External Signing: Hashing Algorithm

Deterministic hashing specification for prepared transactions used by external signing.

External Signing Hashing Algorithm

Introduction

This document specifies the encoding algorithm used to produce a deterministic hash of a com.daml.ledger.api.v2.interactive.PreparedTransaction. The resulting hash is signed by the holder of the external party’s private key. The signature authorizes the ledger changes described by the transaction on behalf of the external party.

The specification can be implemented in any language, but certain encoding patterns are biased due to Canton being implemented in a JVM-based language and using the Java protobuf library. Those biases are made explicit in the specification.

Protobuf serialization is unsuitable for signing cryptographic hashes because it is not canonical. We must define a more precise encoding specification that can be re-implemented deterministically across languages and provide the required cryptographic guarantees. See https://protobuf.dev/programming-guides/serialization-not-canonical/ for more information on the topic.

Versioning

Hashing Scheme Version

The hashing algorithm as a whole is versioned. This enables updates to accommodate changes in the underlying Daml format, or, for instance, to the way the protocol verifies signatures. The implementation must respect the specification of the version it implements.

// The hashing scheme version used when building the hash of the PreparedTransaction
enum HashingSchemeVersion {
  HASHING_SCHEME_VERSION_UNSPECIFIED = 0;
  reserved 1; // Hashing Scheme V1 - unsupported
  HASHING_SCHEME_VERSION_V2 = 2;
  HASHING_SCHEME_VERSION_V3 = 3;
}

The hashing algorithm is tied to the protocol version of the synchronizer used to synchronize the transaction. Specifically, each hashing scheme version is supported on one or several protocol versions. Implementations must use a hashing scheme version supported on the synchronizer on which the transaction is submitted.

Protocol Version	Supported Hashing Schemes
v34	V2
v35	V2, V3
dev	V2, V3, V4

Transaction Nodes

Transaction nodes are additionally individually versioned with a Daml version (also called LF version). The encoding version is decoupled from the LF version and implementations should only focus on the hashing version. However, new LF versions may introduce new fields in nodes or new node types. For that reason, the protobuf representation of a node is versioned to accommodate those future changes. In practice, every new Daml language version results in a new hashing version.

message Node {
  // Required
  string node_id = 1;

  // Versioned node
  //
  // Required
  oneof versioned_node {
    // Start at 1000 so we can add more fields before if necessary
    // When new versions will be added, they will show here
    //
    // Required
    interactive.transaction.v1.Node v1 = 1000;
  }
}

Specification

General approach

The hash of the PreparedTransaction is computed by encoding the fields specified by this section to byte arrays, and feeding those encoded values into a SHA-256 hash builder. The rest of this section details how to deterministically encode every proto message into a byte array. Sometimes during the process, partially encoded results are hashed with SHA-256, and the resulting hash value serves as the encoding in messages further up. This is explicit when necessary.

Big Endian notation is used for numeric values. Furthermore, protobuf numeric values are encoded according to their Java type representation. Refer to the official protobuf documentation for more information about protobuf to Java type mappings: https://protobuf.dev/programming-guides/proto3/#scalar In particular:

In Java, unsigned 32-bit and 64-bit integers are represented using their signed counterparts, with the top bit simply being stored in the sign bit

Additionally, this is the java library used under the hood in Canton to serialize and deserialize protobuf: https://github.com/protocolbuffers/protobuf/tree/v3.25.5/java

Summary of differences between V3 and V4

V4 is currently unstable and subject to changes. For this reason it targets Protocol Version Dev only.

• New fields: Exercise nodes with version dev include a new external_call_results field. This binds the recorded external-call result payloads shown in prepared transaction review to the external party’s signature.
• Final hash: The hashing scheme version byte changes from 0x03 (V3) to 0x04 (V4).

Summary of differences between V2 and V3

The following is a summary of the differences between V2 and V3. The full specification for each version is provided in the sections below, where version-specific details are shown using tabs.

• Removed prefixes: V3 removes the 0x01 “Node encoding version” byte prefix from all node encodings, and the 0x01 “Metadata encoding version” prefix from the metadata encoding.
• New fields: key_opt (optional, composite) is added to Create, Exercise, and Fetch nodes. by_key (boolean) is added to Exercise and Fetch nodes.
• New node type: QueryByKey is introduced in V3.
• Metadata: max_record_time is added to the metadata encoding.
• Final hash: The hashing scheme version byte changes from 0x02 (V2) to 0x03 (V3).

V3 introduces support for contract keys. Usage of contract keys in externally signed transactions requires usage of V3. Contract keys will not work on V2. Also note that V3 is supported from protocol version 35 onwards.

Notation and Utility Functions

• encode: Function that takes a protobuf message or primitive type T and transforms it into an array of bytes: encode: T => byte[]

e.g:

encode(false) = [0x00]

• to_utf_8: Function converting a Java String to its UTF-8 encoded version: to_utf_8: string => byte[]

e.g:

to_utf_8("hello") = [0x68, 0x65, 0x6c, 0x6c, 0x6f]

• len: Function returning the size of a collection (array, list etc…) as a signed 4 bytes integer: len: Col => Int

e.g:

len([4, 2, 8]) = 3

• split: Function converting a Java String to a list of String, by splitting the input using the provided delimiter: split: (string, char) => byte[]

e.g:

split("com.digitalasset.canton", '.') = ["com", "digitalasset", "canton"]

• ||: Symbol representing concatenation of byte arrays

e.g:

[0x00] || [0x01] = [0x00, 0x01]

• []: Empty byte array. Denotes that the value should not be encoded.
• from_hex_string: Function that takes a string in the hexadecimal format as input and decodes it as a byte array: from_hex_string: string => byte[]

e.g:

from_hex_string("08020a") = [0x08, 0x02, 0x0a]

• int_to_string: Function that takes an int and converts it to a string : int_to_string: int => string

e.g:

int_to_string(42) = "42"

• some: Value wrapped in a defined optional. Should be encoded as a defined optional value: some: T => optional T

e.g:

encode(some(5)) = 0x01 || encode(5)

See encoding of optional values below for details.

