k256+p256: handle identity point in GroupEncoding
#446
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`GroupEncoding` uses a fixed-width `Repr` whereas SEC1 is variable-width with respect to compressed points versus the identity. This change allows 33-bytes of zeroes to be used as the identity. Technically that's not a valid SEC1 encoding: the SEC1 encoding for the identity is 1-byte: 0x00. However, this is the best we can do with a fixed-width encoding. See also: #443 and #444
tarcieri
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Oct 2, 2021
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| EncodedPoint::from_bytes(bytes) | ||
| .map(|point| CtOption::new(point, Choice::from(1))) | ||
| .unwrap_or_else(|_| { | ||
| // SEC1 identity encoding is technically 1-byte 0x00, but the | ||
| // `GroupEncoding` API requires a fixed-width `Repr` | ||
| let is_identity = bytes.ct_eq(&Self::Repr::default()); | ||
| CtOption::new(EncodedPoint::identity(), is_identity) | ||
| }) |
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Technically this is a branch, but it's one which only occurs with respect to the SEC1 tag.
The important part to ensure is actually constant-time is the point decoding, which happens regardless of which branch is taken through the initial SEC1 decoding.
Codecov Report
@@ Coverage Diff @@
## master #446 +/- ##
==========================================
+ Coverage 64.58% 65.03% +0.45%
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Files 28 28
Lines 3586 3598 +12
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+ Hits 2316 2340 +24
+ Misses 1270 1258 -12
Continue to review full report at Codecov.
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GroupEncodinguses a fixed-widthReprwhereas SEC1 is variable-width with respect to compressed points versus the identity.This change allows 33-bytes of zeroes to be used as the identity. Technically that's not a valid SEC1 encoding: the SEC1 encoding for the identity is 1-byte: 0x00. However, this is the best we can do with a fixed-width encoding.
See also: #443 and #444