Internet Engineering Task Force                              T. Hardjono
Internet-Draft                                                       MIT
Intended status: Informational                             M. Hargreaves
Expires: 20 December 2024                                  Quant Network
                                                                N. Smith
                                                                   Intel
                                                          V. Ramakrishna
                                                                     IBM
                                                            18 June 2024


       Secure Asset Transfer (SAT) Interoperability Architecture
                    draft-ietf-satp-architecture-05

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
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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 20 December 2024.

Copyright Notice

   Copyright (c) 2024 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Assumptions and Principles  . . . . . . . . . . . . . . . . .   5
     3.1.  Design Principles . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Operational Assumptions . . . . . . . . . . . . . . . . .   5
     3.3.  Assumptions Regarding Gateway Operators . . . . . . . . .   6
   4.  Gateway Interoperability Modes  . . . . . . . . . . . . . . .   7
   5.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Goal of Architecture  . . . . . . . . . . . . . . . . . .   7
     5.2.  Overview of Asset Transfer  . . . . . . . . . . . . . . .   8
     5.3.  Desirable Properties of Asset Transfer  . . . . . . . . .   9
     5.4.  Event log-data, crash recovery and backup gateways  . . .  11
     5.5.  Overview of the Stages in Asset Transfer  . . . . . . . .  11
   6.  Pre-transfer Verification and Context Establishment
           (Stage-0) . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Transfer Initiation and Commencement (Stage-1)  . . . . . . .  15
   8.  Asset Lock Assertion and Receipt (Stage 2)  . . . . . . . . .  16
   9.  Commitment Preparation and Finalization (Stage 3) . . . . . .  18
   10. The Commitment sub-protocol . . . . . . . . . . . . . . . . .  20
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   12. Policy Considerations . . . . . . . . . . . . . . . . . . . .  21
   13. Threat Analysis Considerations  . . . . . . . . . . . . . . .  22
     13.1.  Multiple intentional aborts by the sender gateway  . . .  22
     13.2.  Multiple intentional aborts by the receiver gateway  . .  22
     13.3.  Failure to transmit ACK-Final Receipt  . . . . . . . . .  23
     13.4.  Failure to extinguish asset  . . . . . . . . . . . . . .  23
     13.5.  Identity impersonations  . . . . . . . . . . . . . . . .  23
   14. Compatibility Considerations  . . . . . . . . . . . . . . . .  24
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  24
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  24
     15.2.  Informative References . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26








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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.

   The peer gateways must implement a secure asset transfer protocol
   that must satisfy certain security, privacy, atomicity and liveliness
   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

   The current architcture specification borrows existing terminology
   from NIST and ISO.  New terms have been introduced notably in
   relation to the use of the gateways paradigm and commitment
   subprotocols.

   *  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.



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   *  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 (e.g. sign messages) 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.

   *  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 the DevID
      (IEEE 802.1AR) [DevID] or the EK/AIK keys (in TPM1.2 and TPM2.0)
      [TPMdevID].

   *  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 is the identifier used between the sender
      gateway G1 and the recipient gateway G2 to identify the specific
      transfer instance between them.  The Session-ID must be included
      in all messages between the peer gateways.








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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.

   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.



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   *  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:

   *  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 control of asset by gateway: An asset being transferred
      via SAT will technically be controlled by gateway throughout the
      transfer duration to ensure the state of the asset is not modified
      by another entity.  Gateway owners are liable for the asset
      throughout this duration.

   *  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.






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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
      (burned) in the origin network and be regenerated (minted) 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.

   *  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 (i.e. across) 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 gateways 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.



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   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.

   The abstract construct of the gateway is used to represent endpoints
   that implement the asset transfer protocol interactions and the
   business logic that coordinates the transfer protocol steps until
   completion satisfying the ACID properties and ensuring liveliness of
   the protocol interactions.  In classical distributed databases, this
   business logic is often referred to as the transaction manager or the
   transaction coordinator.  This architecture specification does not
   prescribe any implementations of gateways.

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) at 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 (ACID) 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



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   specification.  The mechanisms used to provide cryptographic proofs
   that an asset has been burned or minted in a given network is also
   out of scope.