• encode_key_with_maintainers (from V3 only): Function encoding a GlobalKeyWithMaintainers composite value. This is used for the key field in Create, Exercise, Fetch, and QueryByKey nodes.

fn encode_key_with_maintainers(key_with_maintainers):
    encode(key_with_maintainers.key.package_name) ||
    encode(key_with_maintainers.key.template_id) ||
    encode(key_with_maintainers.key.key) ||
    encode(key_with_maintainers.key.hash) ||
    encode(key_with_maintainers.maintainers)

• encode_external_call_result: Function encoding an ExternalCallResult composite value. This is used for the external_call_results field in Exercise nodes.

fn encode_external_call_result(result):
    return encode(result.extension_id) ||
        encode(result.function_id) ||
        encode(result.config) ||
        encode(result.input) ||
        encode(result.output)

Primitive Types

Unless otherwise specified, this is how primitive protobuf types should be encoded.

Not all protobuf types are described here, only the ones necessary to encode a PreparedTransaction message.

Even default values must be included in the encoding. For instance if an int32 field is not set in the serialized protobuf, its default value (0) should be encoded. Similarly, an empty repeated field still results in a 0x00 byte encoding (see the repeated section below for more details)

google.protobuf.Empty

fn encode(empty): 0x00

bool

fn encode(bool):
   if (bool)
      0x01
   else
      0x00

int64 - uint64 - sint64 - sfixed64

fn encode(long):
   long # Java `Long` value equivalent: 8 bytes

e.g:

31380 (base 10) == 0x0000000000007a94

int32 - uint32 - sint32 - sfixed32

fn encode(int):
   int # Java `Int` value equivalent: 4 bytes

e.g:

5 (base 10) == 0x00000005

bytes / byte[]

fn encode(bytes):
encode(len(bytes)) || bytes

e.g

0x68656c6c6f ->
    0x00000005 || # length
    0x68656c6c6f # content

string

fn encode(string):
   encode(to_utf8(string))

e.g

"hello" ->
    0x00000005 || # length
    0x68656c6c6f # utf-8 encoding of "hello"

Collections / Wrappers

repeated

repeated protobuf fields represent an ordered collection of values of a specific message of type T. It is critical that the order of values in the list is not modified, both for the encoding process and in the protobuf itself when submitting the transaction for execution. Below is the pseudocode algorithm encoding a protobuf value repeated T list;

fn encode(list):
   # prefix the result with the serialized length of the list
   result = encode(len(list)) # (result is mutable)

   # successively add encoded elements to the result, in order
   for each element in list:
      result = result || encode(element)

   return result

This encoding function also applies to lists generated from utility functions (e.g: split).

optional

fn encode(optional):
   if (is_set(optional))
      0x01 || encode(optional.value)
   else
      0x00

is_set returns true if the value was set in the protobuf, false otherwise.

map

The ordering of map entries in protobuf serialization is not guaranteed, making it problematic for deterministic encoding. To address this, repeated values are used instead of map throughout the protobuf definitions.

gRPC Ledger API Value

Encoding for the Value message defined in com.daml.ledger.api.v2.value.proto For clarity, all value types are exhaustively listed here. Each value is prefixed by a tag unique to its type, which is explicitly specified for each value below.

Unit

fn encode(unit):
    0X00 # Unit Type Tag

Protobuf Definition

Bool

fn encode(bool):
    0X01 || # Bool Type Tag
encode(bool) # Primitive boolean encoding

Protobuf Definition

Int64

fn encode(int64):
    0X02 || # Int64 Type Tag
    encode(int64) # Primitive int64 encoding

Protobuf Definition

Numeric

fn encode(numeric):
    0X03 || # Numeric Type Tag
    encode(numeric) # Primitive string encoding

Protobuf Definition

Timestamp

fn encode(timestamp):
    0X04 || # Timestamp Type Tag
    encode(timestamp) # Primitive sfixed64 encoding

Protobuf Definition

Date

fn encode(date):
    0X05 || # Date Type Tag
    encode(date) # Primitive int32 encoding

Protobuf Definition

Party

fn encode(party):
    0X06 || # Party Type Tag
    encode(party) # Primitive string encoding

Protobuf Definition

Text

fn encode(text):
    0X07 || # Text Type Tag
    encode(text) # Primitive string encoding

Protobuf Definition

Contract_id

fn encode(contract_id):
    0X08 || # Contract Id Type Tag
    from_hex_string(contract_id) # Contract IDs are hexadecimal strings, so they need to be decoded as such. They should not be encoded as classic strings

Protobuf Definition

Optional

fn encode(optional):
   if (optional.value is set)
      0X09 || # Optional Type Tag
      0x01 || # Defined optional
      encode(optional.value)
   else
      0X09 || # Optional Type Tag
      0x00 || # Undefined optional

Protobuf Definition

Note this is conceptually the same as for the primitive optional protobuf modifier, with the addition of the type tag prefix.

