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Long-term Archive And Notary                           A. Jerman Blazic
Services (LTANS)                                                 SETCCE
Internet Draft                                                S. Saljic
Intended status: Standards Track                                 SETCCE
Expires: July 24, 2011                                       T. Gondrom
                                                       January 24, 2011

             Extensible Markup Language Evidence Record Syntax
                      draft-ietf-ltans-xmlers-11.txt


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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on March 18, 2011.

Copyright Notice

   Copyright (c) 2010 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
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   (http://trustee.ietf.org/license-info) in effect on the date of
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   described in the Simplified BSD License.

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. ERS-XML provides
   alternative syntax and processing rules to the ASN.1 (Abstract Syntax
   Notation One) ERS (Evidence Record Syntax) [RFC4998] syntax by using
   XML language.




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Table of Contents


   1. Introduction...................................................5
      1.1. Motivation................................................5
      1.2. General Overview and Requirements.........................7
      1.3. Terminology...............................................8
      1.4. Conventions Used in This Document........................10
   2. Evidence Record...............................................11
      2.1. Structure................................................11
      2.2. Generation...............................................16
      2.3. Verification.............................................17
   3. Archive Time-Stamp............................................18
      3.1. Structure................................................18
         3.1.1. Hash Tree...........................................18
         3.1.2. Time-Stamp..........................................20
         3.1.3. Cryptographic Information List......................21
      3.2. Generation...............................................22
         3.2.1. Generation of Hash Tree.............................23
         3.2.2. Reduction of hash tree..............................27
      3.3. Verification.............................................29
   4. Archive Time-Stamp Sequence and Archive Time-Stamp Chain......30
      4.1. Structure................................................31
         4.1.1. Digest Method.......................................32
         4.1.2. Canonicalization Method.............................33
      4.2. Generation...............................................34
         4.2.1. Time-Stamp Renewal..................................34
         4.2.2. Hash Tree Renewal...................................35
      4.3. Verification.............................................37
   5. Encryption....................................................39
   6. Version.......................................................40
   7. Storage of policies...........................................40
   8. XSD Schema for the Evidence Record............................41
   9. Security Considerations.......................................46
      9.1. Secure Algorithms........................................46
      9.2. Redundancy...............................................47


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      9.3. Secure Time-Stamps.......................................47
      9.4. Time-Stamp verification..................................47
   10. IANA Considerations..........................................48
   11. References...................................................51
      11.1. Normative References....................................51
      11.2. Informative References..................................52
   APPENDIX A: Detailed verification process of an Evidence Record..55
   Author's Addresses...............................................57






























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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 due to for example 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


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   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.

   The XMLERS (Extensible Markup Language Evidence Record Syntax )
   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 language, XMLERS does not present a direct
   transformation of ERS in ASN.1 syntax. The XMLERS syntax 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



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   defined in this draft. XMLERS is a standalone draft and is based on
   [RFC4998] conceptually only.

   Evidence Record Syntax in XML format is based on long term archive
   service requirements as defined in [RFC4810]. XMLERS syntax 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". The XMLERS syntax 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].

   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 draft.

   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


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   [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 timestamp 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 Timestamps are
   comprised of an optional reduced hash tree and a timestamp.

   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 included Time-Stamp(s).

   Due to the fact that digest algorithms or cryptographic methods used
   may become weak or that 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.

1.3. Terminology

   Archive data object: Data unit that is archived and has to be
   preserved for a long time by the Long-term Archive Service.

   Archive data object group: A set of archive data objects, which for
   some reason (logically) belong together, e.g. a group of document



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   files or a document file and a signature file could represent an
   archive data object group.

   Archive object: an archive data object or an archive data object
   group.

   Archive Time-Stamp (ATS): An Archive Time-Stamp 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): holds a sequence of Archive Time-
   Stamps generated during the preservation period.

   Archive Time-Stamp Sequence (ATSSeq): is a sequence of Archive Time-
   Stamp Chains.

   Canonicalization: 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: Data or part of data related to the
   validation process of signed data, e.g. digital certificates, digital
   certificate chains, certificate revocation lists, etc.