   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
   sufficient 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 gateways
   include, but not limited, to the following:

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

   *  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.

   *  Liveliness and safety: The asset transfer protocol results in both
      gateways reaching a non-blocking state (commits or aborts)
      satisfying the above ACID properties.

   *  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.






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   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.

   Performing an asset transfer safely and securely is not simply a
   matter of communicating desire or intent between two systems
   represented by gateways, though such communication is a necessary
   part of asset transfer.  The systems, or at least their gateway
   proxies, must be interoperable in order to transfer assets among
   themselves, but such interoperability imposes strictly more demands
   on systems managing digital assets, especially systems that are built
   on distributed ledgers, compared to conventional communication
   interoperability.

   Communication interoperability, which is concerned with syntax and
   semantics of information geared towards producing a common
   understanding (or knowledge reconciliation) among systems, is
   insufficient to fulfill an asset transfer that requires systems to
   carry out state updates in concert with each other.  But
   communication, or messaging standards, play a necessary and
   complementary role to asset transfer protocols.  An exemplar of this
   is ISO 20022, which is a comprehensive global standard for financial
   messaging that specifies message syntax for common actions occurring
   in financial business processes, including payments, credit card
   transactions, securities settlements, funds, and trade [ISO20022].
   This standard provides the tools to model business processes from
   basic logical building blocks and schemas to construct messages using
   common formats like XML, JSON, and ASN.1.

   As discussed later, such messaging standards are useful to
   communicate information about the states of processes and digital
   assets across systems, to make requests, and to convey intent.  They
   therefore play a necessary and complementary role in asset transfer
   protocols.  However they are by themselves insufficient to ensure the
   ACID and verifiability properties described earlier.  Another way to
   think about the relationship between messaging standards like ISO
   20022 and asset transfer protocols is that the former is concerned
   with the "what" of cross-system interoperability whereas the latter
   is concerned with the "how".  Both kinds of protocols treat systems
   as black boxes, but asset transfer protocols must place some
   responsibility, and depend, on systems to drive a protocol instance
   to successful conclusion.









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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 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.  The gateways are denoted as G1
   and G2 respectively.




















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            Originator                                   Beneficiary
                |                                             |
         +-------------+                               +-------------+
         |   Client    |                               |   Client    |
         | Application |                               | Application |
         |    (App1)   |                               |    (App2)   |
         +-------------+                               +-------------+
                |                                             |
                |                                             |
                |?----------+                     +-----------|
                |           |  |<--(Stage 0)-->|  |           |
                V           |  |               |  |           V
         +-------------+    V  |               |  V    +-------------+
         |    Network  |  +----+<-- Stage 1 -->+----+  |   Network   |
         |     NW1     |  |Gate|               |Gate|  |     NW2     |
         |             |--|way |               |way |--|             |
         | +---------+ |  | G1 |<-- Stage 2 -->| G2 |  | +---------+ |
         | |  State  | |  +----+               +----+  | |   State | |
         | | Data DB1| |       |               |       | | Data DB2| |
         | +---------+ |       |<-- Stage 3--->|       | +---------+ |
         +-------------+                               +-------------+


                                  Figure 1

   The stages are summarized as follows.

   *  Stage 0: Pre-transfer Verification and Context Establishment.  The
      two applications utilized by the originator and beneficiary are
      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 specification.

   *  Stage 1: Transfer Initiation and Commencement.  In this stage
      gateways G1 and G2 must exchange information (claims) regarding
      the asset to be transferred, the identity information of the
      Originator and Beneficiary and other information regarding
      relevant actors (e.g. gateway owner/operator).  The main task in
      this stage is for both gateways to finalize the parameters
      previously negotiated in Stage 0 and to agree to commence the
      transfer.






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   *  Stage 2: Lock Assertion and Receipt.  In this stage, gateway G1
      must provide gateway G2 with a signed assertion that the asset in
      NW1 has been immobilized and under the control on G1.  A signed
      assertion is needed because NW1 may be a private or closed
      network, and therefore the state-database (ledger) in NW1 is not
      readable by external entities including by G2.  This means that
      gateway G1 must make an explicit signed assertion about the state
      in NW1.  Note that the owner/operator of G1 takes on liability in
      signing this assertion.

