure is an optional container for significant values,
needed to unambiguously relate a Time-Stamped value to protected data
objects, and is represented by the <HashTree> element. The root hash
value that is generated from the values of the hash tree MUST be the
same as the Time-Stamped value.
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<HashTree>
<Sequence Order>
<DigestValue>base64 encoded hash value</DigestValue> +
</Sequence> +
</HashTree>
The algorithm by which a root hash value is generated from the
<HashTree> element is as follows: the content of each <DigestValue>
element within the first <Sequence> element is base64 ([RFC4648],
using the base64 alphabet not the base64url alphabet) decoded to
obtain a binary value (representing the hash value). All collected
hash values from the sequence are ordered in binary ascending order,
concatenated and a new hash value is generated from that string.
With one exception to this rule: when the first <Sequence> element
has only one <DigestValue> element, then its binary value is added to
the next list obtained from the next <Sequence> element.
The newly calculated hash value is added to the next list of hashes
obtained from the next <Sequence> element and the previous step is
repeated until there is only one hash value left, i.e., when there
are no <Sequence> elements left. The last calculated hash value is
the root hash value. When an archive object is a group and composed
of more than one data object, the first hash list MUST contain the
hash values of all its data objects.
When a single Time-Stamp is obtained for a set of archive objects,
the LTA MUST construct a hash tree to generate a single hash value to
bind all archive objects from that group and then a reduced hash tree
MUST be calculated from the hash tree for each archive object
respectively (see Section 3.2.1).
For example: A SHA-1 digest value is a 160-bit string. The text
value of the <DigestValue> element shall be the base64 encoding of
this bit string viewed as a 20-octet octet stream. And to continue
the example, using an example message digest value of
A9993E364706816ABA3E25717850C26C9CD0D89D (note this is a HEX encoded
value of the 160-bit message digest), its base64 representation would
be <DigestValue>qZk+NkcGgWq6PiVxeFDCbJzQ2J0=</DigestValue>.
3.1.2. Time-Stamp
Time-Stamp Token is an attestation generated by a TSA that a data
item existed at a certain time. The Time-Stamp Token is a signed
data object that contains the hash value, the identity of the TSA,
and the exact time (obtained from trusted time source) of Time-
Stamping. This proves that the given data existed before the time of
Time-Stamping. For example, [RFC3161] specifies a structure for
signed Time-Stamp Tokens in ASN.1 format. Since at the time being
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there is no standard for an XML Time-Stamp, the following structure
example is provided [TS-ENTRUST], which is a digital signature
compliant to [XMLDSig] specification containing Time-Stamp specific
data, such as Time-Stamped value and time within the <Object> element
of a signature.
<element name="TimeStampInfo">
<complexType>
<sequence>
<element ref="Policy" />
<element ref="Digest" />
<element ref="SerialNumber" minOccurs="0" />
<element ref="CreationTime" />
<element ref="Accuracy" minOccurs="0" />
<element ref="Ordering" minOccurs="0" />
<element ref="Nonce" minOccurs="0" />
<element ref="Extensions" minOccurs="0" />
</sequence>
</complexType>
</element>
A <TimeStamp> element of ATS holds a complete structure of Time-Stamp
Token as provided by a TSA. Time-Stamp Token may be in XML or ASN.1
format. The Attribute type MUST be used to indicate the format for
processing purposes, with values "XMLENTRUST" or "RFC3161"
respectively. For an RFC3161 type Time-Stamp Token, the <TimeStamp>
element MUST contain base64 encoding of a DER-encoded ASN1 data.
These type values are registered by IANA (see Section 10). For
support of future types of Time-Stamps (in particular for future XML
Time-Stamp standards), these need to be registered there as well.
For example:
<TimeStamp Type="RFC3161">MIAGCSqGSIb3DQEH...</TimeStamp>
or
<TimeStamp Type="XMLENTRUST"><dsig:Signature>...</dsig:Signature>
</TimeStamp>
3.1.3. Cryptographic Information List
Digital certificates, CRLs (Certificate Revocation Lists), SCVP
(Server-Based Certificate Validation Protocol), or OCSP-Responses
(Online Certificate Status Protocol) needed to verify the Time-Stamp
Token SHOULD be stored in the Time-Stamp Token itself. When this is
not possible, such data MAY be stored in the
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<CryptographicInformationList> element; each data object is stored
into a separate <CryptographicInformation> element, with a REQUIRED
Order attribute.
The attribute Type is REQUIRED and is used to store processing
information about the type of stored cryptographic information. The
Type attribute MUST use a value registered with IANA, as identifiers:
CRL, OCSP, SCVP, or CERT, and for each type the content MUST be
encoded respectively:
o for type CRL, a base64 encoding of a DER-encoded X.509 CRL
[RFC5280]
o for type OCSP, a base64 encoding of a DER-encoded OCSPResponse
[RFC2560]
o for type SCVP, a base64 encoding of a DER-encoded CVResponse;
[RFC5055]
o for type CERT, a base64 encoding of a DER-encoded X.509
certificate [RFC5280]
The supported type identifiers are registered by IANA (see Section
10). Future supported types can be registered there (for example, to
support future validation standards).
3.2. Generation
An initial ATS relates to a data object or a data object group that
represents an archive object. The generation of the initial ATS
element can be done in a single process pass for one or for many
archived objects. It MUST be done as described in the following
steps:
1. Collect one or more archive objects to be Time-Stamped.
2. Select a canonicalization method C to be used for obtaining binary
representation of archive data and for Archive Time-Stamp at a
later stage in the renewing process (see Section 4). Note that
the selected canonicalization method MUST be used also for archive
data when data is represented in XML format.
3. Select a valid digest algorithm H. The selected secure hash
algorithm MUST be the same as the hash algorithm used in the Time-
Stamp Token and for the hash tree computations.
4. Generate a hash tree for selected archive object (see Section
3.2.1).
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The hash tree may be omitted in the initial ATS, when an archive
object has a single data object; then the Time-Stamped value MUST
match the digest value of that single data object.
5. Acquire Time-Stamp token from TSA for root hash value of a hash
tree (see Section 3.1.1). If the Time-Stamp token is valid, the
initial Archive Time-Stamp may be generated.
