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Secure Asset Transfer (SAT) Interoperability Architecture
draft-hardjono-sat-architecture-00

Document Type Active Internet-Draft (individual)
Authors Thomas Hardjono , Martin Hargreaves , Ned Smith , Venkatraman Ramakrishna
Last updated 2022-06-01
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draft-hardjono-sat-architecture-00
Internet Engineering Task Force                              T. Hardjono
Internet-Draft                                                       MIT
Intended status: Informational                             M. Hargreaves
Expires: December 3, 2022                                  Quant Network
                                                                N. Smith
                                                                   Intel
                                                          V. Ramakrishna
                                                                     IBM
                                                            June 1, 2022

       Secure Asset Transfer (SAT) Interoperability Architecture
                   draft-hardjono-sat-architecture-00

Abstract

   This document proposes an interoperability architecture for the
   secure transfer of assets between two networks or systems based on
   the gateway model.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on December 3, 2022.

Copyright Notice

   Copyright (c) 2022 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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   (https://trustee.ietf.org/license-info) in effect on the date of
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   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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.  Gateway Interoperability Modes  . . . . . . . . . . . . . . .   5
   5.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Goal of Architecture  . . . . . . . . . . . . . . . . . .   6
     5.2.  Overview of Asset Transfer  . . . . . . . . . . . . . . .   7
     5.3.  Desirable Properties of Asset Transfer  . . . . . . . . .   7
     5.4.  Event log-data, crash recovery and backup gateways  . . .   8
     5.5.  Overview of the Phases in Asset Transfer  . . . . . . . .   8
   6.  Pre-transfer Verification of Asset and Identities (Phase 1) .  10
   7.  Evidence of asset locking or escrow (Phase 2) . . . . . . . .  12
   8.  Transfer Commitment (Phase 3) . . . . . . . . . . . . . . . .  13
   9.  Commitment sub-protocol . . . . . . . . . . . . . . . . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  16
   11. Policy Considerations . . . . . . . . . . . . . . . . . . . .  16
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     12.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   This document proposes an interoperability architecture based on
   gateways, which are points of interconnection between networks or
   systems.

   There are several services that may be offered by a gateway, one of
   which being the direct transfer of a digital asset from one network
   to another via pairs of gateways without a mediating third party.

   A given network or system may have one or more gateways to perform a
   unidirectional direct transfer of digital assets to another network
   possessing one or more compatible gateway.

   Both gateways must implement a secure asset transfer protocol that
   must satisfy certain security, privacy and atomicity requirements.

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   The purpose of this architecture document is to provide technical
   framework within which to define the required properties of a gateway
   that supports the secure asset transfer protocol.

2.  Terminology

   There following are some terminology used in the current document.
   We borrow terminology from NIST and ISO as much as possible,
   introducing new terms only when needed:

   o  Asset network (system): The network or system where a digital
      asset is utilized.

   o  Asset Transfer Protocol: The protocol used to transfer (move) a
      digital asset from one network to another using gateways.

   o  Origin network: The current network where the digital asset is
      located.

   o  Destination network: The network to which a digital asset is to be
      transferred.

   o  Resource Domain: The collection of resources and entities
      participating within an asset network.  The domain denotes a
      boundary for permissible or authorized actions on resources.

   o  Interior Resources: The various interior protocols, data
      structures and cryptographic constructs that are a core part of an
      asset network or system.

   o  Exterior Resources: The various protocols, data structures and
      cryptographic constructs that are outside of (external to) the
      network or system.

   o  Gateway: The collection of services which connects to a minimum of
      one network or system, and which implements the secure asset
      transfer protocol.

   o  Entity public-key pair: This the private-public key pairs of an
      entity, where the public-key is available and verifiable outside
      the network.  Among others, it may be utilized for interactions
      other entities from outside the network.  The term is used to
      distinguish this public-key from other key-pairs belonging to the
      same entity, but which is only available within the (private)
      network.