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

   These stages will be further discussed below.

6.  Pre-transfer Verification and Context Establishment (Stage-0)

   Although Stage 0 is out of scope for the current architecture, it is
   discussed here in order to provide background with the regards to the
   various asset verification requirements prior to commencing the
   transfer in Stage 1.

   Several tasks need to be conducted as part of the pre-transfer:

   *  Application level ContextID establishment: The application (App1)
      used by the originator and the application (App2) used by the
      beneficiary may need to establish a transfer context identifier
      (contextID) to uniquely identity the transfer at the application
      level.

   *  Identification of the asset in the origin network: The specific
      asset in the origin network NW1 must be located and identified,
      and its legal ownership be verified by gateway G1.  A gateway
      should not transfer assets whose ownership is unverified.  An
      asset may be identified by way of a combination of the network-
      identifier and a cryptographic hash of the ledger data
      representing the asset.  Other syntax may also be used, such as
      [ISO20022] or ITIN [ITIN].

   *  Verification of the class or type of asset: The receiving gateway
      G2 may need to verify the class or type of asset that is to be
      transferred by gateway G1 in network NW1.  This is to ensure that
      the asset type/class conforms to the governing policies in the
      destination network NW2.  Additionally, gateway G2 must ensure
      that network NW2 can technologically receive (mint) the asset of
      that given type/class.  Asset schema definitons and asset profiles
      may assist in this vertification process.



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   *  Validation of asset ownership status: The gateway G1 in the origin
      network NW1 must ensure that an asset to be transferred to an
      external network is legally owned by the originator who is
      requesting the transfer.

   *  Authorization to transfer: The gateway G1 must obtain
      authorization from the owner of the asset (originator) to perform
      the transfer to the beneficiary in network NW2.  Similarly, the
      gateway G2 serving network NW2 must obtain authorization from the
      beneficiary to receive the transfer and assign the asset to the
      beneficiary in NW2.

   *  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.

   *  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.

   *  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.

   *  Negotiation of transfer protocol and network parameters: Gateway
      G1 and G2 must agree on the parameters to be employed within the
      asset transfer protocol.  Examples include endpoints definitions
      for resources, duration of time-outs of messages, type of
      commitment flows (e.g. 2PC or 3PC), signature algorithms, average
      lock-time durations in their respective networks, and others.

   The current specification seeks to reuse as much as possible the
   existing standards related to digital assets.  We seek to rely on
   existing messaging standards like ISO 20022 [ISO20022] or ITIN [ITIN]
   for gateway ownership validation, owner status validation, asset
   profile identification, and communication of travel rule and transfer
   context information.  For identification of digital assets maintained
   by distributed ledgers or blockchain systems, we can also rely on
   standards like ITIN [ITIN].



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7.  Transfer Initiation and Commencement (Stage-1)

   In Stage 1 the sender gateway (G1) and the receiver gateway (G2) must
   explicitly accept the parameters of the transfer which were
   negotiated in the pre-transfer stage (Stage 0).

   This explicit acceptance of the parameters takes the form of gateway
   G1 sending a signed Transfer Proposal message containing a Transfer
   Initiation Claims set (namely the parameters agreed upon in Stage 0),
   and for the gateway G2 to respond with a signed Proposal Receipt
   message which carries a hash of the proposal message.