3.2.1. Generation of Hash Tree
The <DigestValue> elements within the <Sequence> element MUST be
ordered in binary ascending order to ensure the correct calculation
of digest values at the time of renewal and later for verification
purposes. Note that the text value of the <DigestValue> element is
base64 encoded, so it MUST be base64 decoded in order to obtain a
binary representation of the hash value.
A hash tree MUST be generated when the Time-Stamped value is not
equal to the hash value of the input data object. This is the case
when either of the following is true:
1. When an archive object has more than one data object (i.e., is an
archive data object group), its digest value is the digest value
of binary ascending ordered and concatenated digest values of all
its containing data objects. Note that in this case the first
list of the hash tree MUST contain hash values of all data objects
and only those values.
2. When for more than one archive object a single Time-Stamp Token is
generated, then the hash tree is a reduced hash tree extracted
from the hash tree for that archive object (see Section 3.2.2).
The hash tree for a set of archive objects is built from the leaves
to the root. First the leaves of the tree are collected, each leaf
representing the digest value of an archive object. You MUST use the
following procedure to calculate the hash tree:
1. Collect archive objects and for each archive object its
corresponding data objects.
2. Choose a secure hash algorithm H and calculate the digest values
for the data objects and put them into the input list for the hash
tree as follows: a digest value of an archive object is the digest
value of its data object, if there is only one data object in the
archive object; if there is more than one data object in the
archive object (i.e., it is an archive data object group) the
digest value is the digest value of binary sorted, concatenated
digest values of all its containing data objects.
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Note that for an archive object group (having more than one data
object), lists of their sub-digest values are stored and later,
when creating a reduced hash tree for that archive object, they
will become members of the first hash list.
3. Group together items in the input list by the order of N (e.g.,
for a binary tree group in pairs, for a tertiary tree group in
triplets, and so forth) and for each group: binary ascending sort,
concatenate, and calculate the hash value. The result is a new
input for the next list. For improved processing it is
RECOMMENDED to have the same number of children for each node.
For this purpose you MAY extend the tree with arbitrary values to
make every node have the same number of children.
4. Repeat step 3, until only one digest value is left; this is the
root value of the hash tree, which is Time-Stamped.
Note that the selected secure hash algorithm MUST be the same as the
one defined in the <DigestMethod> element of the ATSChain.
Example: An input list with 18 hash values, where the h'1 is
generated for a group of data objects (d4, d5, d6, and d7) and has
been grouped by 3. The group could be of any size (2, 3...). Note
that the addition of the arbitrary values h''6 and h'''3 are OPTIONAL
and can be used for improved processing as outlined in step 3 above.
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----------
d1 -> h1 \
\
G1 d2 -> h2 |-> h''1
+--------+ / \
|d4 -> h4|\ d3 -> h3 / \
|d5 -> h5| \ ---------- |
| | | -> h'1\ |
|d6 -> h6| / \ |
|d7 -> h7|/ d8 -> h8 |-> h''2 |-> h'''1
+--------+ / | \
d9 -> h9 / | \
---------- | |
d10 -> h10\ / |
\ / |
d11 -> h11 |-> h''3 |
/ |
d12 -> h12/ |-> root hash value
---------- |
d13 -> h13\ |
\ |
d14 -> h14 |-> h''4 |
/ \ |
d15 -> h15/ \ |
---------- |-> h'''2 |
d16 -> h16\ | |
\ | |
d17 -> h17 |-> h''5 | |
/ | |
d18 -> h18/ | |
---------- / |
/ /
(any arbitrary) h''6 /
(any arbitrary) h'''3
Figure 1. Generation of the Merkle Hash Tree
Note that there are no restrictions on the quantity of hash value
lists and of their length. Also note that it is beneficial but not
required to build hash trees and reduce hash trees. An Archive Time-
Stamp may consist only of one list of hash values and a Time-Stamp or
in an extreme case only a Time-Stamp with no hash value lists.
3.2.2. Reduction of Hash Tree
The generated Merkle hash tree can be reduced to lists of hash
values, necessary as a proof of existence for a single archive object
as follows:
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1. For a selected archive object (AO) select its hash value h within
the leaves of the hash tree.
2. Put h as base64 encoded text value of a new <DigestValue> element
within a first <Sequence> element. If the selected AO is a data
object group (i.e., has more than one data object), the first
<Sequence> element MUST in this case be formed from the hash
values of all AOs data objects, each within a separate
<DigestValue> element.
3. Select all hash values that have the same father node as hash
value h. Place these hash values each as a base64 encoded text
value of a new <DigestValue> element within a new <Sequence>
element, increasing its Order attribute value by 1.
4. Repeat step 3 for the parent node until the root hash value is
reached, with each step create a new <Sequence> element and
increase its Order attribute by one. Note that node values are
not saved as they are computable.
The order of <DigestValue> elements within each <Sequence> element
MUST be binary ascending (by base64 decoded values).
Reduced hash tree for data object d4 (from the previous example,
presented in Figure 1):
<HashTree>
<Sequence Order='1'>
<DigestValue>base64 encoded h4</DigestValue>
<DigestValue>base64 encoded h5</DigestValue>
<DigestValue>base64 encoded h6</DigestValue>
<DigestValue>base64 encoded h7</DigestValue>
</Sequence>
<Sequence Order='2'>
<DigestValue>base64 encoded h8</DigestValue>
<DigestValue>base64 encoded h9</DigestValue>
</Sequence>
<Sequence Order='3'>
<DigestValue>base64 encoded h''1</DigestValue>
<DigestValue>base64 encoded h''3</DigestValue>
</Sequence>
<Sequence Order='4'>
<DigestValue>base64 encoded h'''2</DigestValue>
</Sequence>
</HashTree>
Reduced hash tree for data object d2 (from the previous example,
presented in Figure 1):
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<HashTree>
<Sequence Order='1'>
<DigestValue>base64 encoded h2</DigestValue>
</Sequence>
<Sequence Order='2'>
<DigestValue>base64 encoded h1</DigestValue>
<DigestValue>base64 encoded h3</DigestValue>
</Sequence>
<Sequence Order='3'>
<DigestValue>base64 encoded h''2</DigestValue>
<DigestValue>base64 encoded h''3</DigestValue>
</Sequence>
<Sequence Order='4'>
<DigestValue>base64 encoded h'''2</DigestValue>
</Sequence>
</HashTree>
3.3. Verification
The initial Archive Time-Stamp shall prove that an archive object
existed at a certain time, indicated by its Time-Stamp Token. The
verification procedure MUST be as follows:
1. Identify hash algorithm H (from <DigestMethod> element) and
calculate the hash value for each data object of the archive
object.
