Open Digital Asset Protocol
draft-hargreaves-odap-02

Document Type Active Internet-Draft (individual)
Authors Martin Hargreaves  , Thomas Hardjono  , Rafael Belchior 
Last updated 2021-05-08
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Internet Engineering Task Force                            M. Hargreaves
Internet-Draft                                             Quant Network
Intended status: Informational                               T. Hardjono
Expires: November 10, 2021                                           MIT
                                                             R. Belchior
                                                         Technico Lisboa
                                                             May 9, 2021

                      Open Digital Asset Protocol
                        draft-hargreaves-odap-02

Abstract

   This memo describes the Open Digital Asset Protocol (ODAP).  ODAP is
   an asset transfer protocol that operates between two gateway devices.
   The protocol includes a description of virtual or digital assets held
   on distributed ledgers in an open and interoperable format, a session
   negotiation part and message passing flows between gateways
   connecting disparate distributed ledger technologies (DLTs).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on November 10, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions used in this document . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  The Open Digital Asset Protocol . . . . . . . . . . . . . . .   5
     4.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  ODAP Model  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Types of APIs . . . . . . . . . . . . . . . . . . . . . .   6
     4.4.  Types of Flows  . . . . . . . . . . . . . . . . . . . . .   6
     4.5.  Resources and Identifiers . . . . . . . . . . . . . . . .   7
     4.6.  Access Modes  . . . . . . . . . . . . . . . . . . . . . .   7
       4.6.1.  Direct Mode: Simple Client to Gateway . . . . . . . .   8
       4.6.2.  Direct Mode: Client to Multiple Gateway . . . . . . .   8
       4.6.3.  Relay Mode: Client-initiated Gateway to Gateway . . .   9
   5.  ODAP Message Format, identifiers and Descriptors  . . . . . .  10
     5.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  ODAP Message Format . . . . . . . . . . . . . . . . . . .  10
     5.3.  Digital Asset Resource Descriptors  . . . . . . . . . . .  11
       5.3.1.  Organisation Identifier . . . . . . . . . . . . . . .  11
       5.3.2.  DLT Gateway / Endpoint ID . . . . . . . . . . . . . .  12
       5.3.3.  DLT Identifier  . . . . . . . . . . . . . . . . . . .  12
       5.3.4.  Resource  . . . . . . . . . . . . . . . . . . . . . .  12
       5.3.5.  Examples  . . . . . . . . . . . . . . . . . . . . . .  12
     5.4.  Digital Asset Resource Client Descriptors . . . . . . . .  12
       5.4.1.  Organization Identifier . . . . . . . . . . . . . . .  13
       5.4.2.  DLT Gateway / Endpoint ID . . . . . . . . . . . . . .  13
       5.4.3.  Organizational Unit . . . . . . . . . . . . . . . . .  13
       5.4.4.  Name  . . . . . . . . . . . . . . . . . . . . . . . .  13
       5.4.5.  Examples  . . . . . . . . . . . . . . . . . . . . . .  13
     5.5.  Gateway Level Access Control  . . . . . . . . . . . . . .  14
     5.6.  Negotiation of Security Protocols and Parameters  . . . .  14
       5.6.1.  TLS Established . . . . . . . . . . . . . . . . . . .  14
       5.6.2.  Client offers supported credential schemes  . . . . .  14
       5.6.3.  Server selects supported credential scheme  . . . . .  15
       5.6.4.  Client asserts of proves identity . . . . . . . . . .  15
       5.6.5.  Sequence numbers initialized  . . . . . . . . . . . .  15
       5.6.6.  Messages can now be exchanged . . . . . . . . . . . .  15
     5.7.  Asset Profile Negotiation . . . . . . . . . . . . . . . .  15
     5.8.  Application Profile Negotiation . . . . . . . . . . . . .  15
     5.9.  Digital Asset Resource Discovery  . . . . . . . . . . . .  16
     5.10. Accessing Resources via a DLT Gateway . . . . . . . . . .  16
       5.10.1.  CREATE . . . . . . . . . . . . . . . . . . . . . . .  17

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       5.10.2.  WRITE  . . . . . . . . . . . . . . . . . . . . . . .  17
       5.10.3.  INVOKE . . . . . . . . . . . . . . . . . . . . . . .  17
       5.10.4.  LOCK . . . . . . . . . . . . . . . . . . . . . . . .  17
       5.10.5.  UNLOCK . . . . . . . . . . . . . . . . . . . . . . .  17
       5.10.6.  TRANSFER . . . . . . . . . . . . . . . . . . . . . .  17
       5.10.7.  SUBSCRIBE  . . . . . . . . . . . . . . . . . . . . .  17
       5.10.8.  DESTROY  . . . . . . . . . . . . . . . . . . . . . .  17
       5.10.9.  READ . . . . . . . . . . . . . . . . . . . . . . . .  17
       5.10.10. NATIVE_TXN . . . . . . . . . . . . . . . . . . . . .  17
     5.11. Response Codes  . . . . . . . . . . . . . . . . . . . . .  17
     5.12. Backward Compatibility  . . . . . . . . . . . . . . . . .  18
   6.  Transfer Initiation Flow (Phase 1)  . . . . . . . . . . . . .  18
     6.1.  Initialization Request Message  . . . . . . . . . . . . .  19
     6.2.  Initialization Request Message Response (ACK) . . . . . .  20
   7.  Lock-Evidence Verification Flow (Phase 2) . . . . . . . . . .  21
     7.1.  Transfer Commence Message . . . . . . . . . . . . . . . .  22
     7.2.  Transfer Commence Response Message (Ack)  . . . . . . . .  23
     7.3.  Lock Evidence Message . . . . . . . . . . . . . . . . . .  24
     7.4.  Lock Evidence Response Message (Ack)  . . . . . . . . . .  25
   8.  Commitment Establishment Flow (Phase 3) . . . . . . . . . . .  26
     8.1.  Commit Preparation Message  . . . . . . . . . . . . . . .  26
     8.2.  Commit Preparation Response . . . . . . . . . . . . . . .  27
     8.3.  Commit Final Message  . . . . . . . . . . . . . . . . . .  28
     8.4.  Commit Final Response Message . . . . . . . . . . . . . .  28
     8.5.  Transfer Complete Message . . . . . . . . . . . . . . . .  29
   9.  Security Consideration  . . . . . . . . . . . . . . . . . . .  30
   10. IANA Consideration  . . . . . . . . . . . . . . . . . . . . .  30
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     11.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   There is a lack of interoperability between individual blockchains,
   but also a general difficulty building open DLT networks.  Extant
   networks are custom built and relatively closed, usually limited to
   networks of a single DLT type.