       App1  DB1          G1                     G2          DB2    App2
        |     |            |                      |            |     |
        |     |            |      (Stage 0)       |            |     |
        |     |            |                      |            |     |
        |<------------ (transfer context establishment) ------------>|
        |     |            |                      |            |     |
        |---request------->|                      |<------request----|
        |     |            |                      |            |     |
        |     |            |<-Param Negotiation-->|            |     |
        |     |            |                      |            |     |
      ..|.....|............|......................|............|.....|..
        |     |            |       Stage 1        |            |     |
        |     |            |                      |            |     |
        |     |       (1.1)|---Transf. Proposal-->|            |     |
        |     |            |                      |            |     |
        |     |            |                      |            |     |
        |     |       (1.2)|<--Proposal Receipt---|            |     |
        |     |            |                      |            |     |
        |     |            |                      |            |     |
        |     |       (1.3)|---Transf. Commence-->|            |     |
        |     |            |                      |            |     |
        |     |            |                      |            |     |
        |     |       (1.4)|<--- ACK Commence ----|            |     |
        |     |            |                      |            |     |
      ..|.....|............|......................|............|.....|..
        |     |            |                      |            |     |


                               Figure 2

   There are several steps that may occur in Stage 1:





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   *  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.

   *  Transfer Proposal message (1.1): Gateway G1 sends a signed
      transfer proposal message that contains the Transfer Initiation
      Claims to gateway G2.  The claims carry the parameters negotiated
      in Stage 0 (pre-transfer negotiations).

   *  Proposal Receipt (1.2): The gateway G2 indicates acceptance of the
      parameters in the Transfer Initiation Claims by way of sending a
      signed Proposal Receipt message to G1.  If gateway G2 decides not
      to accept parameters in the Transfer Initiation Claims, then G2
      can send an abort message to G1 ignore the message (time-out).

   *  Transfer Commence message (1.3): Once gateway G1 receives the
      signed Proposal Receipt from gateway G2, gateway G1 is ready to
      signal the commencement of the asset transfer.  This is done by
      gateway G1 sending a signed Transfer Commence message to G2.

   *  Commence Acknowledgement message (1.4): Gateway G2 accepts the
      formal commencement of the transfer by responding with a signed
      Commence ACK message.

   It is important to note the logical separation between the transfer
   proposal/receipt messages from the commencement messages.  This
   separation allows the gateways to decline to proceed during the
   proposal finalization (1.1 and 1.2), prior to starting the 2PC
   subprotocol which formally begins at the Commence messages (1.3 and
   1.4).

   This logical separation is useful because in some implementations the
   decision to start the commencement (1.3 and 1.4) implies that the
   gateways and network have sufficient resource to complete the
   transfer.  Gateways that experience extreme loads may use this
   separation to slightly delay the commencement until their loads
   subsides.

   Note that some implementations may choose to enable a multi-round
   interactions for steps 1.1 and 1.2.

8.  Asset Lock Assertion and Receipt (Stage 2)

   In this stage, gateway G1 must issue a signed assertion that the
   asset in origin network NW1 has been immobilized and under the
   control of G1.




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

   *  G1 lock/escrow asset (2.1): 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 (2.2): Gateway G1 sends a digitally signed
      assertion regarding the locked (escrowed or immobilized) 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.  The mechanism
      to lock or immobilize the asset is outside the scope of secure
      asset transfer protocol.

   *  G2 Logs and Broadcasts lock-assertion information (2.3): Gateway
      G2 logs a copy of the signed lock-assertion message received in
      Step 2.4 to its local state data DB2.  G2 may also broadcast the
      fact of the lock-assertion to all members of network NW2.  The
      mechanism to log and to broadcast is also out of scope.

   *  Lock-Assertion Receipt (2.4): 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      |              |     |
         |     |            |                    |              |     |
         |     |<---Lock----|(2.1)               |              |     |
         |     |            |                    |              |     |
         |     |       (2.2)|--Lock-Assertion--->|              |     |
         |     |            |                    |              |     |
         |     |            |               (2.3)|--Broadcast-->|     |
         |     |            |                    |              |     |
         |     |            |                    |              |     |
         |     |            |<-----Receipt-------|(2.4)         |     |
         |     |            |                    |              |     |
       ..|.....|............|....................|..............|.....|..
         |     |            |                    |              |     |



                               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.

9.  Commitment Preparation and Finalization (Stage 3)

   In Stage 3 the gateways G1 and G2 commit to the asset transfer by
   making permanent the changes they made to the respective networks
   (ledgers).  The previous signed receipt message (2.4) from gateway G2
   to gateway G1 signals the start of the commitment subprotocol in
   Stage 3.