2. If the hash tree is present, search for hash values in the first
<Sequence> element. If hash values are not present, terminate
verification process with negative result. If the verifying party
also seeks additional proof that the Archive Time-Stamp relates to
a data object group (e.g., a document and all its digital
signatures), it SHOULD also be verified that only the hash values
of the data objects that are members of the given data object
group are in the first hash value list.
3. If the hash tree is present, calculate its root hash value.
Compare the root hash value with the Time-Stamped value. If they
are not equal, terminate the verification process with negative
result.
4. If the hash tree is omitted, compare the hash value of the single
data object with the Time-Stamped value. If they are not equal,
terminate the verification process with negative result. If an
archive object is having more data objects and the hash tree is
omitted, also exit with negative result.
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5. Check the validity of the Time-Stamp Token. If the needed
information to verify formal validity of the Time-Stamp Token is
not available or found within the <TimeStampToken> element or
within the <CryptographicInformationList> element or in
<SupportingInformationList> (see Section 9.4), exit with a
negative result.
Information for formal verification of the Time-Stamp Token includes
digital certificates, Certificate Revocation Lists, Online
Certificate Status Protocol responses, etc. This information needs
to be collected prior to the Time-Stamp renewal process and protected
with the succeeding Time-Stamp, i.e., included in the
<TimeStampToken> or <CryptographicInformation> element (see Section
9.4 for additional information and Section 4.2.1 for details on the
Time-Stamp renewal process). For the current (latest) Time-Stamp),
information for formal verification of the (latest) Time-Stamp should
be provided by the Time-Stamping Authority. This information can
also be provided with the Evidence Record within the
<SupportingInformation> element, which is not protected by any Time-
Stamp.
4. Archive Time-Stamp Sequence and Archive Time-Stamp Chain
An Archive Time-Stamp proves the existence of single data objects or
a data object group at a certain time. However, the initial Evidence
Record created can become invalid due to losing the validity of the
Time-Stamp Token for a number of reasons: hash algorithms or public
key algorithms used in its hash tree or the Time-Stamp may become
weak or the validity period of the Time-Stamp authority certificate
expires or is revoked.
To preserve the validity of an Evidence Record before such events
occur, the Evidence Record has to be renewed. This can be done by
creating a new ATS. Depending on the reason for renewing the
Evidence Record (the Time-Stamp becomes invalid or the hash algorithm
of the hash tree becomes weak) two types of renewal processes are
possible:
o Time-Stamp renewal: For this process a new Archive Time-Stamp is
generated, which is applied over the last Time-Stamp created. The
process results in a series of Archive Time-Stamps, which are
contained within a single Archive Time-Stamp Chain (ATSC).
o Hash tree renewal: For this process a new Archive Time-Stamp is
generated, which is applied to all existing Time-Stamps and data
objects. The newly generated Archive Time-Stamp is placed in a
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new Archive Time-Stamp Chain. The process results in a series of
Archive Time-Stamp Chains, which are contained within a single
Archive Time-Stamp Sequence (ATSSeq).
After the renewal process, only the most recent (i.e., the last
generated) Archive Time-Stamp has to be monitored for expiration or
validity loss.
4.1. Structure
Archive Time-Stamp Chain and Archive Time-Stamp Sequence are
containers for sequences of Archive Time-Stamp(s) that are generated
through renewal processes. The renewal process results in a series
of Evidence Record elements: the <ArchiveTimeStampSequence> element
contains an ordered sequence of <ArchiveTimeStampChain> elements, and
the <ArchiveTimeStampChain> element contains an ordered sequence of
<ArchiveTimeStamp> elements. Both elements MUST be sorted by time of
the Time-Stamp in ascending order. Order is indicated by the Order
attribute.
When an Archive Time-Stamp must be renewed, a new <ArchiveTimeStamp>
element is generated and depending on the generation process, it is
either placed:
o as the last <ArchiveTimeStamp> child element in a sequence of the
last <ArchiveTimeStampChain> element in case of Time-Stamp renewal
or
o as the first <ArchiveTimeStamp> child element in a sequence of the
newly created <ArchiveTimeStampChain> element in case of hash tree
renewal.
The ATS with the largest Order attribute value within the ATSC with
the largest Order attribute value is the latest ATS and MUST be valid
at the present time.
4.1.1. Digest Method
Digest method is a required element that identifies the digest
algorithm used to calculate hash values of archive data (and node
values of hash tree). The digest method is specified in the
<ArchiveTimeStampChain> element by the required <DigestMethod>
element and indicates the digest algorithm that MUST be used for all
hash value calculations related to the Archive Time-Stamps within the
Archive Time-Stamp Chain.
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The Algorithm attribute contains URIs [RFC3986] for identifiers that
MUST be used as defined in [RFC3275] and [RFC4051]. For example,
when the SHA-1 algorithm is used, the algorithm identifier is:
<DigestMethod Algorithm="
http://www.w3.org/2000/09/xmldsig#sha1"/>
Within a single ATSC, the digest algorithms used for the hash trees
of its Archive Time-Stamps and the Time-Stamp Tokens MUST be the
same. When algorithms used by a TSA are changed (e.g., upgraded) a
new ATSC MUST be started using an equal or stronger digest algorithm.
4.1.2. Canonicalization Method
Prior to hash value calculations of an XML element, a proper binary
representation must be extracted from its (abstract) XML data
presentation. The binary representation is determined by UTF-8
[RFC3629] encoding and canonicalization of the XML element. The XML
element includes the entire text of the start and end tags as well as
all descendant markup and character data (i.e., the text and sub-
elements) between those tags.