   This memo proposes at DLT-agnostic protocol in order to allow the
   creation of business applications that use and modify multiple DLTs,
   through a single programmatic interface.

   The target DLTs can be of any type, operated by different owners and
   managed using different DLT interoperability / management platforms
   that implement ODAP interfaces.

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   These platforms may act as gateways or relays for the application to
   interact with the hosted DLTs.  They are referred to herein as DLT
   Gateways.

   When correctly implemented and deployed, the protocol should provide
   the basis for solutions involving asset migration between two DLT
   systems, as well as use-cases when one side is non-DLT system (e.g.
   legacy system).

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   In this document, these words will appear with that interpretation
   only when in ALL CAPS.  Lower case uses of these words are not to be
   interpreted as carrying significance described in RFC 2119.

3.  Terminology

   The following are some terminology used in the current document.
   Further terminology can be found in [Arch].

   Client application: This is the application employed by a user to
   interact with a gateway node.

   Gateway: The node of the DLT system functionally capable of acting as
   a gateway in an asset transfer.

   Sender gateway: The gateway that initiates a unidirectional asset
   transfer.

   Recipient gateway: The gateway that is the recipient side of a
   unidirectional asset transfer.

   DLT resources: The various interior protocols, data structures and
   cryptographic constructs that are a core part of a DLT system.

   Off-DLT resources: The various resources that are outside a DLT
   system, and are not part of the operations of the DLT system.

   Role: As in the classic client-server roles.  In the gateway-to-
   gateway interaction, one gateway will take the role of the client
   while the other takes the role of the server, depending on the type
   of interaction flow.

   Claim: An assertion made by an Entity [JWT].

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   Claim Type: Syntax used for representing a Claim Value [JWT].

   DLT Claim: An assertion made by a Gateway regarding the status or
   condition of resources (e.g. asset, public keys, etc.) accessible to
   that gateway within its DLT system.

4.  The Open Digital Asset Protocol

4.1.  Overview

   The Open Digital Asset Protocol (ODAP) is a gateway-to-gateway
   protocol used by a sender gateway with a recipient gateway to perform
   a unidirectional transfer of a virtual asset [Arch].

   The protocol defines a number of API endpoints, resources and
   identifier definitions, and message flows corresponding to the asset
   transfer between the two gateways.

                +----------+                +----------+
                |  Client  |                | Off-DLT  |
                | (Applic) |                | Resource |
                +----------+                +----------+
                     |                      |API Type-3|
                     |                      +----------+
                     |                           ^
                     V                           |
                +----------+                     |
                |API Type-1|                     |
     +------+   +----------+----+        +----+----------+   +------+
     |      |   |          |    |        |    |          |   |      |
     | DLT  |   | Gateway  |API |        |API | Gateway  |   | DLT  |
     |  L1  |---|    G1    |Type|<------>|Type|    G2    |---|  L2  |
     |      |   |          | 2  |        | 2  |          |   |      |
     +------+   +----------+----+        +----+----------+   +------+

                                 Figure 1

4.2.  ODAP Model

   Following the gateway interoperability architecture [Arch], the model
   for ODAP is shown in Figure 1.

   The Client (application) interacts with its local gateway (G1) over
   an interface (API Type-1) in order to provide instructions to the
   gateway with regards to actions related resources located in the

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   local DLT system (L1) and resources located in remote DLT system
   (L2).

   Gateways interact with each other over a gateway interface (API Type-
   2).  A given gateway may be required to access resources that are not
   located in DLT system L1 or DLT system L2.  Access to these types of
   resources are performed over an Off-DLT interface (API Type-3).

4.3.  Types of APIs

   The following are the types of APIs in ODAP:

   o  Gateway APIs for client (API Type-1): This the REST APIs that
      permit a Client (application) to interact with a local gateway,
      and issue instructions for actions pertaining to resources
      accessible to the gateway in the local DLT system.

   o  Gateway APIs for peer gateways (API Type-2): This is the REST APIs
      employed by two (2) peer gateways in performing unidirectional
      asset transfers.

   o  APIs for validation of off-DLT resources (API Type-3): This is the
      REST APIs made available by a resource server (resource owner) at
      which a gateway can access Off-DLT resources.

   The use of these APIs is dependent on the mode of access and the type
   of flow in question.

4.4.  Types of Flows

   The ODAP protocol defines the following three (3) flows:

   o  Transfer Initiation flow: This flow deals with the asset profile
      verification, asset ownership evidence verification and identities
      verification.

   o  Lock-Evidence flow: This flow deals with the conveyance of
      evidence regarding the lock (escrow) status of an asset by one
      gateway, and the verification of the evidence by the other
      gateway.

   o  Commitment Establishment flow: This flow deals with the asset
      transfer and commitment establishment between two gateways on
      behalf of their DLT systems.

   These flow will be discussed below.

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4.5.  Resources and Identifiers

   o  (a) Resource addressing for DLTs, using the URL syntax.

   o  (b) Client identification based on the URN format.  These are for
      identifying clients (developers and applications) who access these
      resources, and which in some use-cases require access
      authorization.

   o  (c) Protocol message family for negotiating authentication,
      authorisation, and parameters for confidential channel
      establishment.

   o  (d) Resource discovery mechanism for developers and applications
      to discover DLT-based resources hosted at a DLT gateway.  The
      gateway response is subject to the level of access granted to that
      developer or application.

