Distributed Ledger Time-Stamp

Document Type Active Internet-Draft (individual)
Authors Emanuele Cisbani  , Daniele Ribaudo  , Giuseppe Damiano 
Last updated 2021-11-26
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Network Working Group                                         E. Cisbani
Internet-Draft                                                D. Ribaudo
Intended status: Standards Track                            Intesi Group
Expires: 29 May 2022                                          G. Damiano
                                                        25 November 2021

                     Distributed Ledger Time-Stamp


   This document defines a standard to extend Time Stamp Tokens with
   Time Attestations recorded on Distributed Ledgers.

   The aim is to provide long-term validity to Time Stamp Tokens,
   backward compatible with currently available software.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 29 May 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   4
   3.  Symbols And Abbreviations . . . . . . . . . . . . . . . . . .   5
   4.  DL Attestation  . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  DL Time-Stamp Objects . . . . . . . . . . . . . . . . . . . .   7
     5.1.  DL Time-Stamp Attributes  . . . . . . . . . . . . . . . .   8
       5.1.1.  Response Status . . . . . . . . . . . . . . . . . . .   8
     5.2.  DL Time-Stamp Extensions  . . . . . . . . . . . . . . . .   9
       5.2.1.  Response Status . . . . . . . . . . . . . . . . . . .  10
     5.3.  Use case  . . . . . . . . . . . . . . . . . . . . . . . .  10
       5.3.1.  Promises  . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Attesting that a file existed prior to a specific point in time can
   be useful - for example - to:

   *  prove when an agreement was signed, if it is disputed

   *  validate a signature after a revocation occurred

   *  prove the ownership for copyright

   *  grant record integrity

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   A Time-Stamp Token (TST) provided by a Time-Stamp Authority (TSA)
   compliant with RFC 3161 [RFC3161] can be based on an accurate time
   source linked to Coordinated Universal Time, and can be very precise
   - it can prove the existence also at the second or less.  It is such
   a consolidated standard that - for example - the European Union
   legally enforced its usage by eIDAS Regulation [eIDAS], European
   Standards and Technical Specifications [ETSI.EN.319.422]

   In an in-deep appraisal of Time Stamping Schemes conducted in 2001 by
   Masashi Une [IMES], PKI TSA was evaluated as one of the most
   desirables in term of security against alteration of a time stamp.

   The integrity of the timestamping process that is inevitably bound to
   the integrity of the TSA gave rise to other proposals like ANSI X9.95
   [ANSI.X9.95] and ISO/IEC 18014-4 [ISO.IEC.18014-4].

   Furthermore a TSA TST can be validated for a limited time - usually
   no longer than 20 years for technical reasons such as the TSA
   certificates expiration, or for economic reasons such as the cost of
   providing the validation service by TSA.

   This situation brought about some solutions [ETSI.TS.102.778-4] aimed
   at mitigating the inconvenience by extending the validity of TSA

   Security of a Distributed Ledger (def. in Section 2) is based on
   hashes of data timestamped and widely published.  Each timestamp
   includes the previous timestamp in its hash, forming a chain, with
   each additional timestamp reinforcing the ones before it.

   The advantage of a Distributed Ledger Attestation (DLA) relies on the
   resilience of the distributed system and the overall design whose aim
   is the DL perpetual survival.

   Based on a distributed trust scheme, a Distributed Ledger
   significantly increases security as already noted by Haber and
   Stornetta in 1991 [HaberStornetta].

   In the case of a permissioned DL, security is provided by an
   authoritative network of trust [Hyperledger][NISTIR_8202], while in
   the case of a permissionless DL security is provided by the economic
   incentive for running full nodes [Nakamoto].

   On the other hand, a DLA is not yet a standard solution.
   Furthermore, the bigger the network the less precise the DLA, because
   distributed nodes need time to reach consensus.

