| commit | 31ffc1de13504ebf03125231ddc336dbc6cb2270 | [log] [tgz] |
|---|---|---|
| author | Rupert Swarbrick <rswarbrick@lowrisc.org> | Fri Oct 02 10:13:55 2020 +0100 |
| committer | Rupert Swarbrick <rswarbrick@gmail.com> | Fri Oct 02 16:53:26 2020 +0100 |
| tree | 28aadc35923bfe1ffcd812fb8712e61145f9cf6d | |
| parent | 342e0d7afe71eb875991c94962e080b3feebfbeb [diff] |
[otbn] Fix possible overflow in BN.SUBM This was triggered by comments on commit 840e329 (a similar fix for BN.ADDM). As specced before, you'd get an overflow if wrs1 = 0 and wrs2 > 2**255 because the result would have a zero top bit when truncated to 256 bits. Since we're not using the borrow at all here, I've changed the spec (and ISS code) to be more explicit, showing the subtraction and testing whether the result is less than zero. Note that the detailed behaviour depends on the fact that our negative numbers are 2's-complement (in the case that the intermediate result is less than -MOD), so I've made sure to say so explicitly in the spec change. Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
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