4.6.  Access Modes

   This draft proposes three (3) distinct mode of operation for Clients
   when accessing resources recorded a DLT.  These modes make use of a
   gateway, with the assumption that a gateway has full access to the
   DLT behind the gateway.

                             +----------+
                             |  Client  |
                             | (Applic) |
                             +----------+
                                  |
                                  |
                                 (1)
                                  |
                                  V
                             +----------+
                             |API Type-1|
            +------+         +----------+----+
            |      |         |          |    |
            | DLT  |         | Gateway  |API |
            |  L1  |<--(2)---|    G1    |Type|
            |      |         |          | 2  |
            +------+         +----------+----+

                                 Figure 2

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4.6.1.  Direct Mode: Simple Client to Gateway

   In this mode, the client uses its local gateway known to the client
   in order to access (e.g. local transactions to) the local DLT.  This
   is shown in Figure 2.

   In the direct mode, the simplest case, a client application interacts
   with a single DLT gateway in order to interact with the DLTs hosted
   behind the gateway.

   The application must be recognized and authorized by the gateway.
   Asset transfers between the DLTs behind the gateway are possible, and
   the set of operations specified in section 5.10 MUST be supported by
   the DLT Gateway.

   Additional operations specific to DLT or Gateway implementations MAY
   also be available.

                   +----------+
                   |  Client  |
                   | (Applic) |------(1)---------
                   +----------+                 |
                        |                       |
                       (1)                      |
                        |                       |
                        V                       V
                   +----------+            +----------+
                   |API Type-1|            |API Type-1|
     +------+      +----------+            +----------+      +------+
     |      |      |          |            |          |      |      |
     | DLT  |      | Gateway  |            | Gateway  |      | DLT  |
     |  L1  |<-(2)-|    G1    |            |    G2    |-(2)->|  L2  |
     |      |      |          |            |          |      |      |
     +------+      +----------+            +----------+      +------+

                                 Figure 3

4.6.2.  Direct Mode: Client to Multiple Gateway

   In this mode, the client is interacting with multiple gateways
   simultaneously in order to access the DLTs behind each of those
   gateways.  The client is assumed to be performing the synchronization
   of actions while interacting these gateways.  This is illustrated in
   Figure 3.

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   Direct mode can support connections from a single application to
   multiple DLT gateways.  The applications may assert different
   identities with each gateway, provided it has the relevant
   credentials.

   The applications can interact with the DLTs behind each gateway
   according to the authorizations granted by the gateways.

   Asset transfers between/across DLTs hosted behind different gateways
   are not supported in this mode.

4.6.3.  Relay Mode: Client-initiated Gateway to Gateway

   In this mode, the application interacts with a single Gateway, and
   that Gateway acts as an intermediary to other Gateways.

   Connection types and security methods used in the application to
   gateway connection can differ from those used in the gateway to
   gateway connection(s).  The authorization method and credentials
   presented on behalf of the application must be acceptable to the
   final target gateway(s).

   In Relay Mode, additional functionality is available.  Asset
   transfers, based on a two/three phase commit are available.  These
   rely on evidence of locks on source DLTs, which can be passed from
   Gateway to Gateway, insulating the application from the additional
   complexity and keeping the lock data private from the application.

   Compliant Gateways MUST implement these operations, in order to
   support Relay Mode.

   Multi-hop connections between gateways are out of scope of this
   document.

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                 +----------+
                 |  Client  |
                 | (Applic) |
                 +----------+
                      |
                     (1)
                      |
                      V
                  +----------+
                  |API Type-1|
    +------+      +----------+----+         +----+-------+      +------+
    |      |      |          |    |         |    |       |      |      |
    | DLT  |      | Gateway  |API |--(2)--->|API |Gateway|      | DLT  |
    |  L1  |<-(4)-|    G1    |Type|         |Type|  G2   |-(4)->|  L2  |
    |      |      |          | 2  |<---(3)--|  2 |       |      |      |
    |      |      |          |    |         |    |       |      |      |
    +------+      +----------+----+         +----+-------+      +------+

                                 Figure 4

5.  ODAP Message Format, identifiers and Descriptors

5.1.  Overview

   This section describes (i) the phases of the ODAP protocol; (ii) the
   format of ODAP messages; (iii) the format for resource descriptors;
   (iv) a method for gateways to implement access controls; (iv)
   protocol for negotiating security capabilities; (v) discovery and
   accessing resources and provisions for backward compatibility with
   existing systems.

5.2.  ODAP Message Format

   ODAP messages are exchanged between applications (clients) and DLT
   gateways (servers).  They consist of protocol negotiation and
   functional messages.

   Messages are JSON format, with protocol specific mandatory fields,
   support for arbitrary authentication and authorization schemes and
   support for a free format field for plaintext or encrypted payloads
   directed at the DLT gateway or an underlying DLT.

   JSON format message, mandatory fields are shown below:

   o  Version: ODAP protocol Version (major, minor).

   o  Session ID: unique identifier (UUIDv2) representing a session

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   o  Sequence Number: monotonically increasing counter that uniquely
      represents a message from a session.

   o  ODAP Phase: The current ODAP phase.

   o  Resource URL: Location of Resource to be accessed.

   o  Developer URN: Assertion of developer / application identity.

   o  Action/Response: GET/POST and arguments (or Response Code)

   o  Credential Profile: Specify type of auth (e.g.  SAML, OAuth,
      X.509)

   o  Credential Block: Credential token, certificate, string

   o  Payload Profile: Asset Profile provenance and capabilities

   o  Application Profile: Vendor or Application specific profile

   o  Payload: Payload for POST, responses, and native DLT txns.  The
      payload is specific to the current ODAP phase.

   o  Payload Hash: hash of the current message payload.

   o  Message signature: Gateway EDCSA signature over the message

   Other relevant attributes may exists that need to be captured for
   logging purposes.  See [ODAP-2PC].

5.3.  Digital Asset Resource Descriptors

   Resources are identified by URL [RFC 1738] as described below:

   o  The type is new: application/odapres

   o  The access protocol is ODAP.