List

fn encode(list):
   0X0a || # List Type Tag
   encode(list.elements)

Protobuf Definition

TextMap

fn encode(text_map):
   0X0b || # TextMap Type Tag
   encode(text_map.entries)

Protobuf Definition

TextMap.Entry

fn encode(entry):
   encode(entry.key) || encode(entry.value)

Protobuf Definition

Record

fn encode(record):
   0X0c || # Record Type Tag
   encode(some(record.record_id)) ||
   encode(record.fields)

Protobuf Definition

RecordField

fn encode(record_field):
   encode(some(record_field.label)) || encode(record_field.value)

Protobuf Definition

Variant

fn encode(variant):
   0X0d || # Variant Type Tag
   encode(some(variant.variant_id)) ||
   encode(variant.constructor) || encode(variant.value)

Protobuf Definition

Enum

fn encode(enum):
   0X0e || # Enum Type Tag
   encode(some(enum.enum_id)) ||
   encode(enum.constructor)

Protobuf Definition

GenMap

fn encode(gen_map):
   0X0f || # GenMap Type Tag
   encode(gen_map.entries)

Protobuf Definition

GenMap.Entry

fn encode(entry):
   encode(entry.key) || encode(entry.value)

Protobuf Definition

Identifier

fn encode(identifier):
   encode(identifier.package_id) || encode(split(identifier.module_name, '.')) || encode(split(identifier.entity_name, '.')))

Protobuf Definition

Transaction

A transaction is a forest (list of trees). It is represented with a following protobuf message found here.

The encoding function for a transaction is

fn encode(transaction):
   encode(transaction.version) || encode_node_ids(transaction.roots)

encode_node_ids(node_ids) encodes lists in the same way as described before, except the encoding of a node_id is NOT done by encoding it as a string, but instead uses the following encode(node_id) function:

fn encode(node_id):
    for node in nodes:
        if node.node_id == node_id:
           return sha_256(encode_node(node))
    fail("Missing node") # All node ids should have a unique node in the nodes list. If a node is missing it should be reported as a bug.

encode(node_id) effectively finds the corresponding node in the list of nodes and encodes the node. The node_id is an opaque value only used to reference nodes and is itself never encoded. Additionally, each node’s encoding is hashed using the sha_256 hashing algorithm. This is relevant when encoding root nodes here as well as when recursively encoding sub-nodes of Exercise and Rollback nodes as seen below.

Node

Each node’s encoding is prefixed with additional meta-information about the node, this is made explicit in the encoding of each node.

Exercise and Rollback nodes both have a children field that references other nodes by their NodeId.

The following find_seed: NodeId => optional bytes function is used in the encoding:

fn find_seed(node_id):
    for node_seed in node_seeds:
        if int_to_string(node_seed.node_id) == node_id
            return some(node_seed.seed)
    return none

# There's no need to prefix the seed with its length because it has a fixed length. So its encoding is the identity function
fn encode_seed(seed):
    seed

# Normal optional encoding, except the seed is encoded with `encode_seed`
fn encode_optional_seed(optional_seed):
    if (is_some(optional_seed))
      0x01 || encode_seed(optional_seed.get)
   else
      0x00

some represents a set optional field, none an empty optional field.

Create

V4 (unstable)

fn encode_node(create):
  encode(create.lf_version) || # Node LF version
  0x00 || # Create node tag
  encode_optional_seed(find_seed(node.node_id)) ||
  encode(create.contract_id) ||
  encode(create.package_name) ||
  encode(create.template_id) ||
  encode(create.argument) ||
  encode(create.signatories) ||
  encode(create.stakeholders) ||
  encode_key_with_maintainers(create.key)

V3

fn encode_node(create):
    encode(create.lf_version) || # Node LF version
    0x00 || # Create node tag
    encode_optional_seed(find_seed(node.node_id)) ||
    encode(create.contract_id) ||
    encode(create.package_name) ||
    encode(create.template_id) ||
    encode(create.argument) ||
    encode(create.signatories) ||
    encode(create.stakeholders) ||
    encode_key_with_maintainers(create.key) # new in V3

V2

fn encode_node(create):
    0x01 || # Node encoding version
    encode(create.lf_version) || # Node LF version
    0x00 || # Create node tag
    encode_optional_seed(find_seed(node.node_id)) ||
    encode(create.contract_id) ||
    encode(create.package_name) ||
    encode(create.template_id) ||
    encode(create.argument) ||
    encode(create.signatories) ||
    encode(create.stakeholders)

Exercise

V4 (unstable)

fn encode_node(exercise):
    encode(exercise.lf_version) || # Node LF version
    0x01 || # Exercise node tag
    encode_seed(find_seed(node.node_id).get) ||
    encode(exercise.contract_id) ||
    encode(exercise.package_name) ||
    encode(exercise.template_id) ||
    encode(exercise.signatories) ||
    encode(exercise.stakeholders) ||
    encode(exercise.acting_parties) ||
    encode(exercise.interface_id) ||
    encode(exercise.choice_id) ||
    encode(exercise.chosen_value) ||
    encode(exercise.consuming) ||
    encode(exercise.exercise_result) ||
    encode(exercise.choice_observers) ||
    encode(exercise.by_key) ||
    encode_key_with_maintainers(exercise.key) ||
    encode_external_call_result(exercise.external_call_results) || # new in V4
    encode(exercise.children)

