Internet Engineering Task Force                              T. Hardjono
Internet-Draft                                                       MIT
Intended status: Informational                             M. Hargreaves
Expires: 11 September 2023                                 Quant Network
                                                                N. Smith
                                                                   Intel
                                                          V. Ramakrishna
                                                                     IBM
                                                           10 March 2023


       Secure Asset Transfer (SAT) Interoperability Architecture
                   draft-hardjono-sat-architecture-03

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 September 2023.

Copyright Notice

   Copyright (c) 2023 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
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised 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
     3.3.  Assumptions Regarding Gateway Operators . . . . . . . . .   5
   4.  Gateway Interoperability Modes  . . . . . . . . . . . . . . .   6
   5.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Goal of Architecture  . . . . . . . . . . . . . . . . . .   7
     5.2.  Overview of Asset Transfer  . . . . . . . . . . . . . . .   8
     5.3.  Desirable Properties of Asset Transfer  . . . . . . . . .   8
     5.4.  Event log-data, crash recovery and backup gateways  . . .   9
     5.5.  Overview of the Stages in Asset Transfer  . . . . . . . .  10
   6.  Pre-transfer Verification and Context Establishment . . . . .  11
   7.  Asset Lock Assertion and Receipt (Stage 2)  . . . . . . . . .  13
   8.  Transfer Commitment (Stage 3) . . . . . . . . . . . . . . . .  15
   9.  Commitment sub-protocol . . . . . . . . . . . . . . . . . . .  17
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   11. Policy Considerations . . . . . . . . . . . . . . . . . . . .  18
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     12.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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.



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

   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:

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

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

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

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

   *  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.

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

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

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

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

   *  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.

   *  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].

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

   *  Application Context-ID: The relevant identifier used by
      originator's application and the beneficiary's application to
      identify the context of the asset transfer at the gateway level.
      The context identifier may also be used to bind the application to
      selected gateway for the given transfer instance, identified by a
      Session-ID.

   *  Gateway Session-ID: This the identifier used between the sender
      gateway and the recipient gateway to identify the specific
      transfer instance.  The Session-ID must be included in all
      messages between the gateways.

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

   *  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.

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








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   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.

3.2.  Operational Assumptions

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

   *  Application level context establishment: The transfer request from
      an Originator utilizing an application (App1) in the origin
      network is assumed to have occurred, and that some context-
      identifier has subsequently been derived by the respective
      applications (App1 and App2).  Furthermore, this context-
      identifier is assumed to have been delivered by the each
      application to its corresponding gateway, permiting each gateway
      to internally bind the transfer session-identifier to that
      context-identifier.

   *  Identification of asset to be transferred: The applications at the
      originator and the beneficiary are assumed to have identified the
      digital asset to be transferred.

   *  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.

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

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

3.3.  Assumptions Regarding Gateway Operators

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



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

   *  Gateway liabilities: Gateways and gateway-operators are assumed to
      take on legal and financial liability for their transactions, and
      gateways are assumed to operate under a well-defined legal
      framework (e.g. contractual relationship).  Furthermore, the legal
      framework is assumed to be supported by compatible legislation in
      the relevant jurisdictions where the gateways are operating.

   *  Gateway message signatures: All messages between gateways are
      assumed to be signed and verified (e.g.  X.509).

   *  Transitory ownership of asset by gateway: Assets being transferred
      via SAT will be technically be owned by gateway in transit and
      gateways are liable for them while they have ownership.

   *  Network data: Gateways are assumed to have mechanisms in place to
      trust data returned from their local networks.  This will depend
      on the technical architecture and capabilities of each specific
      network.

   *  Gateways are trusted: The gateways are assumed to be trusted to
      carry-out all the stages of the protocol described in this
      architecture.

4.  Gateway Interoperability Modes

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

   *  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.

   *  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.  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.








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   *  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 gateways aid in
      coordinating the messages pertaining to the swap.

   The remainder of this architecture document will focus on the asset
   transfer flows.

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
   (burned) or marked on the origin network, and for the asset to be
   regenerated (minted) at 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 are achieved in the transfer.