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   Since a DLA turns out to be a complementary element providing long-
   term validity to TST - the aim of this specification is to allow an
   extension of the Time-Stamp Token for Distributed Ledger Attestations

2.  Terms and Definitions

   The key words "*MUST*", "*MUST NOT*", "*REQUIRED*", "*SHALL*",
   RECOMMENDED*", "*MAY*", and "*OPTIONAL*" in this document are to be
   interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only
   when, they appear in all capitals, as shown here.

   This document also refers to the following terms and definitions:

   Public Key Infrastructure
      As defined in [RFC5280]

   Trusted Third Party
      As defined in [RFC3161]

   Time-Stamping Authority
      As defined in [RFC3161]

   Time-Stamp Token
      As defined in [RFC3161]

   Time-Stamping Unit
      As defined in [RFC3628]

   Distributed Ledger
      Various definitions of blockchain and distributed ledger
      technology exist, and some of these stress different technical
      features.  Given the nature and scope of this document and the
      lack of definitional consensus we chose to use the term as defined
      by UK Government Chief Scientific Adviser [UK-GCSA] "A distributed
      ledger is essentially an asset database that can be shared across
      a network of multiple sites, geographies or institutions.  All
      participants within a network can have their own identical copy of
      the ledger.  Any changes to the ledger are reflected in all copies
      in minutes, or in some cases, seconds.  The assets can be
      financial, legal, physical or electronic.  The security and
      accuracy of the assets stored in the ledger are maintained
      cryptographically through the use of 'keys' and signatures to
      control who can do what within the shared ledger.  Entries can
      also be updated by one, some or all of the participants, according
      to rules agreed by the network".

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   Merkle Tree
      As defined in [Merkle], [CrosbyWallach] and Section 2.1 of

   Aggregation Server
      A server providing the aggregation of digests to be timestamped in
      a Merkle Tree.  Digests submitted for aggregation are added to a
      list periodically combined into a single Merkle Tree.  Then the
      digest at the root of that tree is timestamped on a Distributed

   Distributed Ledger Attestation
      A Distributed Ledger (Timestamping) Attestation is a proof or a
      promise of timestamping in a precise Distributed Ledger.

      A calendar is simply a collection of Distributed Ledger

   Calendar Server
      A server providing remote access to a collection of Distributed
      Ledger Attestations.

3.  Symbols And Abbreviations

      Public Key Infrastructure

      Trusted Third Party

      Time-Stamping Authority

      Time-Stamp Token

      Time-Stamping Unit

      Distributed Ledger

      Distributed Ledger Attestation

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4.  DL Attestation

   A Digital Ledger can be seen as an untrusted logger - serving a
   number of clients who wish to store their events in the log - kept
   honest by a number of auditors who will challenge the logger to prove
   its correct behaviour [CrosbyWallach].

   A Merkle Tree data structure accomplishes this in a very efficient
   way by aggregating many requests and submitting periodically to the
   log only the root digest of the tree.  This log is built as a hash
   chain (aka blockchain) of small blocks of data.  Consequently, the
   entire chain can be shared and maintained by a large number of nodes,
   becoming a distributed system.

   In a permissioned DL the number of nodes can be small enough to
   permit a quick synchronization and reach consensus concerning the
   state of the chain.  In a permissionless DL the large number of nodes
   introduces a relevant delay in order to reach consensus.

   In the case of Bitcoin, for example, consensus is reached
   statistically.  Usually in an average elapsed time of one hour six
   new blocks are added to the chain.  A block of data that was added
   before the last six blocks, is considered to be practically
   immutable.  This is due to the high computational power that would be
   required to rewrite the chain.

   As a result of this scenario the elapsed time - from the request of
   aggregation of a digest to the proof consolidated inside the DL, may
   amount to one hour or more.

   This is why we distinguish between a *promise* of attestation and a
   *proof* of attestation.  Generally, an Aggregation Server provides
   only a promise to timestamp the client's digest in the DL.  However,
   when the aggregation is completed and the Merkle Tree root hash
   recorded in a block within the chain, the promise has not yet been

   Only after reaching consensus on that block can attestation be
   considered as proof, and made available by the Calendar Server.