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   o  Originator: Person or organization in an origin network seeking
      the transfer of a digital asset to a beneficiary located in a
      remote network.

   o  Beneficiary: Person or organization in an destination network
      seeking to receive the transfer of a digital asset to from an
      originator located in a remote network.

   o  Gateway device identity: The identity of the device implementing
      the gateway functions.  The term is used in the sense of IDevID
      (IEEE 802.1AR) or EK/AIK (in TPM1.2 and TPM2.0) [IDevID].

   o  Gateway owner: The entity that owns and operates a gateway within
      a network.

3.  Assumptions and Principles

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

3.1.  Design Principles

   o  Opaque network resources: The interior resources of each network
      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 with proper authorization.

   o  Externalization of value: The asset transfer protocol is agnostic
      (oblivious) to the economic or monetary value (if any) of the
      digital asset being transferred.

   The opaque resources principle permits the architecture to be applied
   in cases where one (or both) networks are private (closed
   membership).  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 networks.

   The value-externalization principle permits an asset transfer
   protocol to be designed for efficiency, security and reliability --
   independent of the changes in the perceived economic value of the
   digital asset.  It is the analog of the end-to-end principle in the
   Internet architecture [SRC84], where contextual information is placed
   at the endpoints of the transfer.

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3.2.  Operational Assumptions

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

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

   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 asset networks: The
      origin and destination networks is assumed to have been
      identified.

   o  Selection of gateway: The two corresponding gateways at the origin
      and destination networks is assumed to have been identified and
      selected.

   o  Identification of gateway-owners: The owners of the two
      corresponding gateways are assumed to have been identified and
      their ownership status verified.

4.  Gateway Interoperability Modes

   The current interoperability architecture based on gateways
   recognizes several types of transfer modes:

   o  Asset transfer: This refers to the transfer of a digital asset
      from the origin network to a destination network, where a
      successful asset transfer causes the asset to be extinguished in
      the origin network and be created (generated) at the destination
      network.

   o  Data transfer: This refers to the transfer of data only under
      authorization, in such a way that the data can be verified by a
      third party.

   o  Asset exchange (swap): This refers to the case where two users are
      present in two networks, and they perform concurrent and atomic
      swaps of two assets in the two corresponding networks, without
      transferring the assets outside the networks.

   The data transfer mode addresses the use-cases where the state update
   in one network or system depends on the existence of state
   information recorded in a different network or system.  A key use-

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   case is where a business workflow in one system depends on the
   advancement of a different workflow in another system, without
   requiring an asset transfer across the two systems.  Here it is
   useful to distinguish data from asset, where the former can be copied
   in and across systems (with the authorization) and with the assurance
   of integrity and source-authenticity.

5.  Architecture

5.1.  Goal of Architecture

   The goal of the interoperability architecture is to permit two (2)
   gateways belonging to distinct networks to conduct a transfer of
   digital assets transfer between them, in a secure, atomic and
   verifiable manner.

   The asset as understood by the two gateway is expressed in an
   standard digital format in a way meaningful to the gateway
   syntactically and semantically.

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

   The architecture therefore assumes that in addition to implementing
   the bilateral secure asset transfer protocol, a gateway has the role
   of making opaque (i.e. hiding) the constructs that are local and
   specific to its network.

   Overall this approach ensures a high degree of interoperability
   across these networks, where each network can operate as a true
   autonomous system.  Additionally, this approach permits each network
   to evolve its interior technology implementations without affecting
   other (external) networks.

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

   For simplicity the current architecture employs two (2) gateways per
   transfer as the basic building block, with one gateway in the origin
   and destination networks respectively.  However, the architecture
   seeks to be extensible to address future cases involving multiple
   gateways at both sides.

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5.2.  Overview of Asset Transfer

   An asset transfer between two networks is performed using a secure
   asset transfer protocol implemented by the gateways in the respective
   networks.  The two gateways implement the protocol in a direct
   interaction (unmediated).

   A successful transfer results in the asset being extinguished
   (deleted) or marked on the origin network, and for the asset to be
   introduced (generated) into the destination network.

   The secure asset transfer protocol provides a coordination between
   the two gateways through the various message flows in the protocol
   that is communicated over a secure channel.