<CanonicalizationMethod> is a required element that identifies the
canonicalization algorithm used to obtain binary representation of an
XML element or elements. Algorithm identifiers (URIs) MUST be used
as defined in [RFC3275] and [RFC4051]. For example, when Canonical
XML 1.0 (omits comments) is used, algorithm identifier is
<CanonicalizationMethod Algorithm="
http://www.w3.org/TR/2001/REC-
xml-c14n-20010315"/>
Canonicalization MUST be applied over XML structured archive data and
MUST be applied over elements of Evidence Record (namely, ATS and
ATSC in the renewing process).
The canonicalization method is specified in the <Algorithm> attribute
of the <CanonicalizationMethod> element within the
<ArchiveTimeStampChain> element and indicates the canonicalization
method that MUST be used for all binary representations of the
Archive Time-Stamps within that Archive Time-Stamp Chain. In case of
succeeding ATSC the canonicalization method indicated within the ATSC
must also be used for the calculation of the digest value of the
preceding ATSC. Note that the canonicalization method is unlikely to
change over time as it does not impose the same constraints as the
digest method. In theory, the same canonicalization method can be
used for a whole Archive Time-Stamp Sequence. Although alternative
canonicalization methods may be used, it is recommended to use c14n-
20010315 [XMLC14N].
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4.2. Generation
Before the cryptographic algorithms used within the most recent
Archive Time-Stamp become weak or the Time-Stamp certificates are
invalidated, the LTA has to renew the Archive Time-Stamps by
generating a new Archive Time-Stamp using one of two procedures:
Time-Stamp renewal or hash tree renewal.
4.2.1. Time-Stamp Renewal
In case of Time-Stamp renewal, i.e., if the digest algorithm (H) to
be used in the renewal process is the same as digest algorithm (H')
used in the last Archive Time-Stamp, the complete content of the last
<TimeStamp> element MUST be Time-Stamped and a new <ArchiveTimeStamp>
element created as follows:
1. If the current <ArchiveTimeStamp> element does not contain needed
proof for long-term formal validation of its Time-Stamp Token
within the <TimeStamp> element, collect needed data such as root
certificates, Certificate Revocation Lists, etc., and include them
in the <CryptographicInformationList> element of the last Archive
Time-Stamp (each data object into a separate
<CryptographicInformation> element).
2. Select the canonicalization method from the
<CanonicalizationMethod> element and select the digest algorithm
from the <DigestMethod> element. Calculate hash value from binary
representation of the <TimeStamp> element of the last
<ArchiveTimeStamp> element including added cryptographic
information. Acquire the Time-Stamp for the calculated hash
value. If the Time-Stamp is valid, the new Archive Time-Stamp may
be generated.
3. Increase the value order of the new ATS by one and place the new
ATS into the last <ArchiveTimeStampChain> element.
The new ATS and its hash tree MUST use the same digest algorithm as
the preceding one, which is specified in the <DigestMethod> element
within the <ArchiveTimeStampChain> element. Note that the new ATS
MAY not contain a hash tree. However, the Time-Stamp renewal process
may be optimized to acquire one Time-Stamp for many Archive Time-
Stamps using a hash tree. Note that each hash of the <TimeStamp>
element is treated as the document hash in Section 3.2.1.
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4.2.2. Hash Tree Renewal
The process of hash tree renewal occurs when the new digest algorithm
is different from the one used in the last Archive Time-Stamp (H <>
H'). In this case the complete Archive Time-Stamp Sequence and the
archive data objects covered by existing Archive Time-Stamp must be
Time-Stamped as follows:
1. Select one or more archive objects to be renewed and their current
<ArchiveTimeStamp> elements.
2. For each archive object check the current <ArchiveTimeStamp>
element. If it does not contain the proof needed for long-term
formal validation of its Time-Stamp Token within the Time-Stamp
Token, collect the needed data such as root certificates,
Certificate Revocation Lists, etc., and include them in the
<CryptographicInformationList> element of the last Archive Time-
Stamp (each data object into a separate <CryptographicInformation>
element).
3. Select a canonicalization method C and select a new secure hash
algorithm H.
4. For each archive object select its data objects d(i). Generate
hash values h(i) = H(d(i)), for example: h(1), h(2).., h(n).
5. For each archive object calculate a hash hseq=H(ATSSeq) from
binary representation of the <ArchiveTimeStampSequence> element,
corresponding to that archive object. Note that Archive Time-
Stamp Chains and Archive Time-Stamps MUST be chronologically
ordered, each respectively to its Order attribute, and that the
canonicalization method C MUST be applied.
6. For each archive object sort in binary ascending order and
concatenate all h(i) and the hseq. Generate a new digest value
h(j)=H(h(1)..h(n),hseq).
7. Build a new Archive Time-Stamp for each h(j) (hash tree generation
and reduction is defined in Sections 3.2.1 and 3.2.2). Note that
each h(j) is treated as the document hash in Section 3.2.1. The
first hash value list in the reduced hash tree should only contain
h(i) and hseq.
8. Create the new <ArchiveTimeStampChain> containing the new
<ArchiveTimeStamp> element (with order number 1), and place it
into the existing <ArchiveTimeStampSequence> as a last child with
the order number increased by one.
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Example for an archive object with 3 data objects: Select a new hash
algorithm and canonicalization method. Collect all 3 data objects
and currently generated Archive Time-Stamp Sequence.
AO
/ | \
d1 d2 d3
ATSSeq
ATSChain1: ATS0, ATS1
ATSChain2: ATS0, ATS1, ATS2
The hash values MUST be calculated with the new hash algorithm H for
all data objects and for the whole ATSSeq. Note that ATSSeq MUST be
chronologically ordered and canonicalized before retrieving its
binary representation.
When generating the hash tree for the new ATS, the first sequence
become values: H(d1), H(d2),..., H(dn), H(ATSSeq). Note: hash values
MUST be sorted in binary ascending order.
<HashTree>
<Sequence Order='1'>
<DigestValue>H(d1)</DigestValue>
<DigestValue>H(d2)</DigestValue>
<DigestValue>H(d3)</DigestValue>
<DigestValue>H(ATSSeq)</DigestValue>
</Sequence>
</HashTree>
Note that if the group processing is being performed, the hash value
of the concatenation of the first sequence is an input hash value
into the hash tree.