V3

fn encode_node(exercise):
    encode(exercise.lf_version) || # Node LF version
    0x01 || # Exercise node tag
    encode_seed(find_seed(node.node_id).get) ||
    encode(exercise.contract_id) ||
    encode(exercise.package_name) ||
    encode(exercise.template_id) ||
    encode(exercise.signatories) ||
    encode(exercise.stakeholders) ||
    encode(exercise.acting_parties) ||
    encode(exercise.interface_id) ||
    encode(exercise.choice_id) ||
    encode(exercise.chosen_value) ||
    encode(exercise.consuming) ||
    encode(exercise.exercise_result) ||
    encode(exercise.choice_observers) ||
    encode(exercise.by_key)  || # new in V3
    encode_key_with_maintainers(exercise.key) || # new in V3
    encode(exercise.children)

V2

fn encode_node(exercise):
    0x01 || # Node encoding version
    encode(exercise.lf_version) || # Node LF version
    0x01 || # Exercise node tag
    encode_seed(find_seed(node.node_id).get) ||
    encode(exercise.contract_id) ||
    encode(exercise.package_name) ||
    encode(exercise.template_id) ||
    encode(exercise.signatories) ||
    encode(exercise.stakeholders) ||
    encode(exercise.acting_parties) ||
    encode(exercise.interface_id) ||
    encode(exercise.choice_id) ||
    encode(exercise.chosen_value) ||
    encode(exercise.consuming) ||
    encode(exercise.exercise_result) ||
    encode(exercise.choice_observers) ||
    encode(exercise.children)

For Exercise nodes, the node seed MUST be defined. Therefore it is encoded as a non optional field, as noted via the .get in find_seed(node.node_id).get. If the seed of an exercise node cannot be found in the list of node_seeds, encoding must be stopped and it should be reported as a bug.

Fetch

V4 (unstable)

fn encode_node(fetch):
    encode(fetch.lf_version) || # Node LF version
    0x02 || # Fetch node tag
    encode(fetch.contract_id) ||
    encode(fetch.package_name) ||
    encode(fetch.template_id) ||
    encode(fetch.signatories) ||
    encode(fetch.stakeholders) ||
    encode(fetch.interface_id) ||
    encode(fetch.acting_parties) ||
    encode(exercise.by_key) ||
    encode_key_with_maintainers(exercise.key)

V3

fn encode_node(fetch):
    encode(fetch.lf_version) || # Node LF version
    0x02 || # Fetch node tag
    encode(fetch.contract_id) ||
    encode(fetch.package_name) ||
    encode(fetch.template_id) ||
    encode(fetch.signatories) ||
    encode(fetch.stakeholders) ||
    encode(fetch.interface_id) ||
    encode(fetch.acting_parties) ||
    encode(exercise.by_key)  || # new in V3
    encode_key_with_maintainers(exercise.key) # new in V3

V2

fn encode_node(fetch):
    0x01 || # Node encoding version
    encode(fetch.lf_version) || # Node LF version
    0x02 || # Fetch node tag
    encode(fetch.contract_id) ||
    encode(fetch.package_name) ||
    encode(fetch.template_id) ||
    encode(fetch.signatories) ||
    encode(fetch.stakeholders) ||
    encode(fetch.interface_id) ||
    encode(fetch.acting_parties)

Rollback

V4 (unstable)

fn encode_node(rollback):
   0x03 || # Rollback node tag
   encode(rollback.children)

V3

fn encode_node(rollback):
   0x03 || # Rollback node tag
   encode(rollback.children)

V2

fn encode_node(rollback):
   0x01 || # Node encoding version
   0x03 || # Rollback node tag
   encode(rollback.children)

Rollback nodes do not have an lf version.

QueryByKey

This node type is only present in hashing scheme V3 and higher.

V4 (unstable)

    fn encode_node(query_by_key):
        encode(query_by_key.lf_version) || # Node LF version
        0x04 || # QueryByKey node tag
        encode(query_by_key.package_name) ||
        encode(query_by_key.template_id) ||
        encode(query_by_key.exhaustive) ||
        encode_key_with_maintainers(query_by_key.key) ||
        encode(query_by_key.result)

V3

    fn encode_node(query_by_key):
        encode(query_by_key.lf_version) || # Node LF version
        0x04 || # QueryByKey node tag
        encode(query_by_key.package_name) ||
        encode(query_by_key.template_id) ||
        encode(query_by_key.exhaustive) ||
        encode_key_with_maintainers(query_by_key.key) ||
        encode(query_by_key.result)

Transaction Hash

Once the transaction is encoded, the hash is obtained by running sha_256 over the encoded byte array, with a hash purpose prefix:

fn hash(transaction):
    sha_256(
        0x00000030 || # Hash purpose
        encode(transaction)
    )

Metadata

The final part of PreparedTransaction is metadata. Note that all fields of the metadata need to be signed. Only some fields contribute to the ledger change triggered by the transaction. The rest of the fields are required by the Canton protocol but either have no impact on the ledger change, or have already been signed indirectly by signing the transaction itself.

V4 (unstable)

fn encode(metadata, prepare_submission_request):
    encode(metadata.submitter_info.act_as) ||
    encode(metadata.submitter_info.command_id) ||
    encode(metadata.transaction_uuid) ||
    encode(metadata.mediator_group) ||
    encode(metadata.synchronizer_id) ||
    encode(metadata.min_ledger_effective_time) ||
    encode(metadata.max_ledger_effective_time) ||
    encode(metadata.submission_time) ||
    encode(metadata.disclosed_events) ||
    encode(metadata.max_record_time)

V3

fn encode(metadata, prepare_submission_request):
    encode(metadata.submitter_info.act_as) ||
    encode(metadata.submitter_info.command_id) ||
    encode(metadata.transaction_uuid) ||
    encode(metadata.mediator_group) ||
    encode(metadata.synchronizer_id) ||
    encode(metadata.min_ledger_effective_time) ||
    encode(metadata.max_ledger_effective_time) ||
    encode(metadata.submission_time) ||
    encode(metadata.disclosed_events) ||
    encode(metadata.max_record_time)

