Re: [Bitcoin-development] Malleable booleans

[Peter Todd <pete@petertodd.org>]

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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
0000000000000000147fe2005d7d4490938a7ab96901b8256dcd9d4eac78cb8c

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