   For the sake of simplicity, the Aggregation Server and the Calendar
   Server can be implemented as a unique instance.  In this document we
   will generically refer to a Calendar Server indicating both services.

   The DLA data structure is out of scope in this specification
   document.  Any Calendar Server can define his application protocol
   and data structure.  For this specification the DLA is considered as
   pure data.

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5.  DL Time-Stamp Objects

   The ASN.1 structure of Promise type is as follows:

   Promise ::= SEQUENCE {
       version          INTEGER,
       calendarFormat   UTF8String,
       dlPromise        DLPromise,
       signerIdentifier issuerAndSerialNumber,
       serialNumber     INTEGER }

   DLPromise ::= OCTET STRING

   The ASN.1 structure of Proof type is as follows:

   Proof ::= SEQUENCE {
       version          INTEGER,
       calendarFormat   UTF8String,
       dlProof          DLProof,
       signerIdentifier issuerAndSerialNumber,
       serialNumber     INTEGER }


   The fields of Promise and Proof type have the following meanings:

   *  version is the syntax version number.  It MUST always be 0.  The
      usage is as described in Section 1.3 of [RFC5652]

   *  calendarFormat is the media type format of the DL attestation.  It
      MUST be a registered application media type, in accordance with
      procedures laid out in [RFC6838] - for example, if you wanted to
      use the [OpenTimestamps] format, the calendarFormat value would be
      the string "application/vnd.opentimestamps.ots" (without quotes)
      that is the IANA registered Media Type [OTS]

   *  dlProof and dlPromise are the proof and promise obtained from a
      Calendar Server using as input value the value of the signature
      field of the SignerInfo structure inside the digital signature of
      the TimeStampToken, as described in Section 5.3 of [RFC5652]

   *  signerIdentifier is an IssuerAndSerialNumber type that identifies
      the TSU signing certificate as described in Section 10.2.4 of

   *  serialNumber is an integer assigned by the TSA to each
      TimeStampToken as described in Section 2.4.2 of [RFC3161]

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5.1.  DL Time-Stamp Attributes

   A set of proofs or a set of promises, generated by a Calendar Server,
   MAY be included in a TST, using an unsigned attribute of the per-
   signer information.

   To grant backward compatibility with any currently available software
   the unsigned attribute MUST be compliant with the specifications
   defined in Section 5.3 of [RFC5652] for Attribute type.

   Attributes including a set of promises and a set of proofs MUST be
   unsigned attributes; they MUST NOT be signed attributes,
   authenticated attributes, unauthenticated attributes, or unprotected

   The new objects MUST have the following OIDs where id-ce identifies
   the root of standard extensions as described in [RFC5280].

   The ASN.1 structure of attributes including a set of promises is as

   id-ce-dltsPromises OBJECT IDENTIFIER ::= { id-ce TBD1 }

   Promises            SET OF Promise

   The ASN.1 structure of attributes including a set of proofs is as

   id-ce-dltsProofs OBJECT IDENTIFIER ::= { id-ce TBD2 }

   Proofs              SET OF Proof

   All the proofs and promises that have been returned MUST refer to the
   same parent TimeStampToken issued at the time of the request.

   Note that a TSA can return a set of proofs and promises for the same
   input value as it can use calendar servers operating on different
   Distributed Ledgers.

5.1.1.  Response Status

   The response status code in the TimeStampResp MUST be compliant with
   the specifications described in Section 2.4.2 of [RFC3161] and
   Section 5.2.3 of [RFC4210].

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   According to the TimeStamp policy, when the response contains only a
   subset of the expected proofs and promises, the status field SHOULD
   contain either the value one (grantedWithMods) or the value two

5.2.  DL Time-Stamp Extensions

   Upgrade from a set of promises to a set of proofs MAY be done
   requesting a new TST including inside a non critical extension the
   set of promises previously obtained in an unsigned attribute.

   When the TSA receives a request which has a non critical extension
   containing a set of promises, it MAY request the Calendar Server to
   get the corresponding proof for each of them, and MAY include the set
   of proofs in the TST response, using a non critical extension of the
   TSTInfo sequence.