   Data included in the URL includes the folowing:

5.3.1.  Organisation Identifier

   This Legal Entity Identifier (LEI) or other identifier linking
   resource ownership to real world entity.  Any scheme for identifying
   DLT Gateway owners may be implemented (e.g.  LEI directory, closed
   user group membership, SWIFT BIC, etc.).

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   The developer or application MAY validate the identity with the
   issuing authority.  The identifier is not a trusted identity, but MAY
   be relied on where trust has been established between the two parties
   (e.g. in a closed user group).

   The mechanisms to determine organizations identifiers is out of scope
   for the current specification.

5.3.2.  DLT Gateway / Endpoint ID

   FQDN of the ODAP compliant DLT gateway.  Required to establish IP
   connectivity.  This MUST resolve to a valid IP address.

5.3.3.  DLT Identifier

   Specify to gateway behind which the target DLTs operates.  This field
   is local to the DLT gateway and is used to direct ODAP interactions
   to the correct underlying DLT.

   For example: "Hyperledger1", "Bitcoin, "EU-supply-chain".

5.3.4.  Resource

   Specifies a resource held on the underlying DLT.  This field must be
   meaningful to the DLT in question but is otherwise an arbitrary
   string.  The underlying object it points to may be a DLT address,
   block, transaction ID, alias, etc. or a future object type not yet
   defined.

5.3.5.  Examples

   odapres://quant/api.gateway1.com/ripple

   odapres://quant/api.gateway1.com/bitcoin/xxxxxADDRESSxxxxx

5.4.  Digital Asset Resource Client Descriptors

   Resources are identified by URN as described below:

   o  The type is new: application/odapclient

   The URN format does not imply availability or access protocol.

   Data included in the URN includes the folowing:

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5.4.1.  Organization Identifier

   Legal Entity Identifier (LEI) or other identifier linking resource
   ownership to real world entity.  Any scheme for identifying DLT
   Gateway owners may be implemented (e.g.  LEI directory, closed user
   group membership, BIC, etc.).

   The DLT Gateway MAY validate the identity with the issuing authority.
   The identifier is not a trusted identity, but MAY be relied on where
   trust has been established between the two parties (e.g. in a closed
   user group).

5.4.2.  DLT Gateway / Endpoint ID

   Multi-DLT applications can operate in a mode whereby the application
   connects to its local DLT gateway, which then forwards application
   traffic to local DLTs and to remote DLTs via other ODAP gateways.

   Where this is the case, this field identifies the "home" gateway for
   this application.  This may be required to carry out Gateway to
   Gateway handshaking and protocol negotiation, or for the server to
   look up use case specific data relating to the client.

5.4.3.  Organizational Unit

   The organization unit within the organization that the client
   (application or developer) belongs to.  This assertion should be
   backed up with authentication via the negotiated protocol.

   The purpose of this field is to allow DLT gateways to maintain access
   control mapping between applications and resources that are
   independent of the authentication and authorization schemes used,
   supporting future changes and supporting counterparties that operate
   different schemes.

5.4.4.  Name

   A locally unique (within the OU) identifier, which can identify the
   application, project or individual developer responsible for this
   client connection.  This is the most granular unit of access control,
   and DLT Gateways should ensure appropriate identifiers are used for
   the needs of the application or use case.

5.4.5.  Examples

   odapclient:quant/api.overledger.quant.com/research/luke.riley

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5.5.  Gateway Level Access Control

   Gateways can enforce access rules based on standard naming
   conventions using novel or existing mechanisms such as AuthZ
   protocols using the resource identifiers above, for example:

   odapclient://hsbc/api.overledger.hsbc.com/lending/eric.devloper

   can READ/WRITE

   odapres://quant/api.gateway1.com/bitcoin

   AND

   odapres://quant/api.gateway1.com/ripple

   These rules would allow a client so identified to access resources
   directly, for example:

   odapres://quant/api.gateway1.com/bitcoin/xxxxxADDRESSxxxxx

   This example could be an client subscribing to or writing to an
   address associated with a smart contract as part of its
   functionality.

   This method allows resource owners to easily grant access to
   individuals, groups and organizations.  Individual gateway
   implementations may implement access controls, including subsetting
   and supersetting or applications or resources according to their own
   requirements.

5.6.  Negotiation of Security Protocols and Parameters

5.6.1.  TLS Established

   TLS 1.2 or higher MUST be implemented to protect gateway
   communications.  TLS 1.3 or higher SHOULD be implemented where both
   gateways support TLS 1.3 or higher.

5.6.2.  Client offers supported credential schemes

   Capability negotiation prior to data exchange, follows a scheme
   similar to the Session Description Protocol [RFC 5939].  Initially
   the client (application) sends a JSON block containing acceptable
   credential schemes, e.g.  OAuth2.0, SAML in the "Credential Scheme"
   field of the ODAP message.

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5.6.3.  Server selects supported credential scheme

   The server (DLT Gateway) selects one acceptable credential scheme
   from the offered schemes, returning the selection in the "Credential
   Scheme" field of the ODAP message.

   If no acceptable credential scheme was offered, an HTPP 511 "Network
   Authentication Required" error is returned in the Action/Response
   field of the ODAP message.

5.6.4.  Client asserts of proves identity

   The details of the assertion / verification step are specific to the
   chosen credential scheme and are out of scope of this document.

5.6.5.  Sequence numbers initialized

   Sequence numbers are used to allow the server to correctly order
   operations from the client, some of which may be asynchronous,
   synchronous, idempotent with duplicate requests handled in different
   ways according to the use case.

   The initial sequence number is proposed by the client (Application)
   after the finalization of credential verification.  The server (DLT
   gateway) MUST respond with the same sequence number to indicate
   acceptance.

   The client (application) increments the sequence number with each new
   request.  Sequence numbers can be reused for retries in the event of
   a gateway timeout.

