Distributed Ledger Time-Stamp
draft-intesigroup-dlts-03
Network Working Group E. Cisbani
Internet-Draft D. Ribaudo
Intended status: Standards Track Intesi Group
Expires: 29 May 2022 G. Damiano
Entrust
25 November 2021
Distributed Ledger Time-Stamp
draft-intesigroup-dlts-03
Abstract
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.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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]
[ETSI.TS.101.861].
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
timestamps.
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
(DLA).
2. Terms and Definitions
The key words "*MUST*", "*MUST NOT*", "*REQUIRED*", "*SHALL*",
"*SHALL NOT*", "*SHOULD*", "*SHOULD NOT*", "*RECOMMENDED*", "*NOT
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
[RFC6962]
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
Ledger.
Distributed Ledger Attestation
A Distributed Ledger (Timestamping) Attestation is a proof or a
promise of timestamping in a precise Distributed Ledger.
Calendar
A calendar is simply a collection of Distributed Ledger
Attestations.
Calendar Server
A server providing remote access to a collection of Distributed
Ledger Attestations.
3. Symbols And Abbreviations
PKI
Public Key Infrastructure
TTP
Trusted Third Party
TSA
Time-Stamping Authority
TST
Time-Stamp Token
TSU
Time-Stamping Unit
DL
Distributed Ledger
DLA
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
confirmed.
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 }
DLProof ::= OCTET STRING
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
[RFC5652]
* 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
attributes.
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
follows:
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
follows:
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
(rejection).
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
extension.
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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
(waiting).
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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signature-1
+--- timestampToken
|--- signerIdentifier
|--- serialNumber-1
+--- id-ce-dltsPromises
+--- Promise
|--- version
|--- calendarFormat
|--- dlPromise
|--- signerIdentifier
+--- serialNumber-1
signature-2
+--- 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
extensions.
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,
<https://www.rfc-editor.org/info/rfc2119>.
[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,
<https://www.rfc-editor.org/info/rfc3628>.
[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,
<https://www.rfc-editor.org/info/rfc4210>.
[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,
<https://www.rfc-editor.org/info/rfc5280>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/info/rfc5652>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[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
[ANSI.X9.95]
American National Standards Institute (ANSI), "Trusted
Time Stamp Management And Security", 2005,
<https://webstore.ansi.org/Standards/ASCX9/ANSIX9952005>.
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[CrosbyWallach]
Crosby, S. and D. Wallach, "Efficient Data Structures for
Tamper-Evident Logging", Proceedings of the 18th USENIX
Security Symposium, Montreal, August 2009,
<http://static.usenix.org/event/sec09/tech/full_papers/
crosby.pdf>.
[eIDAS] The European Parliament And The Council Of The European
Union, "Regulation (EU) No 910/2014", 23 July 2014,
<https://eur-lex.europa.eu/legal-content/EN/TXT/
HTML/?uri=CELEX:32014R0910=EN>.
[ETSI.EN.319.422]
European Telecommunications Standards Institute,
"Electronic Signatures and Infrastructures (ESI); Time-
stamping protocol and time-stamp token profiles", March
2016, <https://www.etsi.org/deliver/
etsi_en/319400_319499/319422/01.01.01_60/
en_319422v010101p.pdf>.
[ETSI.TS.101.861]
European Telecommunications Standards Institute,
"Electronic Signatures and Infrastructures (ESI); Time
stamping profile", July 2011,
<https://www.etsi.org/deliver/
etsi_ts/101800_101899/101861/01.04.01_60/
ts_101861v010401p.pdf>.
[ETSI.TS.102.778-4]
European Telecommunications Standards Institute,
"Electronic Signatures and Infrastructures (ESI); PDF
Advanced Electronic Signature Profiles; Part 4: PAdES Long
Term - PAdES-LTV Profile", December 2009,
<https://www.etsi.org/deliver/
etsi_ts/102700_102799/10277804/01.01.02_60/
ts_10277804v010102p.pdf>.
[HaberStornetta]
Haber, S. and W. S. Stornetta, "How to Time-Stamp a
Digital Document", 1991,
<https://www.anf.es/pdf/Haber_Stornetta.pdf>.
[Hyperledger]
The Linux Foundation, "Hyperledger Architecture, Volume 1:
Introduction to Hyperledger Business Blockchain Design
Philosophy and Consensus", August 2017,
<https://www.hyperledger.org/wp-content/uploads/2017/08/
Hyperledger_Arch_WG_Paper_1_Consensus.pdf>.
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[IMES] Une, M., "The Security Evaluation of Time Stamping
Schemes: The Present Situation and Studies (2001)", 2001,
<http://citeseerx.ist.psu.edu/viewdoc/
summary?doi=10.1.1.23.7486>.
[ISO.IEC.18014-4]
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,
<http://www.merkle.com/papers/Thesis1979.pdf>.
[Nakamoto] Nakamoto, S., "Bitcoin: A Peer-to-Peer Electronic Cash
System", 31 October 2008,
<https://bitcoin.org/bitcoin.pdf>.
[NISTIR_8202]
Yaga, D., Mell, P., Roby, N., and K. Scarfone, "Blockchain
Technology Overview", October 2018,
<https://doi.org/10.6028/NIST.IR.8202>.
[OpenTimestamps]
Todd, P., "OpenTimestamps: Scalable, Trust-Minimized,
Distributed Timestamping with Bitcoin", 15 September 2016,
<https://petertodd.org/2016/opentimestamps-announcement>.
[OTS] Cisbani, E., "IANA registered OpenTimestamps Media Type",
24 June 2021, <https://www.iana.org/assignments/media-
types/application/vnd.opentimestamps.ots>.
[RFC6962] Laurie, B., Langley, A., and E. Kasper, "Certificate
Transparency", RFC 6962, DOI 10.17487/RFC6962, June 2013,
<https://www.rfc-editor.org/info/rfc6962>.
[UK-GCSA] UK Government Chief Scientific Adviser, "Distributed
Ledger Technology: beyond block chain", January 2016, <htt
ps://assets.publishing.service.gov.uk/government/uploads/s
ystem/uploads/attachment_data/file/492972/gs-16-1-
distributed-ledger-technology.pdf>.
Authors' Addresses
Emanuele Cisbani
Intesi Group
Via Torino 48
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20123 Milano
Italy
Phone: +39 026 760 641
Email: ecisbani@intesigroup.com
URI: https://www.intesigroup.com
Daniele Ribaudo
Intesi Group
Via Torino 48
20123 Milano
Italy
Phone: +39 026 760 641
Email: dribaudo@intesigroup.com
URI: https://www.intesigroup.com
Giuseppe Damiano
Entrust
One Station Square
Cambridge
CB1 2GA
United Kingdom
Email: giuseppe.damiano@entrust.com
URI: https://www.entrust.com
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