   To grant backward compatibility with any currently available
   software, request and response non critical extensions MUST be
   compliant with the specifications described in Section 2.4 of
   [RFC3161] and Section 4.2 of [RFC5280].

   Conforming TSAs MUST mark these extensions as non-critical.

   The ASN.1 structure of the proof request extension is as follows:

   id-ce-dltsPromises OBJECT IDENTIFIER

   Promises            SET OF Promise

   The ASN.1 structure of the proof response extension is as follows:

   id-ce-dltsProofs OBJECT IDENTIFIER

   Proofs               SET OF Proof

   The proofs returned in the extensions by the TSA MUST NOT refer to
   the TimeStampToken issued at the time of the request.  Each Proof
   MUST contain the explicit reference to the pointing TimeStampToken
   with signerIdentifier (referring to the TSU certificate) and
   serialNumber (referring to the time stamp serial number), which have
   been received in the Promise structure of the proof request

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5.2.1.  Response Status

   The response status code in the TimeStampResp MUST be compliant with
   the specifications described in Section 2.4.2 of [RFC3161] and
   Section 5.2.3 of [RFC4210].

   Compliant servers SHOULD also use the status field as follows:

   *  according to TimeStamp policy, when the response contains only a
      subset of the expected proofs, the status field SHOULD contain
      either the value one (grantedWithMods) or two (rejection)

   *  when in the response no proof can be returned, the status field
      SHOULD contain the value two (rejection)

   *  when all the received promises recognized by the Calendar Server
      are pending, the status field SHOULD contain the value three

5.3.  Use case

   In order to clarify the use of the objects thus defined, the case of
   a subscription made by two actors at different times, using distinct
   time stamps, is illustrated below.

5.3.1.  Promises

   Since each signer applies a time stamp to his signature, the
   structure will be presented according to the following simplified
   scheme, in which each promise is inserted as an unsigned attribute of
   the time stamp to which it refers.

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               +--- timestampToken
                           |--- signerIdentifier
                           |--- serialNumber-1
                           +--- id-ce-dltsPromises
                                   +--- Promise
                                           |--- version
                                           |--- calendarFormat
                                           |--- dlPromise
                                           |--- signerIdentifier
                                           +--- serialNumber-1
               +--- timestampToken
                           |--- signerIdentifier
                           |--- serialNumber-2
                           +--- id-ce-dltsPromises
                                   +--- Promise
                                           |--- version
                                           |--- calendarFormat
                                           |--- dlPromise
                                           |--- signerIdentifier
                                           +--- serialNumber-2

                             Figure 1: Figure 1

   Although replicating the signerIdentifier and serialNumber
   information may seem redundant in the case of a single timestamp, it
   can never be ruled out that a second signature with a new timestamp
   will be added later.

   When you also want to obtain the proof of attestation on the DL, the
   application will be able to collect the two promises and include them
   as extensions in a new timestamp request.  The result would have the
   following structure:

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               +--- timestampToken
                           |--- signerIdentifier
                           |--- serialNumber-3
                           +--- id-ce-dltsPromises
                                   +--- Proof
                                           |--- version
                                           |--- calendarFormat
                                           |--- dlPromise
                                           |--- signerIdentifier
                                           +--- serialNumber-1
                                   +--- Proof
                                           |--- version
                                           |--- calendarFormat
                                           |--- dlPromise
                                           |--- signerIdentifier
                                           +--- serialNumber-2

                             Figure 2: Figure 2

   From this example it is evident that the signerIdentifier and
   serialNumber pair is necessary to uniquely identify the
   TimestampToken to which each Proof obtained refers.

   It is up to the application to choose whether the new timestamp,
   containing the evidence, will be saved within the same document,
   containing the promises, or stored separately.

6.  Security Considerations

   Each security consideration described in Section 4 of [RFC3161] SHALL
   be evaluated designing TSA services that include DL Time-Stamp

   When a TSA executes a request to a Calendar Server the use of a nonce
   is RECOMMENDED because using a nonce always allows the client to
   detect replays.