V2

fn encode(metadata, prepare_submission_request):
    0x01 || # Metadata Encoding Version
    encode(metadata.submitter_info.act_as) ||
    encode(metadata.submitter_info.command_id) ||
    encode(metadata.transaction_uuid) ||
    encode(metadata.mediator_group) ||
    encode(metadata.synchronizer_id) ||
    encode(metadata.min_ledger_effective_time) ||
    encode(metadata.max_ledger_effective_time) ||
    encode(metadata.submission_time) ||
    encode(metadata.disclosed_events) ||
    encode(metadata.max_record_time)

ProcessedDisclosedContract

fn encode(processed_disclosed_contract):
    encode(processed_disclosed_contract.created_at) ||
    encode(processed_disclosed_contract.contract)

Metadata Hash

Once the metadata is encoded, the hash is obtained by running sha_256 over the encoded byte array, with a hash purpose prefix:

fn hash(metadata):
    sha_256(
        0x00000030 || # Hash purpose
        encode(metadata)
    )

Final Hash

Finally, compute the hash that needs to be signed to commit to the ledger changes.

V4 (unstable)

fn encode(prepared_transaction):
    0x00000030 || # Hash purpose
    0x04 || # Hashing Scheme Version
    hash(transaction) ||
    hash(metadata)

V3

fn encode(prepared_transaction):
    0x00000030 || # Hash purpose
    0x03 || # Hashing Scheme Version
    hash(transaction) ||
    hash(metadata)

V2

fn encode(prepared_transaction):
    0x00000030 || # Hash purpose
    0x02 || # Hashing Scheme Version
    hash(transaction) ||
    hash(metadata)

fn hash(prepared_transaction):
    sha_256(encode(prepared_transaction))

This resulting hash must be signed with the protocol signing private key(s) used to onboard the external party. Both the signature along with the PreparedTransaction must be sent to the API to submit the transaction to the ledger.

Example

Example V3 implementation in Python

# Copyright (c) 2026 Digital Asset (Switzerland) GmbH and/or its affiliates. All rights reserved.
# SPDX-License-Identifier: Apache-2.0

# Implements V3 of the transaction hashing specification.
# V3 removes node/metadata encoding version prefixes, adds key_opt to Create/Exercise/Fetch,
# adds by_key to Exercise/Fetch, introduces QueryByKey node type, and adds max_record_time to metadata.

import com.daml.ledger.api.v2.interactive.interactive_submission_service_pb2 as interactive_submission_service_pb2
from daml_transaction_hashing_common import (
    PREPARED_TRANSACTION_HASH_PURPOSE,
    encode_bool,
    encode_hex_string,
    encode_identifier,
    encode_int32,
    encode_int64,
    encode_node_id,
    encode_optional,
    encode_proto_optional,
    encode_repeated,
    encode_string,
    encode_hash,
    encode_value,
    encode_transaction,
    find_seed,
    sha256,
    create_nodes_dict,
)

# Version of the hashing scheme implemented in this file as a byte
HASHING_SCHEME_VERSION_V3 = (
    interactive_submission_service_pb2.HashingSchemeVersion.HASHING_SCHEME_VERSION_V3
)
# Byte version for the encoding (\x03)
HASHING_SCHEME_VERSION = HASHING_SCHEME_VERSION_V3.to_bytes(
    length=1, byteorder="big", signed=False
)


def encode_key_with_maintainers(key_with_maintainers):
    """Encodes a GlobalKeyWithMaintainers composite value."""
    return (
        encode_string(key_with_maintainers.key.package_name)
        + encode_identifier(key_with_maintainers.key.template_id)
        + encode_value(key_with_maintainers.key.key)
        + encode_hash(key_with_maintainers.key.hash)
        + encode_repeated(key_with_maintainers.maintainers, encode_string)
    )


def encode_prepared_transaction(
    prepared_transaction: interactive_submission_service_pb2.PreparedTransaction,
    nodes_dict: dict,
):
    transaction_hash = hash_transaction(prepared_transaction.transaction, nodes_dict)
    metadata_hash = hash_metadata(prepared_transaction.metadata)
    return sha256(
        PREPARED_TRANSACTION_HASH_PURPOSE
        + HASHING_SCHEME_VERSION
        + transaction_hash
        + metadata_hash
    )


def hash_transaction(
    transaction: interactive_submission_service_pb2.DamlTransaction, nodes_dict: dict
):
    encoded_transaction = encode_transaction(
        transaction, nodes_dict, transaction.node_seeds, encode_node
    )
    return sha256(PREPARED_TRANSACTION_HASH_PURPOSE + encoded_transaction)


def encode_node(
    node,
    nodes_dict: dict,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    node_id = node.node_id
    if node.HasField("v1"):
        return encode_node_v1(node.v1, node_id, nodes_dict, node_seeds)
    raise ValueError("Unsupported node version")


def encode_node_v1(
    node,
    node_id,
    nodes_dict: dict,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    if node.HasField("create"):
        return encode_create_node(node.create, node_id, node_seeds)
    elif node.HasField("exercise"):
        return encode_exercise_node(node.exercise, node_id, nodes_dict, node_seeds)
    elif node.HasField("fetch"):
        return encode_fetch_node(node.fetch, node_id)
    elif node.HasField("rollback"):
        return encode_rollback_node(node.rollback, node_id, nodes_dict, node_seeds)
    elif node.HasField("query_by_key"):
        return encode_query_by_key_node(node.query_by_key)
    raise ValueError("Unsupported node type")