   The protocol implements a commitment mechanism between the two
   gateways to ensure that the relevant properties atomicity,
   consistency, isolation, and durability is achieved in the transfer.

   The mechanism to extinguish or introduce an asset from/into a network
   by its gateway is dependent on the specific network and outside the
   scope of the current work.

   As part of the commitment mechanism, the sender gateway in the origin
   network must deliver proof to the received gateway in the destination
   network that asset in question has been extinguished (deleted) in the
   origin network.

   Similarly, the receiver gateway in the destination network must
   deliver proof to the sender gateway in the origin network that the
   asset has been generated in the destination network.

   These two tasks must be performed in a synchronized fashion between
   the two gateways, and the commitment mechanism must provide evidence
   of the asset transfer that is verifiable by a third party.

5.3.  Desirable Properties of Asset Transfer

   The desirable features of asset transfers between two gateway
   include, but not limited, to the following:

   o  Atomicity: A transfer must either commit or entirely fail (failure
      means no change to asset state).

   o  Consistency: A transfer (commit or fail) always leaves the
      networks in a consistent state (i.e. the asset is located in one
      network only at any time).

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   o  Isolation: While the transfer is occurring, the asset state cannot
      be modified in the origin network.

   o  Durability: Once a transfer has been committed by both gateways,
      it must remain so regardless of subsequent gateway crashes.

   o  Verifiable by authorized third parties: The proof that the asset
      has been extinguished in the origin network, and the proof that
      the asset has been generated in the destination network must be
      verifiable by an authorized third party.

   An implementation of the asset transfer protocol should satisfy these
   properties, independent of whether the implementation employs
   stateful messaging or stateless messaging between the two gateways.

5.4.  Event log-data, crash recovery and backup gateways

   Implementations of a gateway should maintain event logs and
   checkpoints for the purpose of gateway crash recovery.  The log-data
   generated by a gateway should be considered as an interior resource
   accessible to other authorized gateways within the same network.

   The mechanism used to provide gateway crash-recovery is dependent on
   the specific network.  For interoperability purposes the information
   contained in the log and the format of the log-data should be
   standardized.

   The resumption of an interrupted transfer session (e.g. due to
   gateway crash, network failure, etc.) should take into consideration
   the aspects of secure channel establishment and the aspects of the
   transfer protocol resumption.  In some cases, a new secure channel
   (e.g.  TLS session) may need to be established between the two
   gateways, before a resumption of the transfer can begin.

   The log-data collected by a gateway acts also as a checkpoint
   mechanism to assist the recovered (or backup) gateway in continuing
   the transfer.  The point at which to re-start the transfer protocol
   flow is dependent on the implementation of the gateway recovery
   strategy.

5.5.  Overview of the Phases in Asset Transfer

   The interaction between two gateways in the secure asset transfer
   protocol is summarized in Figure 1, where the origin network is NW1
   and the destination network is NW2.  T he gateways are denoted as G1
   and G2 respectively.

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            Originator                                   Beneficiary
                |                                             |
         +-------------+                               +-------------+
         |   Client    |                               |   Client    |
         | Application |                               | Application |
         |    (App1)   |                               |    (App2)   |
         +-------------+                               +-------------+
                |                                             |
                |                  (Phases)                   |
                V                                             V
         +-------------+       |<-----(1)----->|       +-------------+
         |    Network  |  +----+               +----+  |   Network   |
         |     NW1     |  |Gate|               |Gate|  |     NW2     |
         |             |--|way |<-----(2)----->|way |--|             |
         | +---------+ |  | G1 |               | G2 |  | +---------+ |
         | |  State  | |  +----+               +----+  | |   State | |
         | | Data DB1| |  +----+               +----+  | | Data DB2| |
         | +---------+ |       |<-----(3)----->|       | +---------+ |
         +-------------+                               +-------------+

                                 Figure 1

   The phases are summarized as follows.

   o  Phase 0: Initiation of transfer at the application layer.  The two
      applications utilized by the originator and beneficiary is assumed
      to interact as part of the asset transfer.  In this phase, the
      applications App1 and App2 may establish some context information
      (e.g.  Session-ID) that will be made available to their respective
      gateways G1 and G2.  The legal verification of the identities of
      the Originator and Beneficiary may occur in this phase [FATF].
      This phase is outside the scope of the current architecture.