4.3. Verification
An Evidence Record shall prove that an archive object existed and has
not been changed from the time of the initial Time-Stamp Token within
the first ATS. In order to complete the non-repudiation proof for an
archive object, the last ATS has to be valid and ATSCs and their
relations to each other have to be proved:
1. Select archive object and re-encrypt its data object or data
object group, if <EncryptionInformation> field is used. Select
the initial digest algorithm specified within the first Archive
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Time-Stamp Chain and calculate the hash value of the archive
object. Verify that the initial Archive Time-Stamp contains
(identical) hash value of the AO's data object (or hash values of
AO's data object group). Note that when the hash tree is omitted,
calculated AO's value MUST match the Time-Stamped value.
2. Verify each Archive Time-Stamp Chain and each Archive Time-Stamp
within. If the hash tree is present within the second and the
next Archive Time-Stamps of an Archive Time-Stamp Chain, the first
<Sequence> MUST contain the hash value of the <TimeStamp> element
before. Each Archive Time-Stamp MUST be valid relative to the
time of the succeeding Archive Time-Stamp. All Archive Time-
Stamps with the Archive Time-Stamp Chain MUST use the same hash
algorithm, which was secure at the time of the first Archive Time-
Stamp of the succeeding Archive Time-Stamp Chain.
3. Verify that the first hash value list of the first Archive Time-
Stamp of all succeeding Archive Time-Stamp Chains contains hash
values of data object and the hash value of Archive Time-Stamp
Sequence of the preceding Archive Time-Stamp Chains. Verify that
Archive Time-Stamp was created when the last Archive Time-Stamp of
the preceding Archive Time-Stamp Chain was valid.
4. To prove the Archive Time-Stamp Sequence relates to a data object
group, verify that the first Archive Time-Stamp of the first
Archive Time-Stamp Chain does not contain other hash values in its
first hash value list than the hash values of those data objects.
For non-repudiation proof for the data object, the last Archive Time-
Stamp MUST be valid at the time of verification process.
5. Encryption
In some archive services scenarios it may be required that clients
send encrypted data only, preventing information disclosure to third
parties, such as archive service providers. In such scenarios it
must be clear that Evidence Records generated refer to encrypted data
objects. Evidence Records in general protect the bit-stream (or
binary representation of XML data), which freezes the bit structure
at the time of archiving. Encryption schemes in such scenarios
cannot be changed afterwards without losing the integrity proof.
Therefore, an ERS record must hold and preserve encryption
information in a consistent manner. To avoid problems when using
Evidence Records in the future, additional special precautions have
to be taken.
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Encryption is a two-way process, whose result depends on the
cryptographic material used, e.g., encryption keys and encryption
algorithms. Encryption and decryption keys as well as algorithms
must match in order to reconstruct the original message or data that
was encrypted. Evidence generated to prove the existence of
encrypted data cannot always be relied upon to prove the existence of
unencrypted data. It may be possible to choose different
cryptographic material, i.e., an algorithm or a key for decryption
that is not the algorithm or key used for encryption. In this case,
the evidence record would not be a non-repudiation proof for the
unencrypted data. Therefore, only encryption methods should be used
that make it possible to prove that archive Time-Stamped encrypted
data objects unambiguously represent unencrypted data objects. In
cases when evidence was generated to prove the existence of encrypted
data the corresponding algorithm and decryption keys used for
encryption must become a part of the Evidence Record and is used to
unambiguously represent original (unencrypted) data that was
encrypted. (Note: In addition, the long-term security of the
encryption schemes should be analyzed to determine if it could be
used to create collision attacks.) Cryptographic material may also
be used in scenarios when a client submits encrypted data to the
archive service provider for preservation but stores himself the data
only in an unencrypted form. In such scenarios cryptographic
material is used to re-encrypt the unencrypted data kept by a client
for the purpose of performing validation of the Evidence Record,
which is related to the encrypted form of client's data. An OPTIONAL
extensible structure <EncryptionInformation> is defined to store the
necessary parameters of the encryption methods. Its
<EncryptionInformationType> element is used to store the type of
stored encryption information, e.g., whether it is an encryption
algorithm or encryption key. The <EncryptionInformationValue>
element then contains the relevant encryption information itself.
The use of encryption elements heavily depends on the cryptographic
mechanism and has to be defined by other specifications.
6. Version
The numbering scheme for XMLERS versions is "<major>.<minor>". The
major and minor numbers MUST be treated as separate integers and each
number MAY be incremented higher than a single digit. Thus, "2.4"
would be a lower version than "2.13", which in turn would be lower
than "12.3". Leading zeros (e.g., "6.01") MUST be ignored by
recipients and MUST NOT be sent.
Blazic, et al. Standards Track [Page 29]
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The major version number will be incremented only if the data format
has changed so dramatically that an older version entity would not be
able to interoperate with a newer version entity if it simply ignored
the elements and attributes it did not understand and took the
actions defined in the older specification.
The minor version number will be incremented if significant new
capabilities have been added to the core format (e.g., new optional
elements).
7. Storage of Policies
As explained above policies can be stored in the Evidence Record in
the <Attribute> or the <SupportingInformation> element. In the case
of storing DSSC policies [RFC5698], the types to be used in the
<Attribute> or <SupportingInformation> element are defined in
Appendix A.2 of [RFC5698] for both ASN.1 and XML representation.
