Internet Engineering Task Force                              T. Hardjono
Internet-Draft                                                       MIT
Intended status: Informational                             M. Hargreaves
Expires: April 5, 2021                                     Quant Network
                                                                N. Smith
                                                         October 2, 2020

        An Interoperability Architecture for Blockchain Gateways


   With the increasing interest in the potential use of blockchain
   systems for virtual assets, there is a need for virtual assets to
   have mobility across blockchain systems.  An interoperability
   architecture is needed to permit the flow of virtual assets between
   blockchain systems.  The architecture must recognize that there are
   different blockchain systems, and that the interior constructs in
   these blockchains maybe incompatible with one another.  Gateway nodes
   perform the transfer of virtual assets between blockchain systems
   while masking the complexity of the interior constructs of the
   blockchain that they represent.

Status of This Memo

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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   ( in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Assumptions and Principles  . . . . . . . . . . . . . . . . .   4
     3.1.  Design Principles . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Operational Assumptions . . . . . . . . . . . . . . . . .   5
   4.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Goal of Architecture  . . . . . . . . . . . . . . . . . .   5
     4.2.  Overview of Asset Transfer  . . . . . . . . . . . . . . .   6
     4.3.  Phases in Asset Transfer  . . . . . . . . . . . . . . . .   7
       4.3.1.  Phase 1: Exchange of security parameters and asset
               information . . . . . . . . . . . . . . . . . . . . .   7
       4.3.2.  Phase 2: Evidence of asset locking  . . . . . . . . .   8
       4.3.3.  Phase 3: Final commitment of transfer . . . . . . . .   8
   5.  Related Open Issues . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Global identification of blockchain systems and public-
           keys  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Selection of gateways nodes within a blockchain system  .   9
     5.3.  Commitment protocols and forms of commitment evidence . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  Policy Considerations . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   Currently there is little technical interoperability between
   blockchain systems.  This results in the difficulty in transferring
   or migrating virtual assets associated with a public-key (address) in
   one blockchain system to another blockchain system.

   The existing solutions involve a third party that mediates the
   transfer.  This mediating third party is typically an asset-exchange
   entity (i.e. crypto-exchange) operating in a centralized hub-spoke
   fashion.  This reliance on a third party results not only delays in

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   transfers, but also in the need for key-holders to open accounts at
   the third party entity.

   This document describes an architecture for blockchain gateways that
   perform the unidirectional transfer of virtual assets between two
   autonomous blockchain systems which employ the gateways.

   The purpose of this architecture document is to provide technical
   framework within which to discuss the various aspects of a transfer
   between two gateways, including security aspects and transfer
   commitment aspects.

2.  Terminology

   There following are some terminology used in the current document:

   o  Blockchain Domain: The collection of resources and entities
      participating within a blockchain system.

   o  Interior Resources: The various interior protocols, data
      structures and cryptographic constructs that are a core part of a
      blockchain system.  Examples of interior resources include the
      ledger, consensus protocol, incentive mechanisms, transaction
      propagation networks, etc.

   o  Exterior Resources: The various resources that are outside a
      blockchain system, and are not part of the operations of a
      blockchain.  Examples include data located at third parties such
      as asset registries, ledgers of other blockchains, PKI
      infrastructures, etc.

   o  Blockchain nodes: The nodes of the blockchain system which form
      the peer-to-peer network, which collectively maintain the shared
      ledger in the blockchain by following a consensus algorithm

   o  Blockchain address: This is the public-key of an entity as known
      within a blockchain system, employed to transact on the blockchain
      network and recorded on the ledger of the blockchain.  Also
      refered to as transaction signing key pair.

   o  Entity public-key: This the private-public key pairs of an entity
      used for interactions outside the blockchain system (e.g.  TL1.2
      key-pairs).  We use this term to distinguish this key pair from
      the blockchain address.

   o  Gateway node: This is the node that implements the asset transfer
      protocol for the purpose of transferring or migrating assets

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      across blockchain systems.  Depending on the blockchain system
      implementation, some or all of the nodes may be gateway-capable.

   o  Asset transfer protocol: The technical protocol used by two
      gateway nodes to transfer a virtual asset.

   o  Virtual Asset: A virtual asset is a digital representation of
      value that can be digitally traded, or transferred as defined by
      the FATF [REF].

   o  Virtual Asset Service Provider (VASP): Legal entity handling
      virtual assets as defined by the FATF [FATF].

   o  Originator: Person or entity seeking the transmittal of virtual
      asset to a beneficiary.

   o  Beneficiary: Person or entity receiving the transmitted virtual
      asset from an originator.