   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: Information that may be used to resolve a dispute about
   various aspects of authenticity, validity and existence of archived
   data objects.

   Evidence Record: 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 Service (LTA): A service responsible for
   generation, collection and maintenance (renewal) of evidence data. A
   LTA service may also preserve data for long periods of time, e.g.
   storage of archive data and associated evidences.

   Hash Tree: 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 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].



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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.

   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
   (ATS). An ATS contains a Time-Stamp Token and optionally other useful
   data for Time-Stamp validation, e.g. certificates, CRLs (certificate
   revocation list) or OCSP (Online Certificate Status Protocol)
   responses and also specific attributes such as service policies.


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   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):

   <EvidenceRecord Version>

      <EncryptionInformation>
         <EncryptionInformationType>
         <EncryptionInformationValue>
      </EncryptionInformation> ?
      <SupportingInformationList>
         <SupportingInformation Type /> +
      </ SupportingInformation> ?
      <ArchiveTimeStampSequence>
         <ArchiveTimeStampChain Order>
            <DigestMethod Algorithm />
            <CanonicalizationMethod Algorithm />
            <ArchiveTimeStamp Order>
               <HashTree /> ?
               <TimeStamp>
                  <TimeStampToken Type />
                  <CryptographicInformationList>
                     <CryptographicInformation Order Type /> +
                  </CryptographicInformationList> ?
               </TimeStamp>
               <Attributes>


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                  <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 the XMLERS. Current version of the XMLERS
     syntax 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 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


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     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 in 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> element.

     <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 archive



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     data object(s), previous Archive Time-Stamp sequence, Time-Stamps
     and within a Time-Stamp Token.

     <CanonicalizationMethod> is a REQUIRED element that specifies which
     canonicalization algorithm is applied to the archive data for XML
     data objects, <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 data object(s)
     preservation and evidence validation at a later stage. Each data
     object is put into a separate child element <Attribute>, with a


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     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 [RFC5698, section A.2].

     The Order attribute is REQUIRED in all cases when one or more XML
     elements with the same name occur on the same level in the 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.

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


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   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 draft 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
   archived object data group and will be protected within 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 TS(s).

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 Timestamp Sequence (details in Section 3.3. and
      Section 4.3.).



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3. Archive Time-Stamp

   An Archive Time-Stamp is a timestamp 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



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   value that is generated from the values of the hash tree MUST be the
   same as the time-stamped value.

   <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 from 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


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   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 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 <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>




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   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 timestamps (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 List), 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
   <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:



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   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.



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   4. Generate a hash tree for selected archive object (see 3.2.1).

      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 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 <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


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   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 are more than one data objects 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.

      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.




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   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,
   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.


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   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:

   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 archive object
      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 AO's data objects, each within a separate
      <DigestValue> element.

   3. Select all hash values, which 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).




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   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):

   <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>


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         <DigestValue>base64 encoded h''3</DigestValue>
     </Sequence>
     <Sequence Order='4'>
         <DigestValue>base64 encoded h'''2</DigestValue>
     </Sequence>
   </HashTree>


3.3. Verification

   The initial Archive Timestamp 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



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     archive object is having more data objects and the hash tree is
     omitted, also exit with negative result.

   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 <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 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 <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 loosing 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 timestamp 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


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   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 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 (ATSS).

   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) which are generated
   through renewal processes. The renewal process results in a series of
   Evidence Record elements: <ArchiveTimeStampSequence> element contains
   an ordered sequence of <ArchiveTimeStampChain> elements and
   <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:




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   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.

   The Algorithm attribute contains URIs [RFC3986] for identifiers which
   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 Token(s) 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.






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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(s). 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 constrains 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 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 <CryptographicInformationList> element of the last Archive
      Time-Stamp (each data object into a separate
      <CryptographicInformation> element).

   2. Select canonicalization method from <CanonicalizationMethod>
      element and select digest algorithm from <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.




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   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 <ArchiveTimeStampChain> element. Note that the new ATS MAY not
   contain a hash tree. However, 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.

4.2.2. Hash Tree Renewal

   The process of hash tree renewal occurs when the new digest algorithm
   is different to 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).