5.6.6.  Messages can now be exchanged

   Handshaking is complete at this point, and the client (application)
   can send ODAP messages to perform actions of DLT resources, which MAY
   reference the ODAP Payload field.

5.7.  Asset Profile Negotiation

   TBD

5.8.  Application Profile Negotiation

   Where an application relies on specific extensions for operation,
   these can be represented in an Application Profile.

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   For example, a payments application tracks payments through the use
   of a cloud based API and will only interact with Gateways that log
   messages to that API, a resource profile can be established:

   Application Name: TRACKER

   X-Tracker_URL: https://api.tracker.com/updates

   X-Tracking-Policy: Always

   As Gateways implement this functionality, they support the TRACKER
   application profile, and the application is able to expand its reach
   by periodically polling for the availability of the profile.

   This is an intentionally generalized extension mechanism for
   application or vendor specific functionality.

5.9.  Digital Asset Resource Discovery

   Applications MUST be able to discover which resources they are
   authorized to access to the level of individual DLTs.  They MAY be
   able to discover lower level resources.

   Resource discovery is handled by the DLT gateway, for instance a GET
   request against the gateway URL with no DLT or resource could return
   a list of URLs available to the requester to DLT level.  This list is
   subject to the access controls above.

   DLT Gateways MAY allow applications to discover resources they do not
   have access to, this should be indicated the free text field, and
   they should implement a process for applications to request access.

   Formal specification of supported resource discovery methods is out
   of scope of this document.

5.10.  Accessing Resources via a DLT Gateway

   The Action field is used to access resources via the gateway.  We
   suggest these interactions use REST semantics however a detailed API
   specification is out of scope of this memo.

   In general, we suggest exposing a common subset of functionality via
   API using the Action field, augmented with DLT specific or smart
   contract specific functionality as needed.

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5.10.1.  CREATE

   Create an object on the target DLT.

5.10.2.  WRITE

   Write to a location on the target DLT.

5.10.3.  INVOKE

   Invoke code on the target DLT (typically a smart contract).

5.10.4.  LOCK

   Lock an object on the target DLT.

5.10.5.  UNLOCK

   Unlock an object on the target DLT.

5.10.6.  TRANSFER

   Transfer an object from one DLT to another.

5.10.7.  SUBSCRIBE

   Subscribe to be notified of transaction affecting an object on the
   target DLT.

5.10.8.  DESTROY

   Destroy an object on the target DLT.

5.10.9.  READ

   Read an object from the target DLT.

5.10.10.  NATIVE_TXN

   Send a signed native transaction of any kind to the target DLT.
   Payload consists of the native transaction.

5.11.  Response Codes

   The DLT Gateway MUST respond with return codes indicating the failure
   or success of the operation.  For DLTs with slow consensus mechanism,
   the Gateway may return codes indicating the operation has been

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   submitted.  The application may carry out further operation in future
   to determine the ultimate status of the operation.

   For Non-native transactions, the Gateway is responsible for
   translating the request into the appropriate native format and
   ensuring correct signing takes place.

5.12.  Backward Compatibility

   It is also possible to send a fully formatted native message to the
   underlying DLT in the Payload field using the NATIVE_TXN operation,
   directed to a resource URL.  This allows existing DLT native code to
   be ported to ODAP infrastructures with minimal change.

6.  Transfer Initiation Flow (Phase 1)

   This section describes ODAP initialization phase, where a sender
   gateway interacts with a target gateway, proposing a session.

   For this, several artifacts need to be validated: asset profile,
   asset ownership evidence, identities, and logging-related operations
   (log profile, access control profile [ODAP-2PC]).

   In this phase, gateways implement the Transfer Initiation Flows
   endpoint.

   In the following, the sender gateway takes instructions from a client
   application, while the recipient gateway may act on behalf of
   clients.

   The flow follows a request-response model.  The sender gateway makes
   a request (POST) to the Transfer Initiation endpoint at the recipient
   gateway.

   Gateways MUST support the use of the HTTP GET and POST methods
   defined in RFC 2616 [RFC2616] for the endpoint.

   Clients MAY use the HTTP GET or POST methods to send messages in this
   phase to the server.  If using the HTTP GET method, the request
   parameters maybe serialized using URI Query String Serialization.

   The client and server may be required to sign certain messages in
   order to provide standalone proof (for non-repudiation) independent
   of the secure channel between the client and server.  This proof
   maybe required for audit verifications post-event.

   (NOTE: Flows occur over TLS.  Nonces are not shown).

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6.1.  Initialization Request Message

   This message is sent from the sender gateway to the recipient
   gateway.  Note that a client (application) can issue an asset
   transfer, that is sent to the gateway and converted into an
   Initialization Request Message.

   The purpose of this message is for the client to initiate an asset
   transfer via its gateway.  Depending on the proposal, multiple rounds
   of communication between clients and gateways, and between gateways
   may happen.

   The parameters of this message consists of the following:

   o  Version: ODAP protocol Version (major, minor).

   o  Developer URN: Assertion of developer / application identity.

   o  Credential Profile: Specify type of auth (e.g.  SAML, OAuth,
      X.509)

   o  Payload Profile: Asset Profile provenance and capabilities

   o  Application Profile: Vendor or Application specific profile

   o  logging_profile REQUIRED: contains the profile regarding the
      logging procedure.  Default is local store.

   o  Access_control_profile REQUIRED: the profile regarding the
      confidentiality of the log entries being stored.  Default is only
      the gateway that created the logs can access them.

   o  Initialization Request Message signature REQUIRED: Gateway EDCSA
      signature over the message

   o  Source_gateway_pubkey REQUIRED: the public key of the gateway
      initiating a transfer

   o  Source_gateway_dlt_system REQUIRED: the ID of the source DLT

   o  Recipient_gateway_pubkey REQUIRED: the public key of the gateway
      involved in a transfer

   o  Recipient_gateway_dlt_system REQUIRED: the ID of the recipient
      gatewayinvolved in a transfer

   o  Escrow type: faucet, timelock, hashlock, hashtimelock, multi-claim
      PC, destroy/burn (escrowed cross-claim).