def encode_create_node(
    create,
    node_id,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    return (
        encode_string(create.lf_version)
        + b"\x00"  # Create node tag
        + encode_optional(find_seed(node_id, node_seeds), encode_hash)
        + encode_hex_string(create.contract_id)
        + encode_string(create.package_name)
        + encode_identifier(create.template_id)
        + encode_value(create.argument)
        + encode_repeated(create.signatories, encode_string)
        + encode_repeated(create.stakeholders, encode_string)
        + encode_proto_optional(create, "key", create.key, encode_key_with_maintainers)
    )


def encode_exercise_node(
    exercise,
    node_id,
    nodes_dict: dict,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    return (
        encode_string(exercise.lf_version)
        + b"\x01"  # Exercise node tag
        + encode_hash(find_seed(node_id, node_seeds))
        + encode_hex_string(exercise.contract_id)
        + encode_string(exercise.package_name)
        + encode_identifier(exercise.template_id)
        + encode_repeated(exercise.signatories, encode_string)
        + encode_repeated(exercise.stakeholders, encode_string)
        + encode_repeated(exercise.acting_parties, encode_string)
        + encode_proto_optional(
            exercise, "interface_id", exercise.interface_id, encode_identifier
        )
        + encode_string(exercise.choice_id)
        + encode_value(exercise.chosen_value)
        + encode_bool(exercise.consuming)
        + encode_proto_optional(
            exercise, "exercise_result", exercise.exercise_result, encode_value
        )
        + encode_repeated(exercise.choice_observers, encode_string)
        + encode_bool(exercise.by_key)
        + encode_proto_optional(exercise, "key", exercise.key, encode_key_with_maintainers)
        + encode_repeated(exercise.children, encode_node_id(nodes_dict, node_seeds, encode_node))
    )


def encode_fetch_node(fetch, node_id):
    return (
        encode_string(fetch.lf_version)
        + b"\x02"  # Fetch node tag
        + encode_hex_string(fetch.contract_id)
        + encode_string(fetch.package_name)
        + encode_identifier(fetch.template_id)
        + encode_repeated(fetch.signatories, encode_string)
        + encode_repeated(fetch.stakeholders, encode_string)
        + encode_proto_optional(
            fetch, "interface_id", fetch.interface_id, encode_identifier
        )
        + encode_repeated(fetch.acting_parties, encode_string)
        + encode_bool(fetch.by_key)
        + encode_proto_optional(fetch, "key", fetch.key, encode_key_with_maintainers)
    )


def encode_rollback_node(
    rollback,
    node_id,
    nodes_dict: dict,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    return (
        b"\x03"  # Rollback node tag
        + encode_repeated(rollback.children, encode_node_id(nodes_dict, node_seeds, encode_node))
    )


def encode_query_by_key_node(query_by_key):
    return (
        encode_string(query_by_key.lf_version)
        + b"\x04"  # QueryByKey node tag
        + encode_string(query_by_key.package_name)
        + encode_identifier(query_by_key.template_id)
        + encode_bool(query_by_key.exhaustive)
        + encode_key_with_maintainers(query_by_key.key)
        + encode_repeated(query_by_key.result, encode_hex_string)
    )


def hash_metadata(metadata):
    encoded_metadata = encode_metadata(metadata)
    return sha256(PREPARED_TRANSACTION_HASH_PURPOSE + encoded_metadata)


def encode_metadata(metadata):
    return (
        encode_repeated(metadata.submitter_info.act_as, encode_string)
        + encode_string(metadata.submitter_info.command_id)
        + encode_string(metadata.transaction_uuid)
        + encode_int32(metadata.mediator_group)
        + encode_string(metadata.synchronizer_id)
        + encode_proto_optional(
            metadata,
            "min_ledger_effective_time",
            metadata.min_ledger_effective_time,
            encode_int64,
        )
        + encode_proto_optional(
            metadata,
            "max_ledger_effective_time",
            metadata.max_ledger_effective_time,
            encode_int64,
        )
        + encode_int64(metadata.preparation_time)
        + encode_repeated(metadata.input_contracts, encode_input_contract)
        + encode_proto_optional(metadata, "max_record_time", metadata.max_record_time, encode_int64)
    )


def encode_disclosed_contract(contract):
    return encode_int64(contract.created_at) + sha256(
        encode_create_node(contract.contract, "unused_node_id", [])
    )


def encode_input_contract(contract):
    return encode_int64(contract.created_at) + sha256(
        encode_create_node(contract.v1, "unused_node_id", [])
    )

Example V2 implementation in Python

# Copyright (c) 2026 Digital Asset (Switzerland) GmbH and/or its affiliates. All rights reserved.
# SPDX-License-Identifier: Apache-2.0

# Implements V2 of the transaction hashing specification defined in the README.md at https://github.com/digital-asset/canton/blob/main/community/ledger-api/src/release-line-3.2/protobuf/com/daml/ledger/api/v2/interactive/README.md

import com.daml.ledger.api.v2.interactive.interactive_submission_service_pb2 as interactive_submission_service_pb2
from daml_transaction_hashing_common import (
    PREPARED_TRANSACTION_HASH_PURPOSE,
    encode_bool,
    encode_hex_string,
    encode_identifier,
    encode_int32,
    encode_int64,
    encode_node_id,
    encode_optional,
    encode_proto_optional,
    encode_repeated,
    encode_string,
    encode_hash,
    encode_value,
    encode_transaction,
    find_seed,
    sha256,
    create_nodes_dict,
)