   o  Phase 1: Pre-transfer Verification of Asset and Identities.  In
      this phase the gateways G1 and G2 must perform mutual
      identification and authentication.  Gateway G1 must communicate to
      G2 the type/information of the asset to be transferred, while G2
      must validate that it has the ability to support this type of
      asset in its network.

   o  Phase 2: Evidence of asset locking or escrow.  In this phase,
      gateway G1 must provide gateway G2 with sufficient evidence that
      the asset on its network NW1 is in a locked state (or escrowed)
      under the control of G1).

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   o  Phase 3: Transfer commitment.  In this phase gateways G1 and G2
      commit to the unidirectional asset transfer using a 3PC (3-phase
      commit) subprotocol.

   These transfer phases will be further discussed below.

6.  Pre-transfer Verification of Asset and Identities (Phase 1)

   The purpose of the first phase is to verify the various information
   relating to the asset to be transferred.  This may include, among
   others, the correct identities of the originator and beneficiary (as
   provided by the respective applications), the identity and legal
   status of the entities who own and operate the gateways, the type of
   the network, and network parameters, and the device-identities of the
   gateways.

      Orig  DB1          G1                   G2          DB2  Benef
      |     |            |                    |            |     |
      |---request------->|                    |            |     |
      |     |            |                    |            |     |
    ..|.....|............|....................|............|.....|..
      |     |            |       Phase 1      |            |     |
      |     |            |                    |            |     |
      |     |       (1.1)|<-----Owner id----->|            |     |
      |     |            |                    |            |     |
      |     |            |                    |            |     |
      |     |       (1.2)|<--Asset Profile--->|            |     |
      |     |            |                    |            |     |
      |     |            |                    |            |     |
      |     |       (1.3)|<--Orig/Benef id--->|            |     |
      |     |            |                    |            |     |
    ..|.....|............|....................|............|.....|..
      |     |            |                    |            |     |

                                 Figure 2

   This phase starts with the assumption that in network NW1 the gateway
   to process the asset transfer has been selected (namely gateway G1).
   It also assumes that the destination network NW2 has been identified
   where the beneficiary is located, and that gateway G2 in NW2 has been
   identified.

   There are several steps that may occur in Phase 1:

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   o  Secure channel establishment between G1 and G2: This includes the
      mutual verification of the gateway device identities and the
      exchange of the relevant parameters for secure channel
      establishment.  In cases where device attestation [RATS] is
      required, the mutual attestation protocol must occur between G1
      and G2 prior to proceeding to the next phase.

   o  Mutual device attestations: In cases where device attestation
      [RATS] is required, each gateway must yield attestation evidence
      to the other regarding its configuration.  A gateway may take on
      the role as a attestation verifier, or it may rely on an external
      verifier to appraise the received evidence.

   o  Validation of the gateway ownership: There must be a means for
      gateway G1 and G2 to verify their respective ownerships (i.e.
      entities owning G1 and G2 respectively).  Examples of ownership
      verification mechanism include X.509 certificates, directories of
      gateways and owners, and others.

   o  Validation of owner status: In some jurisdictions, limitations may
      be placed for regulated asset service providers to transact only
      with other similarly regulated service providers.  Examples of
      mechanisms used to validate legal status of service providers
      include directories, Extended Validation (EV) X.509 certificates,
      and others.

   o  Identification and validation of type/asset profile: Both gateways
      must agree on the type of asset being transferred based on the
      published profile of the asset.  Gateway G1 must communicate the
      asset-profile identification to gateway G2, who in turn must
      validate both the legal status of the asset as well as the
      technical capability of its network to accept the type of asset.
      The policies governing network NW2 with regards to permissible
      incoming assets must be enforced by G2.

   o  Exchange of Travel Rule information and validation: In
      jurisdictions where the Travel Rule policies regarding originator
      and beneficiary information is enforced [FATF], the owners of
      gateways G1 and G2 must comply to the Travel Rule.  Mechanisms
      must be used to permit gateways G1 and G2 to make available
      originator/beneficiary information to one another in such a away
      that the Travel Rule information can be logged as part of the
      asset transfer history.

   o  Negotiation of asset transfer protocol parameters: Gateway G1 and
      G2 must agree on the parameters to be employed within the asset
      transfer protocol.  Examples include endpoints definitions for

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      resources, type of commitment flows (e.g. 2PC or 3PC), lock-time
      durations, and others [SAT].

