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Pad z value to proper size after P-521 scalar multiplication. #245
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Fixes issue #228. After calling ScalarMult for P-521, the output can sometimes be 65 bytes long instead of 66 bytes. This happens when the first bit of the computed value is zero. Calling z.Bytes() on the big integer will then omit the leading zero, giving us a 65-byte value. This subsequently causes the shared secret computation to be incorrect as the input into the KDF function should always be 66 bytes which is the full length for a P-521 coordinate value.
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Will cherry-pick this into |
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| octSize := dSize(priv.Curve) | ||
| if len(zBytes) != octSize { | ||
| zBytes = append(bytes.Repeat([]byte{0}, octSize-len(zBytes)), zBytes...) |
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do we care about any timing info leak here?
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Possibly, though I’m not sure how we’d fix it, and regardless P-521 in Go isn’t constant time anyway (only P-256 is afaik).
alokmenghrajani
approved these changes
Jun 1, 2019
mbyczkowski
approved these changes
Jun 1, 2019
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| octSize := dSize(priv.Curve) |
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I think the comment from #228 (comment) would be great here
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Fixes issue #228. After calling ScalarMult for P-521, the output can sometimes be 65 bytes long instead of 66 bytes. This happens when the first bit of the computed value is zero. Calling z.Bytes() on the big integer will then omit the leading zero, giving us a 65-byte value.
This subsequently causes the shared secret computation to be incorrect as the input into the KDF function should always be 66 bytes which is the full length for a P-521 coordinate value.