8. XSD Schema for the Evidence Record
<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="
http://www.w3.org/2001/XMLSchema"
xmlns="urn:ietf:params:xml:ns:ers"
targetNamespace="urn:ietf:params:xml:ns:ers"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<xs:element name="EvidenceRecord" type="EvidenceRecordType"/>
<!-- TYPE DEFINITIONS-->
<xs:complexType name="EvidenceRecordType">
<xs:sequence>
<xs:element name="EncryptionInformation"
type="EncryptionInfo" minOccurs="0"/>
<xs:element name="SupportingInformationList"
type="SupportingInformationType" minOccurs="0"/>
<xs:element name="ArchiveTimeStampSequence"
type="ArchiveTimeStampSequenceType"/>
</xs:sequence>
<xs:attribute name="Version" type="xs:decimal" use="required"
fixed="1.0"/>
</xs:complexType>
<xs:complexType name="EncryptionInfo">
<xs:sequence>
<xs:element name="EncryptionInformationType"
type="ObjectIdentifier"/>
<xs:element name="EncryptionInformationValue">
Blazic, et al. Standards Track [Page 30]
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<xs:complexType mixed="true">
<xs:sequence>
<xs:any minOccurs="0"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:complexType name="ArchiveTimeStampSequenceType">
<xs:sequence>
<xs:element name="ArchiveTimeStampChain" maxOccurs="unbounded">
<xs:complexType>
<xs:sequence>
<xs:element name="DigestMethod"
type="DigestMethodType"/>
<xs:element name="CanonicalizationMethod"
type="CanonicalizationMethodType"/>
<xs:element name="ArchiveTimeStamp"
type="ArchiveTimeStampType"
maxOccurs="unbounded" />
</xs:sequence>
<xs:attribute name="Order" type="OrderType"
use="required"/>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:complexType name="ArchiveTimeStampType">
<xs:sequence>
<xs:element name="HashTree" type="HashTreeType" minOccurs="0"/>
<xs:element name="TimeStamp" type="TimeStampType"/>
<xs:element name="Attributes" type="Attributes" minOccurs="0"/>
</xs:sequence>
<xs:attribute name="Order" type="OrderType" use="required"/>
</xs:complexType>
<xs:complexType name="DigestMethodType" mixed="true">
<xs:sequence>
<xs:any namespace="##other" minOccurs="0"/>
</xs:sequence>
<xs:attribute name="Algorithm" type="xs:anyURI" use="required"/>
</xs:complexType>
<xs:complexType name="CanonicalizationMethodType" mixed="true">
<xs:sequence minOccurs="0">
<xs:any namespace="##any" minOccurs="0"/>
Blazic, et al. Standards Track [Page 31]
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</xs:sequence>
<xs:attribute name="Algorithm" type="xs:anyURI" use="required"/>
</xs:complexType>
<xs:complexType name="TimeStampType">
<xs:sequence>
<xs:element name="TimeStampToken">
<xs:complexType mixed="true">
<xs:complexContent mixed="true">
<xs:restriction base="xs:anyType">
<xs:sequence>
<xs:any processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="Type" type="xs:NMTOKEN"
use="required"/>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
</xs:element>
<xs:element name="CryptographicInformationList"
type="CryptographicInformationType" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="HashTreeType">
<xs:sequence>
<xs:element name="Sequence" maxOccurs="unbounded">
<xs:complexType>
<xs:sequence>
<xs:element name="DigestValue" type="xs:base64Binary"
maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="Order" type="OrderType"
use="required"/>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:complexType name="Attributes">
<xs:sequence>
<xs:element name="Attribute" maxOccurs="unbounded">
<xs:complexType mixed="true">
<xs:complexContent mixed="true">
<xs:restriction base="xs:anyType">
<xs:sequence>
<xs:any processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
Blazic, et al. Standards Track [Page 32]
RFC 6283 XMLERS July 2011
</xs:sequence>
<xs:attribute name="Order" type="OrderType"
use="required"/>
<xs:attribute name="Type" type="xs:string"
use="optional"/>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:complexType name="CryptographicInformationType">
<xs:sequence>
<xs:element name="CryptographicInformation"
maxOccurs="unbounded">
<xs:complexType mixed="true">
<xs:complexContent mixed="true">
<xs:restriction base="xs:anyType">
<xs:sequence>
<xs:any processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="Order" type="OrderType"
use="required"/>
<xs:attribute name="Type" type="xs:NMTOKEN"
use="required"/>
</xs:restriction>
</xs:complexContent>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:complexType name="SupportingInformationType">
<xs:sequence>
<xs:element name="SupportingInformation"
maxOccurs="unbounded">
<xs:complexType mixed="true">
<xs:complexContent mixed="true">
<xs:restriction base="xs:anyType">
<xs:sequence>
<xs:any processContents="lax" minOccurs="0"
maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute name="Type" type="xs:string"
use="required"/>
</xs:restriction>
</xs:complexContent>
Blazic, et al. Standards Track [Page 33]
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</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:simpleType name="ObjectIdentifier">
<xs:restriction base="xs:token">
<xs:pattern value="[0-2](\.[1-3]?[0-9]?(\.\d+)*)?"/>
</xs:restriction>
</xs:simpleType>
<xs:simpleType name="OrderType">
<xs:restriction base="xs:int">
<xs:minInclusive value="1"/>
</xs:restriction>
</xs:simpleType>
</xs:schema>
9. Security Considerations
9.1. Secure Algorithms
Cryptographic algorithms and parameters that are used within Archive
Time-Stamps must always be secure at the time of generation. This
concerns the hash algorithm used in the hash lists of Archive Time-
Stamp as well as hash algorithms and public key algorithms of the
Time-Stamps. Publications regarding security suitability of
cryptographic algorithms ([NIST.800-57-Part1.2006] and
[ETSI-TS-102-176-1-V2.0.0]) have to be considered during the
verification. A generic solution for automatic interpretation of
security suitability policies in electronic form is not the subject
of this specification.
9.2. Redundancy
Evidence Records may become affected by weakening cryptographic
algorithms even before this is publicly known. Retrospectively this
has an impact on Archive Time-Stamps generated and renewed during the
archival period. In this case the validity of Evidence Records
created may end without any options for retroactive action.
Many TSAs are using the same cryptographic algorithms. While
compromise of a private key of a TSA may compromise the security of
only one TSA (and only one Archive Time-Stamp, for example),
weakening cryptographic algorithms used to generate Time-Stamp Tokens
would affect many TSAs at the same time.
Blazic, et al. Standards Track [Page 34]
RFC 6283 XMLERS July 2011
To manage such risks and to avoid the loss of Evidence Record
validity due to weakening cryptographic algorithms used, it is
RECOMMENDED to generate and manage at least two redundant Evidence
Records for a single data object. In such scenarios redundant
Evidence Records SHOULD use different hash algorithms within Archive
Time-Stamp Sequences and different TSAs using different cryptographic
algorithms for Time-Stamp Tokens.