   Safety and reliability of the DL proofs depends on the robustness of
   the hash algorithms and on the stability of the DL, i.e. how
   expensive or difficult it would be for an attacker to alter the DL.

7.  IANA Considerations

   This document does not require any action by IANA.

8.  References

8.1.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3161]  Adams, C., Cain, P., Pinkas, D., and R. Zuccherato,
              "Internet X.509 Public Key Infrastructure Time-Stamp
              Protocol (TSP)", RFC 3161, DOI 10.17487/RFC3161, August
              2001, <https://www.rfc-editor.org/info/rfc3161>.

   [RFC3628]  Pinkas, D., Pope, N., and J. Ross, "Policy Requirements
              for Time-Stamping Authorities (TSAs)", RFC 3628,
              DOI 10.17487/RFC3628, November 2003,

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

              American National Standards Institute (ANSI), "Trusted
              Time Stamp Management And Security", 2005,

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              Crosby, S. and D. Wallach, "Efficient Data Structures for
              Tamper-Evident Logging", Proceedings of the 18th USENIX
              Security Symposium, Montreal, August 2009,

   [eIDAS]    The European Parliament And The Council Of The European
              Union, "Regulation (EU) No 910/2014", 23 July 2014,

              European Telecommunications Standards Institute,
              "Electronic Signatures and Infrastructures (ESI); Time-
              stamping protocol and time-stamp token profiles", March
              2016, <https://www.etsi.org/deliver/

              European Telecommunications Standards Institute,
              "Electronic Signatures and Infrastructures (ESI); Time
              stamping profile", July 2011,

              European Telecommunications Standards Institute,
              "Electronic Signatures and Infrastructures (ESI); PDF
              Advanced Electronic Signature Profiles; Part 4: PAdES Long
              Term - PAdES-LTV Profile", December 2009,

              Haber, S. and W. S. Stornetta, "How to Time-Stamp a
              Digital Document", 1991,

              The Linux Foundation, "Hyperledger Architecture, Volume 1:
              Introduction to Hyperledger Business Blockchain Design
              Philosophy and Consensus", August 2017,

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   [IMES]     Une, M., "The Security Evaluation of Time Stamping
              Schemes: The Present Situation and Studies (2001)", 2001,

              International Organization for Standardization,
              "Information technology - Security techniques - Time-
              stamping services - Part 4: Traceability of time sources",
              April 2015, <https://www.iso.org/standard/59934.html>.

   [Merkle]   Merkle, R. C., "Secrecy, authentication, and public-key
              systems - Technical Report No. 1979-1", June 1979,

   [Nakamoto] Nakamoto, S., "Bitcoin: A Peer-to-Peer Electronic Cash
              System", 31 October 2008,

              Yaga, D., Mell, P., Roby, N., and K. Scarfone, "Blockchain
              Technology Overview", October 2018,

              Todd, P., "OpenTimestamps: Scalable, Trust-Minimized,
              Distributed Timestamping with Bitcoin", 15 September 2016,

   [OTS]      Cisbani, E., "IANA registered OpenTimestamps Media Type",
              24 June 2021, <https://www.iana.org/assignments/media-

   [RFC6962]  Laurie, B., Langley, A., and E. Kasper, "Certificate
              Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,

   [UK-GCSA]  UK Government Chief Scientific Adviser, "Distributed
              Ledger Technology: beyond block chain", January 2016, <htt

Authors' Addresses

   Emanuele Cisbani
   Intesi Group
   Via Torino 48

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   20123 Milano

   Phone: +39 026 760 641
   Email: ecisbani@intesigroup.com
   URI:   https://www.intesigroup.com

   Daniele Ribaudo
   Intesi Group
   Via Torino 48
   20123 Milano

   Phone: +39 026 760 641
   Email: dribaudo@intesigroup.com
   URI:   https://www.intesigroup.com

   Giuseppe Damiano
   One Station Square
   CB1 2GA
   United Kingdom

   Email: giuseppe.damiano@entrust.com
   URI:   https://www.entrust.com

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