   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 should include
      hashes of the previous messages (message 2.2 and 2.4).






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   *  Temporary asset mint (3.2): 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 (3.3): 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 (3.4): Gateway G1 extinguishes (burns) the asset in
      network NW1 which it has locked since Step 2.3.

   *  Commit-final assertion (3.5): 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 (3.6): Gateway G2 assigns the minted asset (which
      it has been self-holding since Step 3.2) to the Beneficiary.

   *  ACK-final receipt (3.7): Gateway G2 sends a signed assertion that
      it has assigned the asset to the intended Beneficiary.

   *  Receipt broadcast (3.8) Gateway G1 logs a copy of the signed
      receipt message to its local state data DB2.  G1 may also
      broadcast the fact of the signed receipt to all members of network
      NW1.  The mechanism to log and to broadcast is out of scope.

   *  Transfer complete (3.9): 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--->|            |     |
         |     |             |                    |            |     |
         |     |             |               (3.2)|----Mint--->|     |
         |     |             |                    |            |     |
         |     |             |<--Commit Ready ----|(3.3)       |     |
         |     |             |                    |            |     |
         |     |             |                    |            |     |
         |     |<---Burn-----|(3.4)               |            |     |
         |     |             |                    |            |     |
         |     |        (3.5)|-Commit Final Asrt->|            |     |
         |     |             |                    |            |     |
         |     |             |                    |            |     |
         |     |             |               (3.6)|---Assign-->|     |
         |     |             |                    |            |     |
         |     |             |<-----ACK Final-----|(3.7)       |     |
         |     |             |                    |            |     |
         |     |             |                    |            |     |
         |     |<-Broadcast--|(3.8)               |            |     |
         |     |             |                    |            |     |
         |     |        (3.9)|-----Completed----->|            |     |
         |     |             |                    |            |     |
       ..|.....|.............|....................|............|.....|..
         |     |             |                    |            |     |


                               Figure 4

10.  The Commitment sub-protocol

   Within Stage 3, 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 0.

   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.



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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 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.

11.  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].

12.  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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13.  Threat Analysis Considerations

   The SAT protocol faces challenges with regards to the confidentiality
   of a transfer between gateways, and the potential issue related to a
   denial-of-service (and resource waste) when either gateway is not
   compliant with the protocol.

   For confidentiality of a transfer, the secure asset transfer protocol
   must utilize a TLS1.2 (TLS1.3) secure channel that must be
   established between the sender gateway (G1) and the receiver gateway
   (G2).  The two gateways must first establish this secure channel at
   the start of Stage 1 before they can proceed to execute the asset
   transfer protocol.  This includes both gateways verifying all the
   relevant parameters required for their TLS1.2 session (e.g. correct
   TLS endpoints, certificate validation, identity validation, etc.).

   There are several challenges that may arise when gateways are not
   compliant with the SAT protocol.  Some of these are described below.

13.1.  Multiple intentional aborts by the sender gateway

   A dishonest sender gateway G1 may purposely fail to continue the
   protocol run at certain crucial points.  One such crucial point is in
   Stage-3, where the gateway G1 is expected to transmit the Commit-
   Final Assertion message (3.5).  If the gateway G1 intentionally fails
   to transmit this message, gateway G2 may conclude that the message
   has been lost and may proceed to reverse the temporary hold it has
   previously created (temporary asset mint in message 3.2).  Although
   this dishonest behavior by G1 does not cause asset damage to G2 or
   NW2, it may exhaust computing resources at gateway G2.  If network
   NW2 incurs transaction fees, such a reversal may be costly for
   gateway G2.

13.2.  Multiple intentional aborts by the receiver gateway

   In a similar manner, a receiver gateway G2 may also purposely fail to
   continue the protocol run at certain crucial points.  One such point
   is the Commit-Ready message (3.3) that it should transmit to G1 after
   receiving the commit prepare message (3.1) from G1.  In this case,
   gateway G1 may conclude that the message is lost and simply abort the
   protocol run.

   Another possible denial-of-service attack could arise when G2
   purposefully fails to send a Lock-Assertion-Receipt (2.4), thereby
   forcing G1 to reverse its lock that was performed earlier (2.1).






