Bitcoin Message Signing

Two specifications:

BIP137 (Hourglass) — legacy. Works only for P2PKH (uncompressed or compressed). Output: 65-byte recoverable ECDSA + Base64.
BIP322 (modern) — works for any output type (P2PKH, P2WPKH, P2WSH, P2TR, even script-only). Output: a virtual PSBT-like signature.

BIP137 (legacy)

prefix      = "Bitcoin Signed Message:\n"
preamble    = varint(len(prefix)) || prefix || varint(len(msg)) || msg
hash        = SHA256d(preamble)
signature   = recoverable_ecdsa_sign(privkey, hash)   # 65 bytes
encoded     = base64(signature)

Verify: recover pubkey from signature, derive P2PKH address, compare.

Limitations:

Only P2PKH. Cannot prove ownership of a SegWit / Taproot address.
Wallets don't always agree on the prefix (some include trailing newline, others don't).
Recoverable ECDSA implies pubkey isn't directly transmitted.

BIP322 (modern)

Generalizes signing by constructing a virtual transaction:

to_spend = transaction with:
  version=0, locktime=0,
  vin = [{ prevout: 0xffffffff..., scriptSig: OP_0 PUSH msg_hash, sequence: 0 }],
  vout = [{ value: 0, scriptPubKey: <message-target scriptPubKey> }]

to_sign = transaction with:
  version=0, locktime=0,
  vin = [{ prevout: txid_of_to_spend, vout: 0, sequence: 0 }],
  vout = [{ value: 0, scriptPubKey: OP_RETURN }]

Then sign to_sign as a normal transaction spending to_spend's output. The signature can be:

A full BIP322 signature with witness data, OR
A "simple" form where only the witness/scriptSig is encoded.

Address types supported

P2PKH, P2SH-anything, P2WPKH, P2WSH, P2TR.
Multi-key: produce multiple sigs across signers and combine into one BIP322 witness.
Even script-only addresses (no key) can produce a "proof of knowledge" of preimage.

Encoded forms

BIP322 simple — Base64 of a witness stack only. Suitable for P2WPKH/P2WSH/P2TR.
BIP322 full — Base64 of to_sign PSBT-like blob. Required for P2SH or anything with non-witness data.

Verification flow

def verify_bip322(address, message, signature_b64):
    spk = address_to_scriptPubKey(address)
    msg_hash = bip322_hash(message)
    to_spend = build_to_spend(spk, msg_hash)
    to_sign  = build_to_sign(to_spend)
    apply_signature(to_sign, signature_b64)
    return script_executes_successfully(to_sign, vin[0])

Common use cases

Exchange withdraw verification — "prove you own the destination before we send".
Federated/KYC-free login — sign a server-issued nonce with the Bitcoin address you want to associate with the account.
Off-chain attestation — sign a public commitment.
DLC oracle attestation — adapter signatures + BIP322.

Library support

bitcoinjs-lib (since 6.x) — BIP322 sign/verify.
python-bitcoinlib — BIP137 native, BIP322 via plugin.
rust-bitcoin — BIP322 in signed-message crate.
bitcoinj — BIP137; BIP322 incomplete.

Common bugs

Mixing BIP137 prefix conventions across wallets — verify with the same library/wallet that signed where possible.
Trying to BIP137-sign a SegWit address by deriving an arbitrary P2PKH from the same key → semantically wrong; use BIP322 instead.
For Taproot: the witness stack must use SIGHASH_DEFAULT to be byte-identical to a real Taproot spend; some libraries default to SIGHASH_ALL (0x01) which encodes a different sighash.

bitcoin-message-signing