[jws] check signature length #1004
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We have been notified that certain JWS messages with ES256 signatures come with 63 byte signatures instead of the desired 64, and that jwx successfully verifies these. Upond digging, we have found that PR #65 removed the previous check for the key sizes, way back in 2019. Without this check it is possible for an unpadded R value to pass verification. For example, R with 31 bytes worth of data without padding followed by S with 32 bytes can pass the verification because when generating math.BigInt produces a valid result. For example: Regular Signagure -> | R (32 bytes)... | S (32 bytes)... | Invalid Signature -> | R (31 bytes)... | S (32 bytes)... | The type of signature that will get past is a signature where, both values of R and S are valid but R is not padded at byte 0. Since the signature content must match the correct content either way, it is highly unlikely that this can be used for anything other than adding an extra byte at the end of such a signature (that is, the end user who produced the signature must already have the correct signature anwyays). However, omitting the check _does_ allow otherwise invalid signature to be verified correctly.
Codecov ReportAll modified and coverable lines are covered by tests ✅
Additional details and impacted files@@ Coverage Diff @@
## develop/v2 #1004 +/- ##
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- Coverage 72.29% 72.28% -0.01%
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Files 93 93
Lines 13644 13650 +6
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+ Hits 9864 9867 +3
- Misses 2971 2973 +2
- Partials 809 810 +1
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