   The mechanism to extinguish (burn) or regenerate (mint) an asset
   from/into a network by its gateway is dependent on the specific
   network and is outside the scope of the current architecture.

   As part of the commitment mechanism, the sender gateway in the origin
   network must deliver a signed assertion to the receiver gateway at
   the destination network which states that asset in question has been
   extinguished (burned) from the origin network.

   Similarly, the receiver gateway at the destination network must in
   return deliver a signed assertion to the sender gateway at the origin
   network which states that the asset has been regenerated (minted) in
   the destination network.

   These two tasks must be performed in a synchronized fashion between
   the two gateways, and the commitment mechanism must provide sufficent
   evidence of the asset transfer that is verifiable by an authorized
   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:

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






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   *  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).

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

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

   *  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.








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5.5.  Overview of the Stages 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.



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



                                  Figure 1

   The stages are summarized as follows.

   *  Stage 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 stage, the
      applications App1 and App2 may establish some shared transfer
      context information (e.g.  Context-ID) at the application level
      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 stages [FATF].  This stage is
      outside the scope of the current architecture.

   *  Stage 1: Pre-transfer Verification of Asset and Identities.  In
      this stage the gateways G1 and G2 must perform mutual
      identification and authentication.  Gateway G1 must communicate to



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      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.

   *  Stage 2: Evidence of asset locking or escrow.  In this stage,
      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).

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

   These transfer stages will be further discussed below.

6.  Pre-transfer Verification and Context Establishment

   The purpose of the first stage (pre-transfer) is for the respective
   applications to establish a transfer-context between them, and for
   the respective gateways to perform validations related to the
   transfer.  These validations 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.


























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        App1  DB1          G1                   G2          DB2    App2
         |     |            |                    |            |     |
         |     |            |                    |            |     |
         |<------------transfer context establishment-------------->|
         |     |            |                    |            |     |
         |---request------->|                    |<------request----|
         |     |            |                    |            |     |
       ..|.....|............|....................|............|.....|..
         |     |            |       Stage 1      |            |     |
         |     |            |                    |            |     |
         |     |       (1.1)|<-----Owner id----->|            |     |
         |     |            |                    |            |     |
         |     |            |                    |            |     |
         |     |       (1.2)|<--Asset Profile--->|            |     |
         |     |            |                    |            |     |
         |     |            |                    |            |     |
         |     |       (1.3)|<--Orig/Benef id--->|            |     |
         |     |            |                    |            |     |
       ..|.....|............|....................|............|.....|..
         |     |            |                    |            |     |



                                  Figure 2

   This stage starts with the assumption that in network NW1 the gateway
   who processes 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
   network NW2 has been identified.

   There are several steps that may occur in Stage 1:

   *  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 stage.

   *  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.






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   *  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.

   *  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.

   *  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.

   *  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.

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

7.  Asset Lock Assertion and Receipt (Stage 2)

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

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

   *  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



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      cryptographic hash of the information agreed-upon in Stage 1 (e.g.
      asset profile, gateway identities, originator/beneficiary public
      keys, etc.).

   *  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.

   *  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.

   *  Lock Assertion (2.4): Gateway G1 sends a digitally signed
      assertion regarding the locked (escrowed) state on the asset in
      network NW1.  The signature by G1 is performed using its entity
      public-key pair.  This signature signifies that G1 (i.e. its
      owner/operator) is legally standing behind its statement regarding
      the locked/escrowed state on the asset.

   *  G2 logs lock-assertion (2.5): Gateway G2 logs a copy of the signed
      lock-assertion message received in Step 2.4 to its local state
      data DB2.  This may also act as a notification for the beneficiary
      regarding incoming the asset transfer.

   *  Lock-Assertion Receipt (2.6): If gateway G2 accepts the signed
      assertion from G1, then G2 responds with a digitally signed
      receipt message which includes a hash of the previous lock-
      assertion message.  The signature by G2 is performed using its
      entity public-key pair.  Otherwise, if G2 declines accepting the
      assertion then G2 can simply ignore the transfer and let the
      session time-out (i.e. transfer attempt has failed).

















