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Arkade extends the standard Partially Signed Bitcoin Transaction (PSBT) format (BIP 174) to include custom fields required for its advanced contract logic and coordination mechanisms. These fields allow Arkade to encode metadata such as taproot trees, relative timelocks, multi-signer coordination, and custom witness data; features that are not supported by standard PSBTs. Arkade’s PSBT extensions build on BIP 174’s forward-compatible design. Because standard PSBT parsers simply ignore but preserve these unknown fields, Arkade transactions remain fully interoperable while adding richer contract semantics and coordination logic.

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Input Fields

Arkade leverages the unknown field mechanism by introducing its own field namespace under key type 222 (0xDE), enabling protocol-specific functionality while remaining interoperable with standard tooling. Each field appears in the PSBT input map under key type 0xDE, following the PSBT unknown-field encoding rules.
NameTypeKeyPurposeValue Format
taptree0xDE0x74617074726565 (“taptree”)A list of tapscript leavesSequence of tapscript leaves (depth + version + script).
expiry0xDE0x657870697279 (“expiry”)Specifies relative timelock (CSV) for input spendingBIP68 sequence encoding
cosigner0xDE0x636F7369676E6572 (“cosigner”) + <uint32_key_index>Identifies indexed Musig2 cosigner public keys33-byte compressed public key
condition0xDE0x636F6E646974696F6E (“condition”)Adds custom witness elements for script executionraw witness bytes

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Field Details

Field: taptree
Purpose: Embeds a Taproot script tree for complex input spending conditions.
Key Format: 0xDE + "taptree" (7 bytes)
Value Format: TapTree encoding (variable length)The TapTree is encoded as a sequence of tapscript leaves, where each leaf contains:
  • Depth (1 byte): Always 1 for single-level trees
  • Leaf version (1 byte): Always tapscript version (0xC0)
  • Script length (compact size): Length of the script in bytes
  • Script bytes: The actual tapscript
Example:
Key: 0xDE 0x74 0x61 0x70 0x74 0x72 0x65 0x65 ("taptree")
Value: [1][0xC0][script_len][script_bytes][1][0xC0][script_len][script_bytes]...
Field: expiry
Purpose: Defines a relative locktime (CSV) condition for the input.
Key Format: 0xDE + "expiry" (6 bytes)
Value Format: BIP68 sequence encoding (1–5 bytes, little-endian)Example:
Key: 0xDE 0x65 0x78 0x70 0x69 0x72 0x79 ("expiry")
Value: [sequence_bytes] (e.g., 0x80 0x96 0x98 for 10000 blocks)
Field: cosigner
Purpose: Specifies indexed Musig2 cosigner public keys.
Key Format: 0xDE + "cosigner" + <uint32_index> (11 bytes)
Value Format: 33-byte compressed public keyIndex is a 4-byte big-endian integer appended to the base key. This allows multiple cosigner fields per input, each with a unique index for proper ordering.Example:
Key: 0xDE 0x63 0x6F 0x73 0x69 0x67 0x6E 0x65 0x72 0x00 0x00 0x00 0x01 ("cosigner" + index 1)
Value: [33-byte compressed public key]
Field: condition
Purpose: Provides additional witness elements for custom script execution.
Key Format: 0xDE + "condition" (9 bytes)
Value Format: PSBT witness encoding (compact size + data)Each witness includes:
  • Number of witness elements (compact size)
  • For each element: length (compact size) + data
Example:
Key: 0xDE 0x63 0x6F 0x6E 0x64 0x69 0x74 0x69 0x6F 0x6E ("condition")
Value: [witness_encoding]

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Reference Implementations

The reference implementations are available in the arkd codebase:

PSBT fields

Core field definitions and encoding/decoding

TapTree

TapTree encoding/decoding

Expiry

Relative Locktime handling
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