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trezorctl Bitcoin transaction JSON format

Since version 0.11.2, trezorctl allows fully offline signing of Bitcoin and Bitcoin-like altcoin transactions encoded in a custom JSON structure. Starting with version 0.11.6, this is the only supported format for signing.

Structure

The structure of the JSON matches the shape of the relevant protobuf messages. See file messages-bitcoin.proto for up-to-date structure.

The root is an object with the following attributes:

  • coin_name: string representing the coin name as listed in coin defs. If missing, "Bitcoin" is used.
  • inputs: array of TxInputType objects. Must be present.
  • outputs: array of TxOutputType objects. Must be present.
  • details: object whose keys correspond to metadata on the SignTx type. Can be omitted.
  • prev_txes: object whose keys are hex-encoded transaction hashes, and values are objects of type TransactionType. When signing a transaction with non-SegWit inputs, each previous transaction must have an entry in prev_txes. With pure SegWit transactions, this field can be omitted.

See definition of the respective object types in messages-bitcoin.proto for descriptions of individual fields.

Please note that the optional keyword in the protobuf definition does not indicate that the field can be omitted, nor does the default extension mean that the default value will be used if missing.

Derivation paths

A derivation path in the field address_n is encoded as an array of numbers according to the BIP-32 specification. Use trezorlib.tools.parse_path to convert a string derivation path to the corresponding array.

Inputs

enum InputScriptType {
    SPENDADDRESS = 0;       // standard P2PKH address
    SPENDMULTISIG = 1;      // P2SH multisig address
    EXTERNAL = 2;           // reserved for external inputs (coinjoin)
    SPENDWITNESS = 3;       // native SegWit
    SPENDP2SHWITNESS = 4;   // SegWit over P2SH (backward compatible)
}

message TxInputType {
    repeated uint32 address_n = 1;   // BIP-32 path to derive the key from master node
    required bytes prev_hash = 2;    // hash of previous transaction output to spend by this input
    required uint32 prev_index = 3;  // index of previous output to spend
    optional bytes script_sig = 4;   // script signature, unset for tx to sign
    optional uint32 sequence = 5;    // sequence (default=0xffffffff)
    optional InputScriptType script_type = 6   ;    // defines template of input script
    optional MultisigRedeemScriptType multisig = 7; // Filled if input is going to spend multisig tx
    optional uint64 amount = 8;                     // amount of previous transaction output
    optional uint32 decred_tree = 9;                // only for Decred
    optional uint32 decred_script_version = 10;     // only for Decred
}

Each input must have a derivation path (address_n), prev_hash and prev_index refering to the output being spent, sequence number, script_type corresponding to the desired signature type, and amount.

The field script_sig must not be set.

The field multisig can be used for multisig inputs. Documenting the multisig structure is TBD. With regular inputs, multisig must not be set.

decred fields must only be set when relevant to your currency.

Outputs

enum OutputScriptType {
    PAYTOADDRESS = 0;       // string address output; change is a P2PKH address
    PAYTOMULTISIG = 2;      // change output is a multisig address
    PAYTOOPRETURN = 3;      // op_return
    PAYTOWITNESS = 4;       // change output is native SegWit
    PAYTOP2SHWITNESS = 5;   // change output is SegWit over P2SH
}

message TxOutputType {
    optional string address = 1;    // destination address in Base58 encoding
    repeated uint32 address_n = 2;  // derivation path for change address
    required uint64 amount = 3;     // amount to spend in satoshis
    required OutputScriptType script_type = 4;      // output script type
    optional MultisigRedeemScriptType multisig = 5; // multisig output definition
    optional bytes op_return_data = 6;              // defines op_return data
    optional uint32 decred_script_version = 7;      // only for Decred

All outputs must have an amount and a script_type.

For normal (non-change) outputs, the field address must be set to an address string, and the script_type must be set to "PAYTOADDRESS". address_n must not be set.

For outputs returning change, address must not be set, and address_n must be a derivation path of the desired change address. script_type indicates the desired address type of the change output.

For OP_RETURN outputs, script_type must be set to "PAYTOOPRETURN" and op_return_data must be filled appropriately. address_n and address must not be set.

decred fields must only be set when relevant to your currency.

Transaction metadata

The following is a shortened definition of the SignTx protobuf message, containing all possible fields that are accepted in the details object.