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   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.



   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



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         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:



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   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 Time-
      Stamp Chain and calculate 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.





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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 the evidence records
   in the future, additional special precautions have to be taken.

   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


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   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 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.

   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


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   of storing DSSC policies [RFC5698], the types to be used in the
   <Attribute> or <SupportingInformation> element are defined in
   [RFC5698, section A.2] 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">


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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"/>


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   </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"/>
      </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>


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   <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"/>
                     </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>


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   <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"/>


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                  </xs:restriction>
               </xs:complexContent>
            </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
   Timestamp as well as hash algorithms and public key algorithms of the
   timestamps. 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.





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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 on Archive Time-Stamp for example), weakening
   cryptographic algorithms used to generate Time-Stamp Tokens would
   affect many TSAs at the same time.

   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 the
   time of renewal (i.e. when the new Time-Stamp is applied) the


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   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).



10. IANA Considerations

   For all IANA registrations related to this document the
   "Specification Required" [RFC5226] allocation policies MUST be used.





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   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 (ERSXML)". 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 be 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


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               Token

   XMLENTRUST EnTrust XML Schema   http://www.entrust.com

                                   /schemas/timestamp

                                   19protocol-20020207

   The sub-registry "Cryptographic Information Type" contains textual
   names and description, which should refer to 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 be 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



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   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", BPC 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.

   [RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
             January 2004.

   [RFC3275] Eastlake, D., Reagle, J., Solo, D., "XML-Signature Syntax
             and Processing", RFC 3275, March 2002.

   [RFC4051] Eastlake, D., "Additional XML Security Uniform Resource
             Identifiers", RFC 4051, April 2005.

   [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
             Encodings", RFC 4648, October 2006.

   [RFC4998] Gondrom, T., Brandner, R., Pordesch, U., "Evidence Record
             Syntax (ERS)", RFC 4998, August 2007.



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   [RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D. and Polk,
             W., "Server-Based Certificate Validation Protocol (SCVP)",
             RFC 5055, December 2007

   [RFC5280] Cooper, D., Santesson, S., Farell, S., Boyen, S., Housley,
             R.,Polk, W., "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.

11.2. Informative References

   [ANSI.X9-95.2005] American National Standard for Financial Services,
             "Trusted Timestamp Management and Security", ANSI X9.95,
             June 2005.






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   [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.

   [RFC3470] Hollenbeck, S., Rose, M., Masinter, L., "Guidelines for the
             Use of Extensible Markup Language (XML) within IETF
             Protocols", RFC 3470, January 2003.

   [RFC4810] Wallace, C., Pordesch, U., Brandner, R., "Long-Term Archive
             Service Requirements", RFC 4810, March 2007.

   [RFC5126] Pinkas, D., Popoe, N., Ross, J., "CMS Advanced Electronic
             Signatures (CAdES)", RFC 5126, February 2008.

   [TS-ENTRUST]   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.


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   [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)", RFC
             5652, September 2009.

   [RFC5698] Kunz, T., Okunick, S., Pordesch, U., "Data Structure for
             the Security Suitability of Cryptographic Algorithms
             (DSSC)", RFC 5698, November 2009
























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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
      <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.


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   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 digest
               value of previous Time-Stamp element. If not, exit with a
               negative result.

         iii. If this ATS is the initial ATS in 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).




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   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.



Author's Addresses

   Aleksej Jerman Blazic
   SETCCE
   Tehnoloski park 21
   1000 Ljubljana
   Slovenia

   Phone: +386 (0) 1 620 4500
   Fax:   +386 (0) 1 620 4509
   Email: aljosa@setcce.si


   Svetlana Saljic
   SETCCE
   Tehnoloski park 21
   1000 Ljubljana
   Slovenia

   Phone: +386 (0) 1 620 4506
   Fax:   +386 (0) 1 620 4509
   Email: svetlana.saljic@setcce.si






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   Tobias Gondrom
   Kruegerstr. 5A
   85716 Unterschleissheim
   Germany

   Phone: +49 (0) 89 320 5330
   Email: tobias.gondrom@gondrom.org































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