script: Require HMAC comparison to be constant time#43773
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We should compare HMAC signatures when validating user-provided signatures in constant time, to prevent leaking timing information proportional to the number of matching bytes. The WebCrypto specification has also updated to require to use constant-time comparison in HMAC signatures. We update our implementation accordingly. Since we are still using the `aws-lc-rs` crate for our HMAC implementation, we use the function `verify_slices_are_equal` provided by `aws_lc_rs::constant_time` to guarantees the comparison is constant-time. Specification Update: w3c/webcrypto@c962bc7 Testing: Existing tests suffice. Signed-off-by: Kingsley Yung <kingsley@kkoyung.dev>
Taym95
reviewed
Mar 30, 2026
| let mac = hmac::sign(&sign_key, message); | ||
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| // Step 2. Return true if mac is equal to signature and false otherwise. | ||
| Ok(mac.as_ref() == signature) |
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we should cover this with WPT test
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Hrm. Is it possible to test that an algorithm is run in constant time via a WPT test?
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I think it is pretty hard to do it as a WPT test. The time difference between constant-time comparison and naive comparison of less than a hundred bytes can be as small as a few nanosecond. Detecting the difference may need fine-tuned device and/or well-designed framework.
mrobinson
approved these changes
Mar 30, 2026
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We should compare HMAC signatures in constant time when validating user-provided signatures, to prevent leaking timing information proportional to the number of matching bytes. The WebCrypto specification has also updated to require to use constant-time comparison in HMAC signatures.
We update our implementation accordingly. Since we are still using the
aws-lc-rscrate for our HMAC implementation, we use the functionverify_slices_are_equalprovided byaws_lc_rs::constant_timeto guarantees the comparison is constant-time.Specification Update: w3c/webcrypto@c962bc7
Testing: Existing tests suffice.