   Further terminology definitions can be found in [NIST].

3.  Assumptions and Principles

   The following assumptions and principles underlie the design of the
   current interoperability architecture, and correspond to the design
   principles of the Internet architecture.

3.1.  Design Principles

   o  Opaque blockchain resources: The interior resources of each
      blockchain system is assumed to be opaque to (hidden from)
      external entities.  Any resources to be made accessible to an
      external entity must be made explicitly accessible by a gateway
      node with proper authorization.

   o  Externalization of value: The gateway protocol is agnostic
      (oblivious) to the economic or monetary value of the virtual asset
      being transferred.

   The opaque resources principle permits the interoperability
   architecture to be applied in cases where one (or both) blockchain
   systems are permissioned (private).  It is the analog of the
   autonomous systems principle in IP networking [Clar88], where
   interior routes in local subnets are not visible to other external
   autonomous systems

   The value-externalization principle permits asset transfer protocols
   to be designed for efficiency, speed and reliability - independent of

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   the changes in the perceived economic value of the virtual asset.  It
   is the analog of the end-to-end principle in the Internet
   architecture [SRC84], where contextual information (economic value)
   is placed at the endpoints of the transaction.  In the case of
   virtual asset transfers, the originator and beneficiary at the
   respective blockchain systems are assumed to have a common agreement
   regarding the economic value of the asset.

3.2.  Operational Assumptions

   The following conditions are assumed to have occurred, leading to the
   invocation of the asset transfer protocol between two gateway nodes:

   o  Application layer transfer request: The transfer request from an
      originator in the origin blockchain is assumed to have occurred
      prior to the execution of the asset transfer protocol.

   o  Identification of originator and beneficiary: The originator and
      beneficiary are assumed to have been identified and that consent
      has been obtained from both parties regarding the asset transfer.

   o  Identification of origin and destination blockchain: The origin
      and destination blockchain systems is assumed to have been

   o  Selection of gateway nodes: The two gateway nodes at the origin
      and destination blockchain systems respectively is assumed to have
      been selected.

   o  Owners of gateway nodes are known: The legal entity operating the
      gateway nodes are assumed to be known.

4.  Architecture

4.1.  Goal of Architecture

   The goal of the interoperability architecture is to permit two (2)
   gateway nodes belonging to distinct blockchain systems to conduct a
   virtual asset transfer between them, in a secure and non-repudiable
   manner while ensuring the asset does not exist simultaneously on both
   blockchains (double-spend problem).

   The virtual asset as understood by the two gateway nodes is a digital
   representation of value, expressed in an standard digital format in a
   way meaningful to the gateway nodes syntactically and semantically.

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   The syntactic representation of the virtual asset between the two
   gateways need not bear any resemblance to the syntactic asset
   representation within their respective blockchain systems.

   The architecture recognizes that there different blockchain systems
   currently in operation and evolving, and that in many cases the
   interior technical constructs in these blockchains maybe incompatible
   with one another.

   The architecture therefore assumes that certain types of nodes
   (gateway nodes) will be equipped with an asset transfer protocol and
   other relevant resources that permits greater interoperability across
   these incompatible blockchain systems.

   The resources within a blockchain system (e.g. ledgers, public-keys,
   consensus protocols, etc.) are assumed to be opaque to external
   entities in order to permit a resilient and scalable protocol design
   that is not dependent on the interior constructs of particular
   blockchain systems.  This ensures that the virtual asset transfer
   protocol between gateways is not conditioned or dependent on these
   local technical constructs.  The role of a gateway therefore is also
   to mask (hide) the complexity of the interior constructs of the
   blockchain system that it represents.  Overall this approach ensures
   that a given blockchain system operates as a true autonomous system.

   The current architecture focuses on unidirectional asset transfers,
   although the building blocks in this architecture can be used to
   support protocols for bidirectional transfers (conditional two
   unidirectional transfers).

   For simplicity the current architecture employs two (2) gateway nodes
   in the respective blockchains, but collective multi-node transfers
   (i.e. multiple nodes at each side) may be developed based on the
   building blocks and constructs identified in the current

4.2.  Overview of Asset Transfer

   An asset transfer between two blockchain systems is carried out by
   two (2) gateway nodes that represent the two respective blockchain

   A successful transfer results in the asset being extinguished or
   marked on the origin ledger by the origin-gateway, and for the asset
   to be introduced by the destination-gateway into the destination
   ledger.  The mechanism to extinguish or introduce an asset from/into
   a ledger is dependent on the specific blockchain system.