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   o  Expiry time: when will the escrow expire

   o  Multiple claims allowed: true/false

   o  Multiple cancels allowed: true/false

   o  Permissions: list of identities (addresses/X.509 certificates)
      that can perform operations on the escrow

   o  Origin: along with the source gateway DLT, allows identifying from
      where are the funds escrowed/provided

   o  Destination: along with the recipient gateway DLT, allows
      identifying to where are the escrowed funds going

   o  Subsequent calls: details possible escrow actions

   o  History: provides an history of the escrow, in case it has
      previously been initialized.  This includes a list of the
      transactions on that escrow (transaction ID) and which action it
      performed (ActionCategory), the origin and destination, balance,
      current status, and ActionLockSpecificParameters.

   The sender gateway makes the following HTTP request using TLS (with
   extra line breaks for display purposes only).

   Example: TBD.

6.2.  Initialization Request Message Response (ACK)

   After receiving an Initialization Request Message, the recipient
   gateway needs to validate the profiles.

   This validation could be performed automatically (using a defined set
   of rules), or by requiring approval by a client application.

   If one of the profiles is rejected, the recipient gateway constructs
   a Initialization Denied Message, stating what was rejected, and
   proposing an alternative (if applicable).

   Otherwise, if approved, the recipient gateway constructs a
   Initialization Request Message Response.

   The purpose of this message is for the server to indicate agreement
   to proceed with the proposed operations, under the proposed profiles.

   This message is sent from the recipient gateway to the sender gateway
   in response to a Initialization Request from the sender gateway.

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   The message must be signed by the server.

   The parameters of this message consists of the following:

   o  Session ID: unique identifier (UUIDv2) representing a session.

   o  Sequence Number: monotonically increasing counter that uniquely
      represents a message from a session.

   o  ODAP Phase: The current ODAP phase.

   o  Hash of the Initialization Request Message REQUIRED: the hash of
      the proposal.

   o  Destination: if the recipient gateway calculates the destination
      address dynamically.

   o  Timestamp REQUIRED: timestamp referring to when the Initialization
      Request Message was received.

   o  Processed Timestamp REQUIRED: timestamp referring to when the
      Initialization Response Message was constructed.

   Example: TBD.

7.  Lock-Evidence Verification Flow (Phase 2)

   This section describes the conveyance of claims regarding to the
   status of assets or resources from a sender gateway to a recipient
   gateway.

   In this phase, gateways implement the Lock-Evidence Agreement
   endpoint.

   In the following, the sender gateway takes the role of the client
   while the recipient gateway takes the role of the server.

   The flow follows a request-response model.  The client makes a
   request (POST) to the Lock-Evidence Agreement endpoint at the server.

   Gateways MUST support the use of the HTTP GET and POST methods
   defined in RFC 2616 [RFC2616] for the endpoint.

   Clients MAY use the HTTP GET or POST methods to send messages in this
   phase to the server.  If using the HTTP GET method, the request
   parameters maybe serialized using URI Query String Serialization.

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   The client and server may be required to sign certain messages in
   order to provide standalone proof (for non-repudiation) independent
   of the secure channel between the client and server.  This proof
   maybe required for audit verifications post-event.

   (NOTE: Flows occur over TLS.  Nonces are not shown).

7.1.  Transfer Commence Message

   This message is sent from the client (sender gateway) to the Transfer
   Request Endpoint at the server.  It signals to the server that the
   client is ready to start the transfer of the digital asset.

   The message must contain claims related to the information from the
   previous flow (Phase 1).  It must be signed by the client (sender
   gateway).

   The parameters of this message consists of the following:

   o  message_type REQUIRED.  MUST be the value
      urn:ietf:odap:msgtype:transfer-commence-msg

   o  originator_pubkey REQUIRED.  This is the public key of the asset
      owner (originator) in the origin DLT system.

   o  beneficiary_pubkey REQUIRED.  This is the public key of the
      beneficiary in the destination DLT system.

   o  sender_dlt_system REQUIRED.  This is the identifier of the origin
      DLT system behind the client.

   o  recipient_dlt_system REQUIRED.  This is the identifier of the
      destination DLT system behind the server.

   o  client_identity_pubkey REQUIRED.  The client who sent this
      message.

   o  server_identity_pubkey REQUIRED.  The server for whom this message
      is intended.

   o  hash_asset_profile REQUIRED.  This is the hash of the asset
      profile previously agreed upon in Phase 1.

   o  asset_unit OPTIONAL.  If applicable this is the unit amount of the
      asset being transferred, previously agreed upon.

   o  hash_prev_message REQUIRED.  The hash of the last message in Phase
      1.

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   o  client_transfer_number OPTIONAL.  This is the transfer
      identification number chosen by the client.  This number is
      meaningful only the client.

   o  client_signature REQUIRED.  The digital signature of the client.

   For example, the client makes the following HTTP request using TLS
   (with extra line breaks for display purposes only):

    POST /token HTTP/1.1
      Host: server.example.com
      Authorization: Basic awHCaGRSa3F0MzpnWDFmQmF0M2ZG
      Content-Type: application/x-www-form-urlencoded

          {
          "message_type": "urn:ietf:odap:msgtype:transfer-commence-msg",
          "originator_pubkey":"zGy89097hkbfgkjvVbNH",
          "beneficiary_pubkey": "mBGHJjjuijh67yghb",
          "sender_dlt_system": "originDLTsystem",
          "recipient_dlt_system":"recipientDLTsystem",
          "client_identity_pubkey":"fgH654tgeryuryuy",
          "server_identity_pubkey":"dFgdfgdfgt43tetr535teyrfge4t54334",
          "hash_asset_profile":"nbvcwertyhgfdsertyhgf2h3v4bd3v21",
          "asset_unit": "ghytredcfvbhfr",
          "hash_prev_message":"DRvfrb654vgreDerverv654nhRbvder4",
          "client_transfer_number":"ji9876543ewdfgh",
          "client_signature":"fdw34567uyhgfer45"
          }

                                 Figure 5

7.2.  Transfer Commence Response Message (Ack)

   The purpose of this message is for the server to indicate agreement
   to proceed with the asset transfer.

