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  BIP: 322
  Layer: Applications
  Title: Generic Signed Message Format
  Author: Karl-Johan Alm <[email protected]>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-0322
  Status: Draft
  Type: Standards Track
  Created: 2018-09-10
  License: CC0-1.0

Table of Contents

Abstract

A standard for interoperable generic signed messages based on the Bitcoin Script format.

Motivation

The current message signing standard only works for P2PKH (1...) addresses. By extending it to use a Bitcoin Script based approach, it could be made more generic without causing a too big burden on implementers, who most likely have access to Bitcoin Script interpreters already.

Specification

A new structure SignatureProof is added, which is a simple serializable scriptSig & witness container.

Two actions "Sign" and "Verify" are defined along with one purpose, "SignMessage", with the ability to expand in the future to add a potential "ProveFunds" purpose.

SignatureProof container

Type Length Name Comment
Uint32 4 flags standard flags (1-to-1 with standard flags in Bitcoin Core)
Uint8 1 entries number of proof entries[1]

The above is followed by [entries] number of signature entries:

Type Length Name Comment
VarInt 1-8 scriptsiglen Number of bytes in scriptSig data
Uint8* [scriptsiglen] scriptsig ScriptSig data
VarInt 1-8 witlen Number of entries in witness stack
Uint8[]* [witlen] wit Witness stack, as [witlen] uint8* vectors, each one prepended with a varint of its size

In some cases, the scriptsig or wit may be empty. If both are empty, the proof is incomplete.

Result Codes

A verification call will return a result code according to the table below.

Code Description
INCOMPLETE One or several of the given challenges had an empty proof. The prover may need some other entity to complete the proof.
INCONCLUSIVE One or several of the given proofs used unknown opcodes or the scriptPubKey had an unknown witness version, perhaps due to the verifying node being outdated.
VALID All proofs were deemed valid.
INVALID One or more of the given proofs were invalid
ERROR An error was encountered

Signing and Verifying

If the challenge consists of a single address and the address is in the P2PK(H) (legacy) format, sign using the legacy format (further information below). Otherwise continue as stated below.

Let there be an empty set inputs which is populated and tested at each call to one of the actions below.

Purpose: SignMessage

The "SignMessage" purpose generates a sighash based on a scriptPubKey and a message. It emits a VALID verification result code unless otherwise stated.

  1. Return INVALID if scriptPubKey already exists in inputs set, otherwise insert it[2]
  2. Define the message pre-image as the sequence "Bitcoin Message:" concatenated with the message, encoded in UTF-8 using Normalization Form Compatibility Decomposition (NFKD)
  3. Let sighash = sha256(sha256(scriptPubKey || pre-image))
A private key may be used directly to sign a message. In this case, its P2WPKH bech32 address shall be derived, and used as the input.

Action: Sign

The "Sign" action takes as input a purpose. It returns a signature or fails.

  1. Obtain the sighash and scriptPubKey from the purpose; FAIL if not VALID
  2. Derive the private key privkey for the scriptPubKey; FAIL if not VALID
  3. Generate and return a signature sig with privkey=privkey, sighash=sighash
The resulting signature proof should be encoded using base64 encoding.

Action: Verify

The "Verify" action takes as input a standard flags value, a script sig, an optional witness, and a purpose. It emits one of INCONCLUSIVE, VALID, INVALID, or ERROR.

  1. Obtain the sighash and scriptPubKey from the purpose; pass on result code if not VALID
  2. If one or more of the standard flags are unknown, return INCONCLUSIVE
  3. Verify Script with flags=standard flags, scriptSig=script sig, scriptPubKey=scriptPubKey, witness=witness, and sighash=sighash
  4. Return VALID if verify succeeds, otherwise return INVALID

Multiple Proofs

When more than one proof is created or verified, repeat the operation for each proof, retaining the inputs set. As noted, if the same input appears more than once, the operation must fail accordingly.

Note that the order of the entries in the proof must match the order of the entries given by the verifier.

  • If any of the proofs are empty during a verification process, skip the verification and set the INCOMPLETE flag
  • If a verification call returns ERROR or INVALID, return ERROR or INVALID immediately, ignoring as yet unverified entries
  • After all verifications complete,
    • return INCONCLUSIVE if any verification call returned INCONCLUSIVE
    • return INCOMPLETE if the INCOMPLETE flag is set
    • return VALID

Legacy format

The legacy format is restricted to the legacy P2PK(H) address format, and restricted to one single challenge (address).

Any other input (e.g. multiple addresses, or non-P2PK(H) address format(s)) must be signed using the new format described above.

Signing

Given the P2PK(H) address a and the message m:

  1. let p be the pubkey-hash contained in a
  2. let x be the private key associated with p
  3. let digest be SHA56d("Bitcoin Signed Message:\n"||m)
  4. create a compact signature sig (aka "recoverable ECDSA signature") using x on digest
The resulting proof is sig, serialized using the base64 encoding.

Verifying

Given the P2PK(H) address a, the message m, and the compact signature sig:

  1. let p be the pubkey-hash contained in a
  2. let digest be SHA56d("Bitcoin Signed Message:\n"||m)
  3. attempt pubkey recovery for digest using the signature sig and store the resulting pubkey into Q
    1. fail verification if pubkey recovery above fails
  4. let q be the pubkey-hash of Q
  5. if p == q, the proof is valid, otherwise it is invalid

Compatibility

This specification is backwards compatible with the legacy signmessage/verifymessage specification through the special case as described above.

Rationale

  1. ^ Why support multiple proofs? It is non-trivial to check a large number of individual proofs for duplicates. Software could be written to do so, but it seems more efficient to build this check into the specification itself.
  2. ^ Why track duplicates? Because a 3-entry proof is not proving 3 entries unless they are all distinct

Reference implementation

  1. Pull request to Bitcoin Core: bitcoin/bitcoin#16440

Acknowledgements

Thanks to David Harding, Jim Posen, Kalle Rosenbaum, Pieter Wuille, and many others for their feedback on the specification.

References

  1. Original mailing list thread: https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-March/015818.html

Copyright

This document is licensed under the Creative Commons CC0 1.0 Universal license.