Fix a typo in Bigint dart implementation of modInverse.

Fix a typo in Bigint dart implementation of modInverse.

Fixes #24324

Committed: https://github.com/dart-lang/sdk/commit/9ca5ca6316eeddfb6f79526e49528b27f862e2f2

[regis, 2015-09-14 04:03:25]

regis@google.com changed reviewers:
+ ager@google.com

[regis, 2015-09-14 04:03:26]

TBR

The code path with the typo is only used for modInverse (not for gcd), and only
when the modulus is even, which never really happens in practice. It is quite
painful to find a pair of input numbers that will trigger this path. But the fix
is obvious.

Thanks for finding it.
Regis

[regis, 2015-09-14 04:04:01]

Committed patchset #1 (id:1) manually as
9ca5ca6316eeddfb6f79526e49528b27f862e2f2 (presubmit successful).

[Mads Ager (google), 2015-09-14 06:18:32]

LGTM, should we add a test?

[regis, 2015-09-14 06:25:01]

On 2015/09/14 06:18:32, Mads Ager (google) wrote:
> LGTM, should we add a test?

As I mentioned, it is hard to find a test exercising this path. You need to use
modInverse (not gcd) with an even modulus This is never the case in practice,
since crypto always uses an odd modulus (the reason this bug remained hidden).
Feel free to add one if you have the patience to find one :-)

You would need to backtrack from the typo location back to a pair of inputs.
This is not obvious. The code, however, is very obvious and I am confident that
the fix is good.