   This message is sent from the server (recipient gateway) to client
   (sender gateway) in response to a Transfer Commence Request from the
   client.

   The message must be signed by the server.

   The parameters of this message consists of the following:

   o  message_type REQUIRED urn:ietf:odap:msgtype:transfer-commenceack-
      msg

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   o  client_identity_pubkey REQUIRED.  The client for whom this message
      is intended.

   o  server_identity_pubkey REQUIRED.  The server who sent this
      message.

   o  hash_commence_request REQUIRED.  The hash of previous message.

   o  server_transfer_number OPTIONAL.  This is the transfer
      identification number chosen by the server.  This number is
      meaningful only to the server.

   o  server_signature REQUIRED.  The digital signature of the server.

   An example of a success response could be as follows:

 ```
      HTTP/1.1 200 OK
      Content-Type: application/json;charset=UTF-8
      Cache-Control: no-store
      Pragma: no-cache

      {
        "message_type":"urn:ietf:odap:msgtype:transfer-commenceack-msg",
        "client_identity_pubkey":"fgH654tgeryuryuy",
        "server_identity_pubkey":"dFgdfgdfgt43tetr535teyrfge4t54334",
        "hash_commence_request":"DRvfrb654vgreDerverv654nhRbvder4",
        "server_transfer_number":"ji9876543ewdfgh",
        "server_signature":"aaw34567uyhgfer66"
      }
 ```

                                 Figure 6

7.3.  Lock Evidence Message

   The purpose of this message is for the client (sending gateway) to
   deliver the relevant asset-lock (or escrow) evidence to the server
   (recipient gateway).

   The format of the evidence is dependent on the DLT fronted by the
   client and is outside the scope of this specification.

   The message must be signed by the client.

   This message is used client (sender gateway) to convey lock/escrow
   evidence to the server (recipient gateway).

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   This message is sent from the client to the Evidence Validation
   Endpoint at the server.  The server must validate the lock evidence
   claims in this message.

   The message must be signed by the client (sender gateway).

   The parameters of this message consists of the following:

   o  message_type REQUIRED urn:ietf:odap:msgtype:lock-evidence-req-msg

   o  client_identity_pubkey REQUIRED.  The client who sent this
      message.

   o  server_identity_pubkey REQUIRED.  The server for whom this message
      is intended.

   o  lock_evidence_claim REQUIRED.  The lock or escrow evidence (on the
      ledger L1 fronted by the client G1).

   o  lock_claim_format OPTIONAL.  The format of the evidence.

   o  lock_evidence_expiration REQUIRED.  The duration of time of lock
      on ledger L1 (after which the lock is released).

   o  hash_commence_ack_request REQUIRED.  The hash of previous message.

   o  client_transfer_number OPTIONAL.  This is the transfer
      identification number chosen by the client.  This number is
      meaningful only to the client.

   o  client_signature REQUIRED.  The digital signature of the client.

7.4.  Lock Evidence Response Message (Ack)

   The purpose of this message is for the server (recipient gateway) to
   indicate accaptance of the asset-lock (or escrow) evidence delivered
   by the client (sending gateway) in the previous message.

   The message must be signed by the server.

   The parameters of this message consists of the following:

   o  message_type REQUIRED urn:ietf:odap:msgtype:lock-evidence-ack-msg

   o  client_identity_pubkey REQUIRED.  The client for whom this message
      is intended.

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   o  server_identity_pubkey REQUIRED.  The server who sent this
      message.

   o  hash_lockevidence_request REQUIRED.  The hash of previous message.

   o  server_transfer_number OPTIONAL.  This is the transfer
      identification number chosen by the server.  This number is
      meaningful only to the server.

   o  server_signature REQUIRED.  The digital signature of the server.

8.  Commitment Establishment Flow (Phase 3)

   This section describes the transfer commitment agreement between the
   sender gateway to a recipient gateway.

   This phase must be completed within the asset-lock duration time
   specificied in the previous lock_evidence_expiration parameter
   (message 7.3).

   In this phase gateways implement the Transfer Commitment endpoint.

   In the following, the sender gateway takes the role of the client
   while the recipient gateway takes the role of the server.

   The flow follows a request-response model.  The client makes a
   request (POST) to the Transfer Commitment endpoint at the server.

   Gateways MUST support the use of the HTTP GET and POST methods
   defined in RFC 2616 [RFC2616] for the endpoint.

   Clients MAY use the HTTP GET or POST methods to send messages in this
   phase to the server.  If using the HTTP GET method, the request
   parameters maybe serialized using URI Query String Serialization.

   The client and server may be required to sign certain messages in
   order to provide standalone proof (for non-repudiation) independent
   of the secure channel between the client and server.  This proof
   maybe required for audit verifications post-event.

   (NOTE: Flows occur over TLS.  Nonces are not shown).

8.1.  Commit Preparation Message

   The purpose of this message is for the client to indicate its
   readiness to begin the commitment of the transfer.

   The message must be signed by the client.

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   The parameters of this message consists of the following:

   o  message_type REQUIRED.  It MUST be the value
      urn:ietf:odap:msgtype:commit-prepare-msg

   o  client_identity_pubkey REQUIRED.  The client who sent this
      message.

   o  server_identity_pubkey REQUIRED.  The server for whom this message
      is intended.

   o  hash_lockevidence_ack REQUIRED.  The hash of previous message.

   o  client_transfer_number OPTIONAL.  This is the transfer
      identification number chosen by the client.  This number is
      meaningful only the client.

   o  client_signature REQUIRED.  The digital signature of the client.

8.2.  Commit Preparation Response

   The purpose of this message is for the server to indicate to the
   client its readiness to proceed with the commitment finalization
   step..

   The message must be signed by the server.