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4.3.  Phases in Asset Transfer

   The interaction between two gateways in an asset transfer is
   summarized in Figure 1, where the origin blockchain is B1 and the
   destination blockchain is B2.  The gateways are denoted as G1 and G2

          The three phases of an asset transfer between gateways

      +-------------+                              +-------------+
      | Application |                              | Application |
      |      A1     |                              |      A2     |
      +-------------+                              +-------------+
             |                                            |
             |                                            |
             v                   Phases                   v
      +-------------+      |<-----(1)----->|       +-------------+
      | Blockchain  | +----+               +----+  | Blockchain  |
      | System B1   | |Gate|               |Gate|  | System B2   |
      |             |-| Way|<-----(2)----->|Way |--|             |
      | +---------+ | | G1 |               | G2 |  | +---------+ |
      | |Ledger L1| | +----+               +----+  | |Ledger L2| |
      | +---------+ |      |<-----(3)----->|       | +---------+ |
      +-------------+                              +-------------+

                                 Figure 1

4.3.1.  Phase 1: Exchange of security parameters and asset information

   In this phase the gateways G1 and G2 initiate a connection to each
   other in order to perform a number of functions.  Some of these are
   as follows:

   o  Exchange of parameters for secure channel establishment between G1
      and G2.

   o  Delivery of asset-related information and asset-holder
      information, including originator and beneficiary identities and
      public keys, and the node-owner (VASP) identities and public keys.

   o  Exchange of parameters related to commitment mechanism employed
      within the flows of the asset transfer protocol.

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4.3.2.  Phase 2: Evidence of asset locking

   In this phase, gateway G1 must provide gateway G2 with sufficient
   evidence that the asset on blockchain B1 is in a locked state on
   ledger L1 and safe from double-spend on the part of its current owner
   (the originator).

   The precise form of the evidence is dependent on the blockchain
   system in B1, and must be previously agreed upon in Phase 1.

   The purpose of this evidence is for dispute resolution between G1 and
   G2 (i.e. entities who own and operate G1 and G2 respectively) in the
   case that double-spend is later detected.

   The gateway G2 must return a signed receipt to G1 of this evidence in
   order to cover G1 in the case of later denial by G2.

4.3.3.  Phase 3: Final commitment of transfer

   In this phase gateway G1 indicates to G2 its readiness to finally
   commit to the transfer, and vice versa.  Both messages must be signed
   by G1 and G2 respectively in case of later (post-transfer) disputes.

   Gateway G1 marks the ledger L1 that the virtual asset is no longer
   associated with the public-key of previous owner (originator) and
   that the asset no longer exists on the blockchain system B1.

   Similarly, gateway G1 marks the ledger L2 in blockchain system B2 to
   indicate that henceforth the asset is associated with the public-key
   of the new owner (beneficiary).

   Optionally, both G1 and G2 may exchange the local ledger marking
   information (e.g. block number and transaction number) with each
   other.  This information may aid in future audit and accountability
   purposes from a legal perspective.

5.  Related Open Issues

   There are a number of open issues that are related to the asset
   transfer protocol between gateway nodes.  Some of the issues are due
   to the fact that blockchain technology is relatively new, and that
   technical constructs designed for interoperability have yet to be
   addressed.  Some of the issues are due to the nascency of the virtual
   asset industry and lack of conventions, and therefore require
   industry collaboration to determine these.

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5.1.  Global identification of blockchain systems and public-keys

   There is currently no standard nomenclature to identify blockchain
   systems in a globally unique manner.  The analog to this is the AS-
   numbers associated with IP routing autonomous systems.

   Furthermore, an address (public-key) may not be unique to one
   blockchain system.  An entity (e.g. user) may in fact employ the same
   public-key at multiple distinct blockchain systems simultaneously.

   However, in order to perform an asset transfer from one blockchain
   system to another, there needs to be mechanism that resolves the
   beneficiary identifier (as known to the originator) to the correct
   public-key and blockchain system as intended by the originator.

5.2.  Selection of gateways nodes within a blockchain system

   A given blockchain system must possess the capability to select or
   designate gateway nodes that will perform an asset transfer across
   blockchain systems.

   A number of blockchain systems already employ consensus mechanisms
   that elect a node to perform the transaction processing (e.g. proof
   of stake in Ethereum).  The same consensus mechanisms may be used to
   elect the gateway node.