All fields are optional unless required by your currency.

message SignTx {
    optional uint32 version = 4;           // transaction version
    optional uint32 lock_time = 5;         // transaction lock_time
    optional uint32 expiry = 6;            // only for Decred and Zcash
    optional uint32 version_group_id = 8;  // only for Zcash, nVersionGroupId when overwintered is set
    optional uint32 timestamp = 9;         // only for Peercoin, transaction timestamp
    optional uint32 branch_id = 10;        // only for Zcash, BRANCH_ID when overwintered is set
}

Previous transactions

For inputs that do not use BIP-143 (SegWit) signing, each input transaction must have an entry in the prev_txes object. The following object definitions are used:

message TxInputType {
    required bytes prev_hash = 2;     // hash of previous transaction output to spend by this input
    required uint32 prev_index = 3;   // index of previous output to spend
    optional bytes script_sig = 4;    // script signature, unset for tx to sign
    optional uint32 sequence = 5;     // sequence (default=0xffffffff)
    optional uint32 decred_tree = 9;  // only for Decred
}

message TxOutputBinType {
    required uint64 amount = 1;
    required bytes script_pubkey = 2;
    optional uint32 decred_script_version = 3;  // only for Decred
}

message TransactionType {
    optional uint32 version = 1;
    repeated TxInputType inputs = 2;
    repeated TxOutputBinType bin_outputs = 3;
    optional uint32 lock_time = 4;
    optional bytes extra_data = 8;          // only for Zcash
    optional uint32 expiry = 10;            // only for Decred and Zcash
    optional bool overwintered = 11;        // only for Zcash
    optional uint32 version_group_id = 12;  // only for Zcash, nVersionGroupId when overwintered is set
    optional uint32 timestamp = 13;         // only for Peercoin, transaction timestamp
    optional uint32 branch_id = 14;         // only for Zcash, BRANCH_ID when overwintered is set
}

Encoding

Object types are encoded by a variant of proto3 JSON mapping. The following notable differences exist:

  1. due to the fact that Trezor protocol uses proto2, the logic for omitted fields is different. If a value is missing or null in JSON, it is considered unset for the corresponding protobuf.
  2. proto3 JSON mapping encodes bytes as Base64. The transaction format encodes them as hexadecimal strings. This will be changed in a future revision, but the hex strings will still be understood.
  3. Field names are expected in snake_case, identical to the protobuf definition. In the future, support for camelCase field names will be added.

Otherwise the encoding is identical:

  • numeric fields (uint32, uint64) are encoded as JSON numbers
  • bool fields are encoded as JSON booleans (true, false)
  • string fields are encoded as JSON strings
  • bytes fields are encoded as JSON strings with hex representation of the bytes content
  • repeated fields are JSON arrays of the inner type
  • enum fields can be either a JSON number of the value, or a JSON string of the name
  • nested objects are JSON objects

Example

The JSON below encodes a transaction with the following inputs:

And the following outputs:

  • 12 345 sat to address 3DDEgt7quAq7XqoG6PjVXi1eeAea4rfWck
  • 562 825 sat to a P2SH-SegWit change address at derivation path m/49'/0'/0'/1/99
  • fee of 10 000 sat

(Note that Trezor does not support change addresses when mixing input types. The example is designed purely to showcase the JSON structure. Usually, all inputs should have the same script_type.)

Transaction version is 2, other metadata is not set.

{
  "coin_name": "Bitcoin",
  "details": {
    "version": 2
  },
  "inputs": [
    {
      "address_n": [
        2147483692,
        2147483648,
        2147483648,
        0,
        282
      ],
      "amount": 85170,
      "prev_hash": "e9cec1644db8fa95fe639a9b503a63ea587d2f4e480d3847703e3ec73adf6b5a",
      "prev_index": 0,
      "script_type": "SPENDADDRESS",
      "sequence": 4294967293
    },
    {
      "address_n": [
        2147483697,
        2147483648,
        2147483648,
        0,
        55
      ],
      "amount": 500000,
      "prev_hash": "1f545c0ca1f2c055e199c70457025c1e393edd013a274a976187115a5c601155",
      "prev_index": 0,
      "script_type": "SPENDP2SHWITNESS",
      "sequence": 4294967293
    }
  ],
  "outputs": [
    {
      "address": "3DDEgt7quAq7XqoG6PjVXi1eeAea4rfWck",
      "amount": 12345,
      "script_type": "PAYTOADDRESS"
    },
    {
      "address_n": [
        2147483697,
        2147483648,
        2147483648,
        1,
        99
      ],
      "amount": 562825,
      "script_type": "PAYTOP2SHWITNESS"
    }
  ],
  "prev_txes": {
    "e9cec1644db8fa95fe639a9b503a63ea587d2f4e480d3847703e3ec73adf6b5a": {
      "bin_outputs": [
        {
          "amount": 85170,
          "script_pubkey": "76a914c4b4272ca6d3b069dcf7afdda172a7dae677d4c988ac"
        },
        {
          "amount": 2375277,
          "script_pubkey": "a914115125511fa9f301ecdda8bb73401644c260c61b87"
        }
      ],
      "inputs": [
        {
          "prev_hash": "59ef8b5633c2a8bf0a21edcbc4b9f271572061f81d42b366fe3b8bc0ec68014e",
          "prev_index": 1,
          "script_sig": "1600149043ed42ab198d95067d8760c247f164c4933f3f",
          "sequence": 4294967295
        }
      ],
      "lock_time": 0,
      "version": 1
    }
  }
}