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13.3.  Failure to transmit ACK-Final Receipt

   Another possible point of attack by a dishonest gateway G2 may occur
   by the gateway intentionally failing to transmit the ACK-Final-
   Receipt (3.7) in response to the Commit-Final Assertion message (3.5)
   from gateway G1.  Here, the sender gateway G1 may conclude that the
   message is lost and will assume that the transaction has reach
   completion in network NW2.  The sender gateway G1 has retained the
   previous Lock-Assertion Receipt (2.4) in Stage-2 that was signed by
   G2, indicating that the gateway G2 has accepted the responsibility of
   ensuring that the asset-assignment (3.6) by G2 will be correctly
   executed.  Failure by G2 to complete this task becomes a financial
   and legal liability for the owner of gateway G2.

   In general, it is recommended that multiple redundant gateways be
   utilized within a network to mitigate a single gateway's malicious
   behavior.  Furthermore, there are gateway recovery and failover
   mechanisms that have been defined in [BCH21].

13.4.  Failure to extinguish asset

   Another potential attack may come from a dishonest gateway G1 who
   intentionally fails to extinguish the asset in network NW1 (in step
   3.4).  This means G1 is henceforth in control of the asset.  This
   type of denial-of-service is network specific because it implies that
   G1 was able to perform a lock on the asset on behalf of the asset-
   owner (i.e. originator) through one or more mechanisms supported by
   network NW1 independent of (and prior to) the secure asset transfer
   protocol.

   This denial-of-service is out of scope for the current architecture
   specification because it represents a weakness on the part of the
   network NW1.

13.5.  Identity impersonations

   Another vector of attack may involve a gateway that impersonates an
   asset holder in a given network.  For example, a gateway G1 may
   pretend to be the owner of an asset (originator) in network NW1 and
   proceed to transfer it to a beneficiary located in network NW2.

   The verification of the identity of the originator and beneficiary
   must be performed as part of the set-up stage (Stage 0) as described
   in Section 6, which is currently out-of-scope for the secure asset
   transfer protocol.






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   The identity verification includes that of the owner of gateways G1
   and G2 respectively.  Standard protocols for federated identity
   management already exist and have wide deployment.

14.  Compatibility Considerations

   As the asset transfer protocol must be agnostic to the anatomy of a
   digital asset and to the type of ledger technology underlying a
   system maintaining digital assets, it must be compatible with
   different asset identification standards like ISO 20022 and ITIN, and
   with standards for communicating information about business processes
   (like ISO 20022).  Keeping the Stage-0 specification open and not
   tied to a specific messaging or identification standard allows the
   Secure Asset Transfer architecture to be flexible and inclusive, and
   thereby meet compatibility goals.

15.  References

15.1.  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>.

   [ISO20022] ISO, "Universal Financial Industry Message Scheme (ISO
              20022).", July 2023, <https://www.iso20022.org>.

   [ITIN]     ITSA, "International Token Identification Number.", July
              2023, <https://my.itsa.global/what-we-do>.

   [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>.

15.2.  Informative References




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   [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>.

   [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.

   [DevID]    IEEE, "802.1AR: Secure Device Identity", August 2018,
              <https://doi.org/10.1109/IEEESTD.2018.8423794>.

   [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>.



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   [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>.

   [RATS]     Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote ATtestation procedureS (RATS)
              Architecture", January 2023,
              <https://datatracker.ietf.org/doc/rfc9334/>.

   [SATcore]  Hargreaves, M., Hardjono, T., and R. Belchior, "IETF
              Secure Asset Transfer Protocol (SATP)", 9 July 2023,
              <https://datatracker.ietf.org/doc/draft-ietf-satp-
              core/>.>.

   [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.

   [TPMdevID] TCG, "TPM 2.0 Keys for Device Identity and Attestation", 8
              October 2021, <https://trustedcomputinggroup.org/resource/
              tpm-2-0-keys-for-device-identity-and-attestation/>.

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



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   Email: vramakr2@in.ibm.com


















































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