   However, there are some blockchain systems that do not elect a single
   node and which employ a race-to-process strategy (e.g. proof of work
   in Bitcoin).  Since the winner of the proof of work can be any node
   in the blockchain system, this implies that all the nodes in these
   types of blockchains must be gateway-capable.

5.3.  Commitment protocols and forms of commitment evidence

   Within Phase 2, the gateway nodes must implement one (or more)
   transactional commitment protocols that permit the coordination
   between two gateways, and the final commitment of the asset transfer.

   The choice of the commitment protocol (type/version) and the
   corresponding commitment evidence must be negotiated between the
   gateways during Phase 1.

   For example, in Phase 2 and Phase 3 discussed above the gateways G1
   and G2 may implement the classic 2 Phase Commit (2PC) protocol
   [Gray81] as a means to ensure efficient and non-disputable
   commitments to the asset transfer.

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   Historically, transactional commitment protocols employ locking
   mechanisms to prevent update conflicts on the data item in question.
   When used within the context of virtual asset transfers across
   blockchain systems, the fact that an asset has been locked by G1 (as
   the 2PC coordinator) must be communicated to G2 (as the 2PC
   participant) in an indisputable manner.

   The exact form of this evidence of asset-locking must be standardized
   (for the given transactional commitment protocol) to eliminate any

6.  Security Considerations

   Although the current interoperability architecture for blockchain
   gateways assumes the externalization of the value of assets, as a
   blockchain system holds an increasing number of virtual assets it
   becomes attractive to attackers seeking to obtain cryptographic keys
   of its nodes and its end-users.

   Gateway nodes are of particular interest to attackers because they
   enable the transferal of virtual assets to external blockchain
   systems, which may or may not be regulated.  As such, hardening
   technologies and tamper-resistant crypto-processors (e.g.  TPM, SGX)
   should be used for implementations of gateways [HS19].

7.  Policy Considerations

   Virtual asset transfers must be policy-driven in the sense that it
   must observe and enforce the policies defined for the blockchain
   domain.  Resources that make-up a blockchain systems are owned and
   operated by entities (e.g. legal persons or organizations), and these
   entities typically operate within regulatory jurisdictions [FATF].
   It is the responsibility of these entities to translate regulatory
   policies into functions on blockchain systems that comply to the
   relevant regulatory policies.

   At the application layer, asset transfers must take into
   consideration the legal status of assets and incorporate relevant
   asset-related policies into their business logic.  These policies
   must permeate down to the nodes that implement the functions of asset
   transaction processing.

   The smart contract abstraction, based on replicated shared code/state
   on the ledger [Herl19], must additionally incorporate the notion of
   policy into the abstraction.

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8.  References

8.1.  Normative References

   [FATF]     FATF, "International Standards on Combating Money
              Laundering and the Financing of Terrorism and
              Proliferation - FATF Revision of Recommendation 15",
              October 2018, <http://www.fatf-

   [NIST]     Yaga, D., Mell, P., Roby, N., and K. Scarfone, "NIST
              Blockchain Technology Overview (NISTR-8202)", October
              2018, <>.

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

8.2.  Informative References

   [Clar88]   Clark, D., "The Design Philosophy of the DARPA Internet
              Protocols, ACM Computer Communication Review, Proc SIGCOMM
              88, vol. 18, no. 4, pp. 106-114", August 1988.

   [Gray81]   Gray, J., "The Transaction Concept: Virtues and
              Limitations, in VLDB Proceedings of the 7th International
              Conference, Cannes, France, September 1981, pp. 144-154",
              September 1981.

   [Herl19]   Herlihy, M., "Blockchains From a Distributed Computing
              Perspective, Communications of the ACM, vol. 62, no. 2,
              pp. 78-85", February 2019,

   [HLP19]    Hardjono, T., Lipton, A., and A. Pentland, "Towards and
              Interoperability Architecture for Blockchain Autonomous
              Systems, IEEE Transactions on Engineering Management",
              June 2019, <https://doi:10.1109/TEM.2019.2920154>.

   [HS2019]   Hardjono, T. and N. Smith, "Decentralized Trusted
              Computing Base for Blockchain Infrastructure Security,
              Frontiers Journal, Sepcial Issue on Blockchain Technology,
              Vol. 2, No. 24", December 2019,

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   [SRC84]    Saltzer, J., Reed, D., and D. Clark, "End-to-End Arguments
              in System Design, ACM Transactions on Computer Systems,
              vol. 2, no. 4, pp. 277-288", November 1984.

Authors' Addresses

   Thomas Hardjono


   Martin Hargreaves
   Quant Network


   Ned Smith


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