[bitcoin-dev] RFC: BIP 322: Generic Signed Message Format

[Karl-Johan Alm <karljohan-alm@garage.co.jp>]

Hi.

[note: BIP number was assigned to PR before this email was sent; I did
not self-assign the BIP number]

Below is a proposal to extend the existing sign/verifymessage format
to a more generalized variant relying on the script verification
mechanism in Bitcoin itself for message signing/verification, based on
the original discussion
(https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-March/015818.html)
.

PR is here: https://github.com/bitcoin/bips/pull/725

A formatted version of this text can be seen here:
https://github.com/kallewoof/bips/blob/bip-generic-signmessage/bip-generic-signmessage.mediawiki

Note: I am not sure how to best deal with CLTV/CSV stuff here, ultimately.

Note 2: I have received suggestions from several people to use a
Bitcoin transaction instead. If someone could explain why this is
beneficial, it would be very helpful. I'm not against it, just feels
like the whole transaction part is unnecessary complexity/overhead.

---
<pre>
  BIP: 322
  Layer: Applications
  Title: Generic Signed Message Format
  Author: Karl-Johan Alm <karljohan-alm@garage.co.jp>
  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
</pre>

== 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 <code>SignatureProof</code> is added, which is a
simple serializable scriptSig & witnessProgram container.

Two actions "Sign" and "Verify" are defined.

=== SignatureProof container ===

{|class="wikitable" style="text-align: center;"
|-
!Type
!Length
!Name
!Comment
|-
|Uint32||4||flags||standard flags (1-to-1 with standard flags in Bitcoin Core)
|-
|VarInt||1-8||msglen||Number of bytes in message string, excluding NUL
termination
|-
|Char*||[msglen]||msg||The message being signed for all subjects,
excluding NUL termination
|-
|Uint8||1||entries||Number of proof entries<ref><strong>Why support
multiple proofs?</strong> In particular with proof of funds, it is
non-trivial to check a large number of individual proofs (one per
UTXO) for duplicates. Software could be written to do so, but it seems
more efficient to build this check into the specification
itself.</ref>
|}

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

{|class="wikitable" style="text-align: center;"
|-
!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 bytes in witness program data
|-
|Uint8*||[witlen]||wit||Witness program
|}

In some cases, the scriptsig may be empty (scriptsiglen=0).

=== Signing ===

The "Sign" action takes as input a scriptPubKey and a message (e.g.
"hello world"). It succeeds or fails.

# FAIL if scriptPubKey already exists in scriptPubKeys set, otherwise
insert it<ref><strong>Why track duplicates?</strong> Because a 3-entry
proof is not proving 3 scriptPubKeys unless they are all distinct, or
unless they are proving different UTXO:s (see Future Extensions)</ref>
# Derive the private key privkey for the scriptPubKey, or FAIL
# Define the message pre-image as the sequence "Bitcoin Message:"
concatenated with the message, encoded in UTF-8 using Normalization
Form Compatibility Decomposition (NFKD)
# Let sighash = sha256(sha256(scriptPubKey || pre-image))
# Generate a signature sig with privkey=privkey, sighash=sighash

Repeat the above operation for each scriptPubKey, retaining the
scriptPubKeys set. As noted, if the same scriptPubKey appears more
than once, the sign operation must fail.

=== Verifying ===

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

# Return ERROR if scriptPubKey already exists in scriptPubKeys set,
otherwise insert it
# If one or more of the standard flags are unknown, return INCONCLUSIVE
# Define the message pre-image as the sequence "Bitcoin Message:"
concatenated with the message, encoded in UTF-8 using Normalization
Form Compatibility Decomposition (NFKD).
# Let sighash = sha256(sha256(scriptPubKey || pre-image))
# Verify Script with flags=standard flags, scriptSig=script sig,
scriptPubKey=scriptPubKey, witness=witness program, and
sighash=sighash
# Return VALID if verify succeeds, otherwise return INVALID

Repeat the above operation for each scriptPubKey, retaining the
scriptPubKeys set. As noted, if the same scriptPubKey appears more
than once, the verify operation must fail with an ERROR.

* 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 VALID if and only if every verification returned VALID.

== Future Extensions ==

=== Proof of Funds ===

The specification can be extended to handle proof of funds in the
following manner:

* Let the message be prefixed with "POF:", followed by a
newline-terminated string<ref><strong>Why not just the UTXO
data?</strong> We want the verifier to be able to challenge the prover
with a custom message to sign, or anyone can reuse the POF proof for a
set of UTXO:s once they have seen it, and the funds have not yet been
spent</ref>, followed by [entries] series of hex-encoded transaction
ID:vout pairs, separated by a single space (" ") character
* Fail if the number of txid:vout pairs is not exactly equal to [entries]
* Retain the message as is for all sighash operations (i.e. all sign
and verify operations should sign and verify the entire list of
UTXO:s)<ref><strong>Why use same sighash?</strong> The prover is
proving that they have a set of UTXO:s at their disposal. Taking a
sub-set of the proofs and turning them into a new proof should not be
valid.</ref>
* Add a verification that the txid/vout is a valid UTXO according to a
synced up Bitcoin node, and that its corresponding scriptPubKey
matches the one given by the proof. Return ERROR if scriptPubKey
mismatch, and SPENT error if spent
* Extend the scriptPubKeys set check to only fail if the same
scriptPubKey and proof-of-funds txid/vout combination is encountered

== Compatibility ==

This specification is not backwards compatible with the legacy
signmessage/verifymessage specification. However, legacy addresses
(1...) may be used in this implementation without any problems.

== Rationale ==

<references/>

== Reference implementation ==

To do.

== Acknowledgements ==

TODO

== References ==

# 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.
---

-Kalle.