9.3. Secure Time-Stamps
Archive Time-Stamps depend upon the security of normal Time-Stamping
provided by TSA and stated in security policies. Renewed Archive
Time-Stamps MUST have the same or higher quality as the initial
Archive Time-Stamp of archive data. Archive Time-Stamps used for
signed archive data SHOULD have the same or higher quality than the
maximum quality of the signatures.
9.4. Time-Stamp Verification
It is important to consider for renewal and verification that when a
new Time-Stamp is applied, it MUST be ascertained that prior to the
time of renewal (i.e., when the new Time-Stamp is applied) the
certificate of the before current Time-Stamp was not revoked due to a
key compromise. Otherwise, in the case of a key compromise, there is
the risk that the authenticity of the used Time-Stamp and therefore
its security in the chain of evidence cannot be guaranteed. Other
revocation reasons like the revocation for cessation of activity do
not necessarily pose this risk, as in that case the private key of
the Time-Stamp unit would have been previously destroyed and thus
cannot be used nor compromised.
Both elements <CryptographicInformationList> and <Attribute> are
protected by future Archive Time_Stamp renewals and can store
information as outlined in Section 2.1 that is available at or before
the time of the renewal of the specific Archive Time-Stamp. At the
time of renewal all previous Archive Time-Stamp data structures
become protected by the new Archive Time-Stamp and frozen by it,
i.e., no data MUST be added or modified in these elements afterwards.
If, however, some supporting information is relevant for the overall
Evidence Record or information that only becomes available later,
this can be provided in the Evidence Record in the
<SupportingInformationList> element. Data in the
<SupportingInformatonList> can be added later to an Evidence Record,
but it must rely on its own authenticity and integrity protection
mechanism, like, for example, signed by current strong cryptographic
means and/or provided by a trusted source (for example, this could be
the LTA providing its current system DSSC policy, signed with current
strong cryptographic means).
Blazic, et al. Standards Track [Page 35]
RFC 6283 XMLERS July 2011
10. IANA Considerations
For all IANA registrations related to this document, the
"Specification Required" [RFC5226] allocation policies MUST be used.
This document defines the XML namespace "urn:ietf:params:xml:ns:ers"
according to the guidelines in [RFC3688]. This namespace has been
registered in the IANA XML Registry.
This document defines an XML schema (see Section 8) according to the
guidelines in [RFC3688]. This XML schema has been registered in the
IANA XML Registry and can be identified with the URN
"urn:ietf:params:xml:schema:ers".
This specification defines a new IANA registry entitled "XML Evidence
Record Syntax (XMLERS)". This registry contains two sub-registries
entitled "Time-Stamp Token Type" and "Cryptographic Information
Type". The policy for future assignments to both sub-registries is
"RFC Required".
The sub-registry "Time-Stamp Token Type" contains textual names and
description, which should refer to the specification or standard
defining that type. It serves as assistance when validating a Time-
Stamp Token.
When registering a new Time-Stamp Token type, the following
information MUST be provided:
o The textual name of the Time-Stamp Token type (value). The value
MUST conform to the XML datatype "xs:NMTOKEN".
o A reference to a publicly available specification that defines the
Time-Stamp Token type (description).
The initial values for the "Time-Stamp Token Type" sub-registry are:
Value
Description
Reference
-------------
RFC3161
RFC3161 Time-Stamp
RFC 3161
XMLENTRUST
EnTrust XML Schema
http://www.si-tsa.gov.si/dokumenti/timestamp-protocol-20020207.xsd
Blazic, et al. Standards Track [Page 36]
RFC 6283 XMLERS July 2011
The sub-registry "Cryptographic Information Type" contains textual
names and description, which should refer to a specification or
standard defining that type. It serves as assistance when validating
cryptographic information such as digital certificates, CRLs, or
OCSP-Responses.
When registering a new cryptographic information type, the following
information MUST be provided:
o The textual name of the cryptographic information type (value).
The value MUST conform to the XML datatype "xs:NMTOKEN".
o A reference to a publicly available specification that defines the
cryptographic information type (description).
The initial values for the "Cryptographic Information Type" sub-
registry are:
Value Description Reference
----- ------------------ -----------------
CERT DER-encoded X.509 Certificate RFC 5280
CRL DER-encoded X.509 RFC 5280
Certificate Revocation List
OCSP DER-encoded OCSPResponse RFC 2560
SCVP DER-encoded SCVP response RFC 5055
(CVResponse)
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2560] Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.
Adams, "X.509 Internet Public Key Infrastructure Online
Certificate Status Protocol - OCSP", RFC 2560, June
1999.
[RFC3161] Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
"Internet X.509 Public Key Infrastructure Time-Stamp
Protocol (TSP)", RFC 3161, August 2001.
Blazic, et al. Standards Track [Page 37]
RFC 6283 XMLERS July 2011
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
January 2004.
[RFC3275] Eastlake 3rd, D., Reagle, J., and D. Solo, "(Extensible
Markup Language) XML-Signature Syntax and Processing",
RFC 3275, March 2002.
[RFC4051] Eastlake 3rd, D., "Additional XML Security Uniform
Resource Identifiers (URIs)", RFC 4051, April 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC4998] Gondrom, T., Brandner, R., and U. Pordesch, "Evidence
Record Syntax (ERS)", RFC 4998, August 2007.
[RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D., and
W. Polk, "Server-Based Certificate Validation Protocol
(SCVP)", RFC 5055, December 2007.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 5280, May 2008.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[XMLC14N] Boyer, J., "Canonical XML", W3C Recommendation, March
2001.
[XMLDSig] Eastlake, D., Reagle, J., Solo, D., Hirsch, F.,
Roessler, T., "XML-Signature Syntax and Processing",
XMLDSig, W3C Recommendation, July 2006.
[XMLName] Layman, A., Hollander, D., Tobin, R., and T. Bray,
"Namespaces in XML 1.0 (Second Edition)", W3C
Recommendation, August 2006.
[XMLSchema] Thompson, H., Beech, D., Mendelsohn, N., and M. Maloney,
"XML Schema Part 1: Structures Second Edition", W3C
Recommendation, October 2004.
Blazic, et al. Standards Track [Page 38]
RFC 6283 XMLERS July 2011
11.2. Informative References
[ANSI.X9-95.2005]
American National Standard for Financial Services,
"Trusted Timestamp Management and Security", ANSI X9.95,
June 2005.