7.  Evidence of asset locking or escrow (Phase 2)

   The asset transfer protocol can commence when both gateways G1 and G2
   have completed the verifications in Phase 1.

   The steps of Phase 2 are summarized in Figure 4, and broadly consists
   of the following:

   o  Commencement (2.1): Gateway G1 indicates the start of the asset
      transfer protocol by sending a transfer-commence message to
      gateway G2.  Among others, the message must include a
      cryptographic hash of the information agreed-upon in Phase 1 (e.g.
      asset profile, gateway identities, originator/beneficiary public
      keys, etc.).

   o  Acknowledgement (2.2): The gateway G2 must send an explicit
      acknowledgement of the receipt of the commence message, which
      should include a hash of commencement message (2.1) and other
      relevant session parameters.

   o  G1 lock/escrow asset (2.3): Gateway G1 proceeds to establish a
      lock or escrow the asset belonging to the originator.  This
      prevents other local transactions in NW1 from changing the state
      of the asset until such time the lock by G1 is finalized or
      released.  A time-lock or escrow may also be employed.

   o  G2 logs incoming asset (2.4): Gateway G2 logs a notification to
      its local state data DB2 that the arrival of the asset is
      imminent.  This may also act as a notification for the beneficiary
      regarding incoming the asset transfer.

   o  Lock Evidence (2.5): Gateway G1 sends a digitally signed evidence
      regarding the locked (escrowed) state on the asset in network NW1.
      The signature by G1 is performed using its entity public-key pair.
      This signifies that G1 (i.e. its owner) is legally standing behind
      its assertion regarding the locked/escrowed state on the asset.

   o  Evidence receipt (2.6): If gateway G2 accepts the evidence, G2
      then responds with a digitally signed receipt message which
      includes a hash of the previous lock-evidence message.  Otherwise,
      if G2 declines the evidence then G2 can ignore the transfer and
      let it time-out (i.e. transfer failed).  The signature by G2 is
      performed using its entity public-key pair.

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      Orig DB1           G1                   G2          DB2  Benef
      |     |            |      (Phase 1)     |            |     |
      |     |            |                    |            |     |
    ..|.....|............|....................|............|.....|..
      |     |            |       Phase 2      |            |     |
      |     |            |                    |            |     |
      |     |       (2.1)|-----Commence------>|            |     |
      |     |            |                    |            |     |
      |     |            |<-------ACK---------|(2.2)       |     |
      |     |            |                    |            |     |
      |     |            |                    |            |     |
      |     |<---Lock----|(2.3)               |            |     |
      |     |            |               (2.4)|----Log---->|     |
      |     |            |                    |            |     |
      |     |            |                    |            |     |
      |     |       (2.5)|---Lock Evidence--->|            |     |
      |     |            |                    |            |     |
      |     |            |<-----Receipt-------|(2.6)       |     |
      |     |            |                    |            |     |
    ..|.....|............|....................|............|.....|..
      |     |            |                    |            |     |

                                 Figure 3

   The precise form of the evidence in step 2.5 is dependent on the type
   of network NW1, and must be previously agreed upon between G1 and G2
   in Phase 1.

   The purpose of this evidence is for dispute resolution between G1 and
   G2 (i.e. the entities who own and operate G1 and G2 respectively) in
   the case that asset state inconsistencies in NW1 and NW2 are
   discovered later.

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

8.  Transfer Commitment (Phase 3)

   In Phase 3 the gateways G1 and G2 finalizes to the asset transfer by
   performing a commitment protocol (e.g. 2PC or 3PC) as a process (sub-
   protocol) embedded within the overall asset transfer protocol.