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


                                  Figure 3

   The purpose of the signed lock-assertion 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 regarding
   the earlier signed lock-assertion in order to cover G1 (exculpatory
   proof) in the case of later denial by G2.

8.  Transfer Commitment (Stage 3)

   In Stage 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 signed receipt message from G2 in the previous
   stage, G1 begins the commitment (see Figure 5):

   *  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).




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

   *  Temporary asset mint (3.3): Gateway G2 creates (mints) an
      equivalent asset in NW2 assigned to itself as the owner.  This
      step can be reversed (i.e. asset destroyed) in the case of the
      failure in the commitment steps because G2 is still the owner of
      the asset in NW2.

   *  Commit-ready (3.4): Gateway G2 sends a commit-ready message to G1
      indicating that it is ready to carry-out the last steps of the
      commitment subprotocol.  Note that that the entire asset transfer
      session can be aborted before this step without affecting the
      asset state in the respective networks.

   *  Asset burn (3.5): Gateway G1 extinguishes (burns) the asset in
      network NW1 which it has locked since Step 2.3.

   *  Commit-final (3.6): Gateway G1 indicates to G2 that G1 has
      performed the extinguishment of the asset in NW1.  This message
      must be digitally signed by G1.

   *  Asset-assignment (3.7): Gateway G2 assigns the minted asset (which
      it has been holding since Step 3.3) to the Beneficiary.

   *  Ack-final (3.8): Gateway G2 sends a signed Asset-Mint Assertion to
      G2 to indicate that it has completed the asset creation (minting)
      in NW2 and that it has assigned the asset to the intended
      Beneficiary.

   *  G1 logs asset-mint assertion (3.9): Gateway G1 logs a copy of the
      signed Asset-Mint Assertion message received in Step 3.8 to its
      local state data DB2.

   *  Transfer complete (3.10): Gateway G1 must explicitly close the
      asset transfer session with gateway G2.  This allows both sides to
      close down the secure channel established earlier in Stage 1.














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         Orig DB1          G1                   G2           DB2  Benef
         |     |            |      (Stage 2)     |            |     |
         |     |            |                    |            |     |
       ..|.....|............|....................|............|.....|..
         |     |            |       Stage 3      |            |     |
         |     |            |                    |            |     |
         |     |       (3.1)|--Commit Prepare--->|            |     |
         |     |            |                    |            |     |
         |     |            |<-----ACK-Prep------|(3.2)       |     |
         |     |            |                    |            |     |
         |     |            |                    |            |     |
         |     |            |               (3.3)|----Mint--->|     |
         |     |            |                    |            |     |
         |     |            |<--Commit Ready ----|(3.4)       |     |
         |     |            |                    |            |     |
         |     |            |                    |            |     |
         |     |<---Burn----|(3.5)               |            |     |
         |     |            |                    |            |     |
         |     |       (3.6)|----Commit Final--->|            |     |
         |     |            |                    |            |     |
         |     |            |                    |            |     |
         |     |            |               (3.7)|---Assign-->|     |
         |     |            |                    |            |     |
         |     |            |<-----ACK Final-----|(3.8)       |     |
         |     |            |                    |            |     |
         |     |            |                    |            |     |
         |     |<----Log----|(3.9)               |            |     |
         |     |            |                    |            |     |
         |     |      (3.10)|-----Complete------>|            |     |
         |     |            |                    |            |     |
       ..|.....|............|....................|............|.....|..
         |     |            |                    |            |     |


                                  Figure 4

9.  Commitment sub-protocol

   Within Stage 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 Stage 1.





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

   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 via an assertion to G2 (as the 3PC participant) in an
   indisputable manner.

   Similarly, G2 must return a signed assertion to G1 that the asset has
   been regenerated (minted) in NW2.

   These signed assertions 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 precise form of these assertions 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.







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   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.

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.", 5 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>.




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   [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>.

   [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>.






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   [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>.

   [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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