On Mon, Oct 13, 2014 at 07:34:16PM -0700, Pieter Wuille wrote: > Hi all, > > while working on a BIP62 implementation I discovered yet another type > of malleability: the interpretation of booleans. > > Any byte array with non-zero bytes in it (ignoring the highest bit of > the last byte, which is the sign bit when interpreting as a number) is > interpreted as true, anything else as false. Other than numbers, > they're not even restricted to 4 bytes. Worse, the code for dealing > with booleans is not very consistent: OP_BOOLAND and OP_BOOLOR first > interpret their arguments as numbers, and then compare them to 0 to > turn them into boolean values. > > This means that scripts that use booleans as inputs will be inherently > malleable. Given that that seems actually useful (passing in booleans > to guide some OP_IF's during execution of several alternatives), I > would like to change BIP62 to also state that interpreted booleans > must be of minimal encoded size (in addition to numbers). > > Any opinions for or against?
I noticed this awhile back myself. More interestingly, I remember
noticing some non-std scripts on mainnet that had opcodes that appeared
to be attempts to solve this issue with variations of the following:
DUP
IF
1 EQUALVERIFY
<do stuff>
ELSE
0 EQUALVERIFY
<do stuff>
ENDIF
I'll have to admit, I decided to keep quiet about it because it's a good
example of how relying on BIP62 for specialty contract applications that
absolutely need to avoid malleability for security reasons is a dubious
idea; it's hard to be sure that we've really gotten every relevant case
correct.
I think a decent argument *for* doing this is that if a script author
fails to properly 'bool-ize' every boolean-using path that can have
non-minimal encodings in normal execution, you can always create a
nVersion=1 transaction manually to spend the output, preventing funds
from getting lost. Meanwhile in the general case of a compenent script
author having the canonical bool testing in every boolean-using opcode
saves a lot of bytes.
--
'peter'[:-1]@petertodd.org
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