   Upon receiving the evidence-receipt message in the previous phase, G1
   begins the commitment (see Figure 5):

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   o  Commit-prepare (3.1): Gateway G1 indicates to G2 to prepare for
      the commitment of the transfer.  This message must include a hash
      of the previous messages (message 2.5 and 2.6).

   o  Ack-prepare (3.2): Gateway G2 acknowledges the commit-prepare
      message.

   o  Lock-final (3.3): Gateway G1 updates its local state data to
      indicate lock-finalization or escrow finalization on the asset in
      network NW1.  This signals the permanent extinguishment of the
      asset from network NW1.  This state data must include a hash
      reference to the lock transaction previously in step (2.3).  This
      indicates that the asset is no longer associated with its previous
      owner (originator) and that the asset instance is no longer
      recognized within network NW1.

   o  Commit-final (3.4): Gateway G1 indicates to G2 that G1 has
      performed a local lock/escrow finalization in NW1.  This message
      must be digitally signed by G1.

   o  Asset-create (3.5): Gateway G2 issues a local state update in
      network NW2 to create (re-generate) the asset, associated with the
      beneficiary.  This transaction must include a hash of the previous
      message (3.4) and hash reference to the log-incoming data
      previously in step (2.4).  These hash references connect the newly
      re-generated asset with the overall transfer event originating
      from gateway G1.

   o  Ack-final (3.6): Gateway G2 indicates to gateway G1 that gateway
      G2 has performed an asset-regeneration in network NW2.  This
      message must be digitally signed by G2.

   o  Location-record (3.7): Gateway G1 has the option to record the
      locally the information provided by G2 in the previous step.  This
      record should include a hash reference to the confirmed lock-
      finalization data from step 3.3.  This information may aid in
      future search, audit and accountability purposes from a legal
      perspective.

   o  Transfer complete (3.8): Gateway G1 must explicitly close the
      asset transfer session with gateway G2.  This allows both sides to
      close down the secure channel established in Phase 1.

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      Orig DB1          G1                   G2           DB2  Benef
      |     |            |      (Phase 2)     |            |     |
      |     |            |                    |            |     |
    ..|.....|............|....................|............|.....|..
      |     |            |       Phase 3      |            |     |
      |     |            |                    |            |     |
      |     |       (3.1)|--Commit Prepare--->|            |     |
      |     |            |                    |            |     |
      |     |            |<-----ACK-Prep------|(3.2)       |     |
      |     |            |                    |            |     |
      |     |            |                    |            |     |
      |     |<--Final----|(3.3)               |            |     |
      |     |            |                    |            |     |
      |     |       (3.4)|----Commit Final--->|            |     |
      |     |            |                    |            |     |
      |     |            |               (3.5)|---Create-->|     |
      |     |            |                    |            |     |
      |     |            |<-----ACK-Final-----|(3.6)       |     |
      |     |            |                    |            |     |
      |     |            |                    |            |     |
      |     |<--Record---|(3.7)               |            |     |
      |     |            |                    |            |     |
      |     |       (3.8)|---Complete/End---->|            |     |
      |     |            |                    |            |     |
    ..|.....|............|....................|............|.....|..
      |     |            |                    |            |     |

                                 Figure 4

9.  Commitment sub-protocol

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

   In the case that there are multiple commitment subprotocols supported
   by the gateways, the choice of the sub-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 or 3 Phase Commit (2PC or 3PC)
   sub-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 digital asset transfers across
   networks, the fact that an asset has been locked in NW1 must be
   communicated to G2 (as the 3PC participant) in an indisputable
   manner.

   Similarly, G2 must return evidence to G1 that the asset has been re-
   generated in NW2.

   These evidences must be verifiable by an authorized third party, in
   the case that disputes occur (post event) or where legal audit is
   required on the asset transfer.

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

10.  Security Considerations

   As an asset network holds an increasing number of digital assets, it
   may become attractive to attackers seeking to compromise the
   cryptographic keys of the entities, services and its end-users.

