[bitcoin-dev] New BIP: Low S values signatures

[Johnson Lau <jl2012@xbt.hk>]

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As discussed in the 11 Aug 2016 IRC meeting (https://bitcoincore.org/en/meetings/2016/08/11/#softfork-to-make-low-s-required), a new BIP with implementation is prepared to make low S value signature as a consensus rule:

https://github.com/jl2012/bips/blob/biplows/bip-lows.mediawiki

https://github.com/bitcoin/bitcoin/pull/8514

The softfork is proposed to be deployed with segwit (BIP141), likely in v0.13.1

The text is copied below

  BIP: ?
  Title: Low S values signatures
  Author: Pieter Wuille <pieter.wuille@gmail.com>
          Johnson Lau <jl2012@xbt.hk>
  Status: Draft
  Type: Standards Track
  Created: 2016-08-16

Abstract

This document specifies proposed changes to the Bitcoin transaction validity rules to restrict signatures to using low S values.

Motivation

ECDSA signatures are inherently malleable as taking the negative of the number S inside (modulo the curve order) does not invalidate it. This is a nuisance malleability vector as any relay node on the network may transform the signature, with no access to the relevant private keys required. For non-segregated witness transactions, this malleability will change the txid and invalidate any unconfirmed child transactions. Although the txid of segregated witness (BIP141) transactions is not third party malleable, this malleability vector will change the wtxid and may reduce the efficiency of compact block relay (BIP152).

To fix this malleability, we require that the S value inside ECDSA signatures is at most the curve order divided by 2 (essentially restricting this value to its lower half range). The value S in signatures must be between 0x1 and 0x7FFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF 5D576E73 57A4501D DFE92F46 681B20A0 (inclusive). If S is too high, simply replace it by S' = 0xFFFFFFFF FFFFFFFF FFFFFFFF FFFFFFFE BAAEDCE6 AF48A03B BFD25E8C D0364141 - S.

Specification

Every signature passed to OP_CHECKSIG, OP_CHECKSIGVERIFY, OP_CHECKMULTISIG, or OP_CHECKMULTISIGVERIFY, to which ECDSA verification is applied, MUST use a S value between 0x1 and 0x7FFFFFFF FFFFFFFF FFFFFFFF FFFFFFFF 5D576E73 57A4501D DFE92F46 681B20A0 (inclusive) with strict DER encoding (see BIP66).

These operators all perform ECDSA verifications on pubkey/signature pairs, iterating from the top of the stack backwards. For each such verification, if the signature does not pass the IsLowDERSignature check, the entire script evaluates to false immediately. If the signature is valid DER with low S value, but does not pass ECDSA verification, opcode execution continues as it used to, causing opcode execution to stop and push false on the stack (but not immediately fail the script) in some cases, which potentially skips further signatures (and thus does not subject them to IsLowDERSignature).

Deployment

This BIP will be deployed by "version bits" BIP9 using the same parameters for BIP141 and BIP143, with the name "segwit" and using bit 1.

For Bitcoin mainnet, the BIP9 starttime will be midnight TBD UTC (Epoch timestamp TBD) and BIP9 timeout will be midnight TBD UTC (Epoch timestamp TBD).

For Bitcoin testnet, the BIP9 starttime will be midnight 1 May 2016 UTC (Epoch timestamp 1462060800) and BIP9 timeout will be midnight 1 May 2017 UTC (Epoch timestamp 1493596800).

Compatibility

The reference client has produced compatible signatures since v0.9.0, and the requirement to have low S value signatures has been enforced as a relay policy by the reference client since v0.11.1. As of August 2016, very few transactions violating the requirement are being added to the chain. In addition, every non-compliant signature can trivially be converted into a compliant one, so there is no loss of functionality by this requirement. This proposal has the added benefit of reducing transaction malleability.

Implementation

An implementation for the reference client is available at https://github.com/bitcoin/bitcoin/pull/8514

Acknowledgements

This document is extracted from the previous BIP62 proposal which had input from various people.

Copyright

This document is placed in the public domain.

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