# Version of the hashing scheme implemented in this file as a byte
HASHING_SCHEME_VERSION_V2 = (
    interactive_submission_service_pb2.HashingSchemeVersion.HASHING_SCHEME_VERSION_V2
)
# Byte version for the encoding (\x02)
HASHING_SCHEME_VERSION = HASHING_SCHEME_VERSION_V2.to_bytes(
    length=1, byteorder="big", signed=False
)
# Version of the protobuf encoding the transaction nodes
NODE_ENCODING_VERSION = b"\x01"


def encode_prepared_transaction(
    prepared_transaction: interactive_submission_service_pb2.PreparedTransaction,
    nodes_dict: dict,
):
    transaction_hash = hash_transaction(prepared_transaction.transaction, nodes_dict)
    metadata_hash = hash_metadata(prepared_transaction.metadata)
    return sha256(
        PREPARED_TRANSACTION_HASH_PURPOSE
        + HASHING_SCHEME_VERSION
        + transaction_hash
        + metadata_hash
    )


def hash_transaction(
    transaction: interactive_submission_service_pb2.DamlTransaction, nodes_dict: dict
):
    encoded_transaction = encode_transaction(
        transaction, nodes_dict, transaction.node_seeds, encode_node
    )
    return sha256(PREPARED_TRANSACTION_HASH_PURPOSE + encoded_transaction)


def encode_node(
    node,
    nodes_dict: dict,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    node_id = node.node_id
    if node.HasField("v1"):
        return encode_node_v1(node.v1, node_id, nodes_dict, node_seeds)
    raise ValueError("Unsupported node version")


def encode_node_v1(
    node,
    node_id,
    nodes_dict: dict,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    if node.HasField("create"):
        return encode_create_node(node.create, node_id, node_seeds)
    elif node.HasField("exercise"):
        return encode_exercise_node(node.exercise, node_id, nodes_dict, node_seeds)
    elif node.HasField("fetch"):
        return encode_fetch_node(node.fetch, node_id)
    elif node.HasField("rollback"):
        return encode_rollback_node(node.rollback, node_id, nodes_dict, node_seeds)
    raise ValueError("Unsupported node type")


def encode_create_node(
    create,
    node_id,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    return (
        NODE_ENCODING_VERSION
        + encode_string(create.lf_version)
        + b"\x00"  # Create node tag
        + encode_optional(find_seed(node_id, node_seeds), encode_hash)
        + encode_hex_string(create.contract_id)
        + encode_string(create.package_name)
        + encode_identifier(create.template_id)
        + encode_value(create.argument)
        + encode_repeated(create.signatories, encode_string)
        + encode_repeated(create.stakeholders, encode_string)
    )


def encode_exercise_node(
    exercise,
    node_id,
    nodes_dict: dict,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    return (
        NODE_ENCODING_VERSION
        + encode_string(exercise.lf_version)
        + b"\x01"  # Exercise node tag
        + encode_hash(find_seed(node_id, node_seeds))
        + encode_hex_string(exercise.contract_id)
        + encode_string(exercise.package_name)
        + encode_identifier(exercise.template_id)
        + encode_repeated(exercise.signatories, encode_string)
        + encode_repeated(exercise.stakeholders, encode_string)
        + encode_repeated(exercise.acting_parties, encode_string)
        + encode_proto_optional(
            exercise, "interface_id", exercise.interface_id, encode_identifier
        )
        + encode_string(exercise.choice_id)
        + encode_value(exercise.chosen_value)
        + encode_bool(exercise.consuming)
        + encode_proto_optional(
            exercise, "exercise_result", exercise.exercise_result, encode_value
        )
        + encode_repeated(exercise.choice_observers, encode_string)
        + encode_repeated(exercise.children, encode_node_id(nodes_dict, node_seeds, encode_node))
    )


def encode_fetch_node(fetch, node_id):
    return (
        NODE_ENCODING_VERSION
        + encode_string(fetch.lf_version)
        + b"\x02"  # Fetch node tag
        + encode_hex_string(fetch.contract_id)
        + encode_string(fetch.package_name)
        + encode_identifier(fetch.template_id)
        + encode_repeated(fetch.signatories, encode_string)
        + encode_repeated(fetch.stakeholders, encode_string)
        + encode_proto_optional(
            fetch, "interface_id", fetch.interface_id, encode_identifier
        )
        + encode_repeated(fetch.acting_parties, encode_string)
    )


def encode_rollback_node(
    rollback,
    node_id,
    nodes_dict: dict,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
):
    return (
        NODE_ENCODING_VERSION
        + b"\x03"  # Rollback node tag
        + encode_repeated(rollback.children, encode_node_id(nodes_dict, node_seeds, encode_node))
    )


def hash_metadata(metadata):
    encoded_metadata = encode_metadata(metadata)
    return sha256(PREPARED_TRANSACTION_HASH_PURPOSE + encoded_metadata)


def encode_metadata(metadata):
    return (
        b"\x01"  # Metadata encoding version
        + encode_repeated(metadata.submitter_info.act_as, encode_string)
        + encode_string(metadata.submitter_info.command_id)
        + encode_string(metadata.transaction_uuid)
        + encode_int32(metadata.mediator_group)
        + encode_string(metadata.synchronizer_id)
        + encode_proto_optional(
            metadata,
            "min_ledger_effective_time",
            metadata.min_ledger_effective_time,
            encode_int64,
        )
        + encode_proto_optional(
            metadata,
            "max_ledger_effective_time",
            metadata.max_ledger_effective_time,
            encode_int64,
        )
        + encode_int64(metadata.preparation_time)
        + encode_repeated(metadata.input_contracts, encode_input_contract)
    )


def encode_input_contract(contract):
    return encode_int64(contract.created_at) + sha256(
        encode_create_node(contract.v1, "unused_node_id", [])
    )