   Gateways are of particular interest to attackers because they enable
   the transferal of digital assets to external networks, 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].

11.  Policy Considerations

   Digital asset transfers must be policy-driven in the sense that it
   must observe and enforce the policies defined for the network.
   Resources that make-up a network 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 networks 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 gateways that implement the functions of
   asset transaction processing.

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12.  References

12.1.  Normative References

   [FATF]     FATF, "International Standards on Combating Money
              Laundering and the Financing of Terrorism and
              Proliferation - FATF Revision of Recommendation 15
              (Updated June 2021)", October 2018, <http://www.fatf-
              gafi.org/publications/fatfrecommendations/documents/fatf-
              recommendations.html>.

   [ISO]      ISO, "Blockchain and distributed ledger technologies-
              Vocabulary (ISO:22739:2020)", July 2020,
              <https://www.iso.org>.

   [NIST]     Yaga, D., Mell, P., Roby, N., and K. Scarfone, "NIST
              Blockchain Technology Overview (NISTR-8202)", October
              2018, <https://doi.org/10.6028/NIST.IR.8202>.

   [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>.

   [SAT]      Hargreaves, M., Hardjono, T., and R. Belchior, "Secure
              Asset Transfer Protocol, IETF, draft-hargreaves-sat-core-
              00.", May 2022, <https://datatracker.ietf.org/doc/draft-
              hargreaves-sat-core/>.

12.2.  Informative References

   [ABCH20]   Ankenbrand, T., Bieri, D., Cortivo, R., Hoehener, J., and
              T. Hardjono, "Proposal for a Comprehensive Crypto Asset
              Taxonomy", May 2020, <https://arxiv.org/abs/2007.11877>.

   [Abebe19]  Abebe, E., Behl, D., Govindarajan, C., Hu, Y.,
              Karunamoorthy, D., Novotny, P., Pandit, V., Ramakrishna,
              V., and C. Vecchiola, "Enabling Enterprise Blockchain
              Interoperability with Trusted Data Transfer (Middleware
              2019, Industry Track)", December 2019,
              <https://arxiv.org/abs/1911.01064>.

   [Abebe21]  Abebe, E., Hu, Y., Irvin, A., Karunamoorthy, D., Pandit,
              V., Ramakrishna, V., and J. Yu, "Verifiable Observation of
              Permissioned Ledgers (ICBC2021)", May 2021,
              <https://arxiv.org/abs/2012.07339>.

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   [BCH21]    Belchior, R., Correia, M., and T. Hardjono, "DLT Gateway
              Crash Recovery Mechanism, IETF, draft-belchior-gateway-
              recovery-01.", March 2021,
              <https://datatracker.ietf.org/doc/draft-belchior-gateway-
              recovery/>.

   [BVGC20]   Belchior, R., Vasconcelos, A., Guerreiro, S., and M.
              Correia, "A Survey on Blockchain Interoperability: Past,
              Present, and Future Trends", May 2020,
              <https://arxiv.org/abs/2005.14282v2>.

   [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.

   [DLVIEW]   Ramakrishna, V., Pandit, V., Nishad, S., Narayanam, K.,
              and D. Vinayagamurthy , "Views and View Addresses for
              Blockchain/DLT Interoperability, IETF Draft", November
              2021.

   [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,
              <https://doi.org/10.1145/3209623>.

   [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, Special Issue on Blockchain Technology,
              Vol. 2, No. 24", December 2019,
              <https://doi.org/10.3389/fbloc.2019.00024>.

   [IDevID]   Richardson, M. and J. Yang, "A Taxonomy of operational
              security of manufacturer installed keys and anchors. IETF
              draft-richardson-t2trg-idevid-considerations-01", August
              2020, <https://tools.ietf.org/html/draft-richardson-t2trg-
              idevid-considerations-01>.

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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
   MIT

   Email: hardjono@mit.edu

   Martin Hargreaves
   Quant Network

   Email: martin.hargreaves@quant.network

   Ned Smith
   Intel

   Email: ned.smith@intel.com

   Venkatraman Ramakrishna
   IBM

   Email: vramakr2@in.ibm.com

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