Fix truncated Karatsuba carry#1305
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Note: this was originally privately disclosed by @tob-joe as part of his work for Trail of Bits on the Patch the Planet project in collaboration with OpenAI, having used Codex to discover it. I am propagating it into a public PR shortly before a release. Fix `karatsuba::wrapping_mul` when a truncated wide output cuts through the initial `z0 = x0 * y0` placement window. The previous path copied or added only the available prefix of `z0`, then returned no carry for omitted high `z0` limbs. The fix routes that short assignment window through `schoolbook::wrapping_mul_add(...)`, clearing it first for overwrite mode, so the returned carry matches the truncated schoolbook operation.
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## master #1305 +/- ##
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+ Coverage 90.92% 90.93% +0.01%
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Files 189 189
Lines 22493 22523 +30
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+ Hits 20451 20481 +30
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This was referenced Jun 22, 2026
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## Added
- `BoxedUint::{from_be_slice_truncated, from_le_slice_truncated}` (#1266)
- `Encoding::{from_bytes, to_bytes}` (#1297)
- `Uint::{from_be_slice_truncated, from_le_slice_truncated}` (#1299)
- `Limb::{from_*_bytes, from_*_slice_truncated, to_*_bytes}` as
`const fn`s (#1300)
- `Encoding::{from_be_slice_truncated, from_le_slice_truncated}` trait
methods (#1301)
- `ArrayEncoding::{from_byte_array, to_byte_array}` (#1302)
## Fixed
- Preserve `NonZero` and `Odd` invariants in `Zeroize` impls (#1287)
- `bitlen` performance (#1298)
- `floor_sqrt` regression (#1304)
- Truncated Karatsuba carry (#1305)
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Note: this was originally privately disclosed by @tob-joe as part of his work for Trail of Bits on the Patch the Planet project in collaboration with OpenAI, having used Codex to discover it. I am propagating it into a public PR shortly before a release.
Fix
karatsuba::wrapping_mulwhen a truncated wide output cuts through the initialz0 = x0 * y0placement window.The previous path copied or added only the available prefix of
z0, then returned no carry for omitted highz0limbs.The fix routes that short assignment window through
schoolbook::wrapping_mul_add(...), clearing it first for overwrite mode, so the returned carry matches the truncated schoolbook operation.