Both versions make use of the following common code:

# Copyright (c) 2026 Digital Asset (Switzerland) GmbH and/or its affiliates. All rights reserved.
# SPDX-License-Identifier: Apache-2.0

# Common encoding utilities shared between hashing scheme versions V2 and V3.

import com.daml.ledger.api.v2.interactive.interactive_submission_service_pb2 as interactive_submission_service_pb2
import hashlib
import struct

# Hash purpose reserved for prepared transaction
PREPARED_TRANSACTION_HASH_PURPOSE = b"\x00\x00\x00\x30"


def encode_bool(value):
    return b"\x01" if value else b"\x00"


def encode_int32(value):
    if not (-(2**31) <= value < 2**31):
        raise ValueError(f"Value {value} out of range for int32")
    return struct.pack(">i", value)


def encode_int64(value):
    return struct.pack(">q", value)


def encode_string(value):
    utf8_bytes = value.encode("utf-8")
    return encode_bytes(utf8_bytes)


def encode_bytes(value):
    length = encode_int32(len(value))
    return length + value

# Like encode_bytes but without the length prefix, as hashes have a fixed size
def encode_hash(value):
    return value


def encode_hex_string(value):
    return encode_bytes(bytes.fromhex(value))


def encode_optional(value, encode_fn):
    if value is not None:
        return b"\x01" + encode_fn(value)
    else:
        return b"\x00"


def encode_proto_optional(parent_value, field_name, value, encode_fn):
    if parent_value.HasField(field_name):
        return b"\x01" + encode_fn(value)
    else:
        return b"\x00"


def encode_repeated(values, encode_fn):
    length = encode_int32(len(values))
    encoded_values = b"".join(encode_fn(v) for v in values)
    return length + encoded_values


def sha256(data):
    return hashlib.sha256(data).digest()


def find_seed(
    node_id, node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed]
):
    for node_seed in node_seeds:
        if str(node_seed.node_id) == node_id:
            return node_seed.seed
    return None


def encode_identifier(identifier):
    return (
        encode_string(identifier.package_id)
        + encode_repeated(identifier.module_name.split("."), encode_string)
        + encode_repeated(identifier.entity_name.split("."), encode_string)
    )


def encode_node_id(
    nodes_dict: dict,
    node_seeds: [interactive_submission_service_pb2.DamlTransaction.NodeSeed],
    encode_node_fn,
):
    def encode(node_id):
        node = nodes_dict[node_id]
        return sha256(encode_node_fn(node, nodes_dict, node_seeds))

    return encode


def encode_transaction(transaction, nodes_dict, node_seeds, encode_node_fn):
    version = encode_string(transaction.version)
    roots = encode_repeated(
        transaction.roots, encode_node_id(nodes_dict, node_seeds, encode_node_fn)
    )
    return version + roots


def encode_value(value):
    if value.HasField("unit"):
        return b"\x00"
    elif value.HasField("bool"):
        return b"\x01" + encode_bool(value.bool)
    elif value.HasField("int64"):
        return b"\x02" + encode_int64(value.int64)
    elif value.HasField("numeric"):
        return b"\x03" + encode_string(value.numeric)
    elif value.HasField("timestamp"):
        return b"\x04" + encode_int64(value.timestamp)
    elif value.HasField("date"):
        return b"\x05" + encode_int32(value.date)
    elif value.HasField("party"):
        return b"\x06" + encode_string(value.party)
    elif value.HasField("text"):
        return b"\x07" + encode_string(value.text)
    elif value.HasField("contract_id"):
        return b"\x08" + encode_hex_string(value.contract_id)
    elif value.HasField("optional"):
        return b"\x09" + encode_proto_optional(
            value.optional, "value", value.optional.value, encode_value
        )
    elif value.HasField("list"):
        return b"\x0a" + encode_repeated(value.list.elements, encode_value)
    elif value.HasField("text_map"):
        return b"\x0b" + encode_repeated(value.text_map.entries, encode_text_map_entry)
    elif value.HasField("record"):
        return (
            b"\x0c"
            + encode_proto_optional(
                value.record, "record_id", value.record.record_id, encode_identifier
            )
            + encode_repeated(value.record.fields, encode_record_field)
        )
    elif value.HasField("variant"):
        return (
            b"\x0d"
            + encode_proto_optional(
                value.variant, "variant_id", value.variant.variant_id, encode_identifier
            )
            + encode_string(value.variant.constructor)
            + encode_value(value.variant.value)
        )
    elif value.HasField("enum"):
        return (
            b"\x0e"
            + encode_proto_optional(
                value.enum, "enum_id", value.enum.enum_id, encode_identifier
            )
            + encode_string(value.enum.constructor)
        )
    elif value.HasField("gen_map"):
        return b"\x0f" + encode_repeated(value.gen_map.entries, encode_gen_map_entry)
    raise ValueError("Unsupported value type")


def encode_text_map_entry(entry):
    return encode_string(entry.key) + encode_value(entry.value)


def encode_record_field(field):
    return encode_optional(field.label, encode_string) + encode_value(field.value)


def encode_gen_map_entry(entry):
    return encode_value(entry.key) + encode_value(entry.value)


def create_nodes_dict(prepared_transaction):
    nodes_dict = {}
    for node in prepared_transaction.transaction.nodes:
        nodes_dict[node.node_id] = node
    return nodes_dict