From b196b685c9089b74fd4ff3d9a28ea847ab36179b Mon Sep 17 00:00:00 2001 From: Gregory Maxwell Date: Tue, 6 Oct 2015 03:19:12 +0000 Subject: [PATCH] Test LowS in standardness, removes nuisance malleability vector. MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This adds SCRIPT_VERIFY_LOW_S to STANDARD_SCRIPT_VERIFY_FLAGS which will make the node require the canonical 'low-s' encoding for ECDSA signatures when relaying or mining. Consensus behavior is unchanged. The rational is explained in a81cd96805ce6b65cca3a40ebbd3b2eb428abb7b: Absent this kind of test ECDSA is not a strong signature as given a valid signature {r, s} both that value and {r, -s mod n} are valid. These two encodings have different hashes allowing third parties a vector to change users txids. These attacks are avoided by picking a particular form as canonical and rejecting the other form(s); in the of the LOW_S rule, the smaller of the two possible S values is used. If widely deployed this change would eliminate the last remaining known vector for nuisance malleability on boring SIGHASH_ALL p2pkh transactions. On the down-side it will block most transactions made by sufficiently out of date software. Unlike the other avenues to change txids on boring transactions this one was randomly violated by all deployed bitcoin software prior to its discovery. So, while other malleability vectors where made non-standard as soon as they were discovered, this one has remained permitted. Even BIP62 did not propose applying this rule to old version transactions, but conforming implementations have become much more common since BIP62 was initially written. Bitcoin Core has produced compatible signatures since a28fb70e in September 2013, but this didn't make it into a release until 0.9 in March 2014; Bitcoinj has done so for a similar span of time. Bitcoinjs and electrum have been more recently updated. This does not replace the need for BIP62 or similar, as miners can still cooperate to break transactions. Nor does it replace the need for wallet software to handle malleability sanely[1]. This only eliminates the cheap and irritating DOS attack. [1] On the Malleability of Bitcoin Transactions Marcin Andrychowicz, Stefan Dziembowski, Daniel Malinowski, Łukasz Mazurek http://fc15.ifca.ai/preproceedings/bitcoin/paper_9.pdf --- src/policy/policy.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/src/policy/policy.h b/src/policy/policy.h index 1551aecde..0ea0d435a 100644 --- a/src/policy/policy.h +++ b/src/policy/policy.h @@ -37,7 +37,8 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY SCRIPT_VERIFY_NULLDUMMY | SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS | SCRIPT_VERIFY_CLEANSTACK | - SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY; + SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY | + SCRIPT_VERIFY_LOW_S; /** For convenience, standard but not mandatory verify flags. */ static const unsigned int STANDARD_NOT_MANDATORY_VERIFY_FLAGS = STANDARD_SCRIPT_VERIFY_FLAGS & ~MANDATORY_SCRIPT_VERIFY_FLAGS;