[ETSI-TS-102-176-1-V2.0.0]
ETSI, "Electronic Signatures and Infrastructures (ESI);
Algorithms and Parameters for Secure Electronic
Signatures; Part 1: Hash functions and asymmetric
algorithms", ETSI TS 102 176-1 V2.0.0 (2007-11),
November 2007.
[ISO-18014-1.2002]
ISO/IEC JTC 1/SC 27, "Time stamping services - Part 1:
Framework", ISO ISO-18014-1, February 2002.
[ISO-18014-2.2002]
ISO/IEC JTC 1/SC 27, "Time stamping services - Part 2:
Mechanisms producing independent tokens", ISO
ISO-18014-2, December 2002.
[ISO-18014-3.2004]
ISO/IEC JTC 1/SC 27, "Time stamping services - Part 3:
Mechanisms producing linked tokens", ISO ISO-18014-3,
February 2004.
[MER1980] Merkle, R., "Protocols for Public Key Cryptosystems,
Proceedings of the 1980 IEEE Symposium on Security and
Privacy (Oakland, CA, USA)", pages 122-134, April 1980.
[NIST.800-57-Part1.2006]
National Institute of Standards and Technology,
"Recommendation for Key Management - Part 1: General
(Revised)", NIST 800-57 Part1, May 2006.
[RFC3470] Hollenbeck, S., Rose, M., and L. Masinter, "Guidelines
for the Use of Extensible Markup Language (XML) within
IETF Protocols", BCP 70, RFC 3470, January 2003.
[RFC4810] Wallace, C., Pordesch, U., and R. Brandner, "Long-Term
Archive Service Requirements", RFC 4810, March 2007.
[RFC5126] Pinkas, D., Pope, N., and J. Ross, "CMS Advanced
Electronic Signatures (CAdES)", RFC 5126, March 2008.
Blazic, et al. Standards Track [Page 39]
RFC 6283 XMLERS July 2011
[TS-ENTRUST] The Slovenian Time Stamping Authority, Entrust XML
Schema for Time-Stamp,
http://www.si-tsa.gov.si/
dokumenti/timestamp-protocol-20020207.xsd.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[XAdES] Cruellas, J. C., Karlinger, G., Pinkas, D., Ross, J.,
"XML Advanced Electronic Signatures", XAdES, W3C Note,
February 2003.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD
70, RFC 5652, September 2009.
[RFC5698] Kunz, T., Okunick, S., and U. Pordesch, "Data Structure
for the Security Suitability of Cryptographic Algorithms
(DSSC)", RFC 5698, November 2009.
Blazic, et al. Standards Track [Page 40]
RFC 6283 XMLERS July 2011
Appendix A. Detailed Verification Process of an Evidence Record
To verify the validity of an Evidence Record start with the first ATS
till the last ATS (ordered by attribute Order) and perform
verification for each ATS, as follows:
1. Select corresponding archive object and its data object or a group
of data objects.
2. Re-encrypt data object or data object group, if the
<EncryptionInformation> field is used (see Section 5 for more
details)
3. Get a canonicalization method C and a digest method H from the
<DigestMethod> element of the current chain.
4. Make a new list L of digest values of (binary representation of)
objects (data, ATS, or sequence) that MUST be protected with this
ATS as follows:
a. If this ATS is the first in the Archive Time-Stamp Chain:
i. If this is the first ATS of the first ATSC (the initial ATS)
in the ATSSeq, calculate digest values of data objects with
H and add each digest value to the list L.
ii. If this ATS is not the initial ATS, calculate a digest value
with H of ordered ATSSeq without this and successive chains.
Add value H and digest values of data objects to the list L.
b. If this ATS is not the first in the ATSC:
i. Calculate the digest value with H of the previous
<TimeSatmp> element and add this digest value to the list L.
5. Verify the ATS's Time-Stamped value as follows. Get the first
sequence of the hash tree for this ATS.
a. If this ATS has no hash tree elements then:
ii. If this ATS is not the first in the ATSSeq (the initial
ATS), then the Time-Stamped value must be equal to the
digest value of previous Time-Stamp element. If not, exit
with a negative result.
Blazic, et al. Standards Track [Page 41]
RFC 6283 XMLERS July 2011
iii. If this ATS is the initial ATS in the ATSC, there must be
only one data object of the archive object. The digest
value of that data object must be the same as its Time-
Stamped value. If not, exit with a negative result.
b. If this ATS has a hash tree then: If there is a digest value in
the list L of digest values of protected objects, which cannot
be found in the first sequence of the hash tree or if there is
a hash value in the first sequence of the hash tree which is
not in the list L of digest values of protected objects, exit
with a negative result.
i. Get the hash tree from the current ATS and use H to
calculate the root hash value (see Sections 3.2.1 and
3.2.2).
ii. Get Time-Stamped value from the Time-Stamp Token. If
calculated root hash value from the hash tree does not match
the Time-Stamped value, exit with a negative result.
6. Verify Time-Stamp cryptographically and formally (validate the
used certificate and its chain, which may be available within
the Time-Stamp Token itself or <CryptographicInformation>
element).
7. If this ATS is the last ATS, check formal validity for the
current time (now), or get "valid from" time of the next ATS
and verify formal validity at that specific time.
8. If the needed information to verify formal validity is not
found within the Time-Stamp or within its Cryptographic
Information section of ATS, exit with a negative result.
Blazic, et al. Standards Track [Page 42]
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Authors' Addresses
Aleksej Jerman Blazic
SETCCE
Tehnoloski park 21
1000 Ljubljana
Slovenia
Phone: +386 (0) 1 620 4500
Fax: +386 (0) 1 620 4509
EMail:
[email protected]
Svetlana Saljic
SETCCE
Tehnoloski park 21
1000 Ljubljana
Slovenia
Phone: +386 (0) 1 620 4506
Fax: +386 (0) 1 620 4509
EMail:
[email protected]
Tobias Gondrom
Kruegerstr. 5A
85716 Unterschleissheim
Germany
Phone: +49 (0) 89 320 5330
EMail:
[email protected]
Blazic, et al. Standards Track [Page 43]
