fix: JA4_a ALPN handling (non-alphanumeric + single-byte)#9
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Pull request overview
This PR aligns JA4_a ALPN handling with the FoxIO JA4 spec by (1) correctly skipping GREASE ALPNs, (2) handling 1-byte ALPNs as valid (non-GREASE), and (3) emitting hex-derived characters when ALPN boundary bytes are not ASCII alphanumeric.
Changes:
- Skip GREASE ALPN entries explicitly and accept 1-byte ALPNs as valid candidates.
- When the first/last ALPN bytes are non-ASCII-alphanumeric, emit the first/last hex characters without encoding the whole string.
- Add/update tests covering non-alphanumeric ALPN bytes and single-character ALPN behavior.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.
| File | Description |
|---|---|
| ja4plus.go | Updates ALPN selection and encoding logic to match JA4 spec edge cases; adds isASCIIAlphanumeric. |
| ja4plus_test.go | Adds/updates test cases validating the new ALPN edge-case behavior. |
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totallyunknown
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Jul 6, 2026
Per the JA4 spec, when the first or last byte of the first ALPN value is not ASCII alphanumeric, JA4_a uses the first and last characters of the value's hex representation instead of the raw bytes. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
A 1-byte ALPN protocol cannot be a GREASE value, but the previous grease check discarded any proto shorter than 2 bytes, falling back to "00". Such protocols are now used, matching the JA4 spec. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Fixes two JA4_a ALPN deviations from the FoxIO JA4 spec.
value isn't ASCII alphanumeric, use the first/last characters of the
value's hex representation instead of emitting the raw bytes.
used (with the single char as both first and last) instead of falling
back to "00".
Tests added/updated for both cases; benchmark unchanged (1 alloc, ~203ns/op).