   The parameters of this message consists of the following:

   o  message_type REQUIRED.  It MUST be the value
      urn:ietf:odap:msgtype:commit-prepare-ack-msg

   o  client_identity_pubkey REQUIRED.  The client for whom this message
      is intended.

   o  server_identity_pubkey REQUIRED.  The server who sent this
      message.

   o  hash_commitprep REQUIRED.  The hash of previous commit preparation
      message.

   o  server_transfer_number OPTIONAL.  This is the transfer
      identification number chosen by the server.  This number is
      meaningful only the server.

   o  server_signature REQUIRED.  The digital signature of the server.

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8.3.  Commit Final Message

   The purpose of this message is for the client to indicate to the
   server that the client (sender gateway) has completed local
   extinguishment of the asset on its DLT (L1), and that now on its part
   the server (recipient gateway) must re-generated the asset on its DLT
   (L2).

   The message must contain claims related to the extinguishment of the
   asset by the client.  It must be signed by the client.

   The parameters of this message consists of the following:

   o  message_type REQUIRED.  It MUST be the value
      urn:ietf:odap:msgtype:commit-final-msg

   o  client_identity_pubkey REQUIRED.  The client who sent this
      message.

   o  server_identity_pubkey REQUIRED.  The server for whom this message
      is intended.

   o  commit_final_claim REQUIRED.  This is one or more claims signed by
      the client that the asset in question has been extinguished by the
      client in its local DLT.

   o  commit_final_claim_format OPTIONAL.  This is the format of the
      claim provided by the client in this message.

   o  hash_commitprepare_ack REQUIRED.  The hash of previous message.

   o  client_transfer_number OPTIONAL.  This is the transfer
      identification number chosen by the client.  This number is
      meaningful only the client.

   o  client_signature REQUIRED.  The digital signature of the client.

8.4.  Commit Final Response Message

   The purpose of this message is for the server to indicate to the
   client that it has completed the asset re-generation at its DLTS
   (L2).

   The message must contain claims related to the re-generated of the
   asset by the server.  It must be signed by the server.

   The parameters of this message consists of the following:

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   o  message_type REQUIRED.  It MUST be the value
      urn:ietf:odap:msgtype:commit-final-ack-msg

   o  client_identity_pubkey REQUIRED.  The client for whom this message
      is intended..

   o  server_identity_pubkey REQUIRED.  The server who sent this
      message.

   o  commit_acknowledgement_claim REQUIRED.  This is one or more claims
      signed by the server that the asset in question has been
      regenerated by the server in its local DLT.

   o  commit_acknowledgement_claim_format OPTIONAL.  This is the format
      of the claim provided by the server in this message.

   o  hash_commitfinal REQUIRED.  The hash of previous commit final
      message.

   o  server_transfer_number OPTIONAL.  This is the transfer
      identification number chosen by the server.  This number is
      meaningful only the server.

   o  server_signature REQUIRED.  The digital signature of the server.

8.5.  Transfer Complete Message

   The purpose of this message is for the client to indicate to the
   server that the asset transer has been completed and that no further
   messages are to be expected from the client in regards to this
   transfer instance.

   The message closes the first message of Phase 2 (Transfer Commence
   Message).  It must be signed by the client.

   The parameters of this message consists of the following:

   o  message_type REQUIRED.  It MUST be the value
      urn:ietf:odap:msgtype:commit-transfer-complete-msg

   o  client_identity_pubkey REQUIRED.  The client who sent this
      message.

   o  server_identity_pubkey REQUIRED.  The server for whom this message
      is intended.

   o  hash_commit_final_ack REQUIRED.  The hash of previous message.

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   o  hash_transfer_commence REQUIRED.  The hash of the Transfer
      Commence message at the start of Phase 2 (see section 7.1).

   o  client_transfer_number OPTIONAL.  This is the transfer
      identification number chosen by the client.  This number is
      meaningful only the client.

   o  client_signature REQUIRED.  The digital signature of the client..

9.  Security Consideration

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

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

   Due to the consensus-based nature of the underlying DLT technologies,
   gateway responses may be conditional and require verification, for
   instance if the DLT is undergoing a byzantine attack at the time of
   the request.

   The application must evaluate the correctness of responses from the
   gateway in context and may need to perform further verification steps
   with later ODAP calls.  The application may base this evaluation on
   the number of DLT nodes the gateway has interacted with in order to
   fulfil the request.

10.  IANA Consideration

   (TBD)

11.  References

11.1.  Normative References

   [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>.

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   [RFC2234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 2234, DOI 10.17487/RFC2234,
              November 1997, <https://www.rfc-editor.org/info/rfc2234>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

11.2.  Informative References

   [Arch]     Hardjono, T., Hargreaves, M., and N. Smith, "An
              Interoperability Architecture for Blockchain Gateways.
              draft-hardjono-blockchain-interop-arch-02", April 2021,
              <https://datatracker.ietf.org/doc/draft-hardjono-
              blockchain-interop-arch/>.

   [HS2019]   Hardjono, T. and N. Smith, "Decentralized Trusted
              Computing Base for Blockchain Infrastructure Security,
              Frontiers Journal, Sepcial Issue on Blockchain Technology,
              Vol. 2, No. 24", December 2019,
              <https://doi.org/10.3389/fbloc.2019.00024>.

   [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>.

   [ODAP-2PC]
              Belchior, R., Correia, M., and T. Hardjono, "Gateway Crash
              Recovery Mechanism. draft-belchior-gateway-recovery-01",
              March 2021, <https://datatracker.ietf.org/doc/draft-
              belchior-gateway-recovery/>.

   [RFC5939]  Andreasen, F., "Session Description Protocol (SDP)
              Capability Negotiation", RFC 5939, DOI 10.17487/RFC5939,
              September 2010, <https://www.rfc-editor.org/info/rfc5939>.

Authors' Addresses

   Martin Hargreaves
   Quant Network

   Email: martin.hargreaves@quant.network

   Thomas Hardjono
   MIT

   Email: hardjono@mit.edu

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   Rafael Belchior
   Technico Lisboa

   Email: rafael.belchior@tecnico.ulisboa.pt

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