fix(core/thp): correct Failure_InvalidProtocol constant message

[romanz]

Otherwise, the host will receive b'\x08\x11' + b'\x00'*18.
The trailing bytes are skipped by trezorlib, since our Python protobuf parser ignores unknown fields:

trezor-firmware/python/src/trezorlib/protobuf.py

Lines 443 to 453 in 53ac49c

	if ftag not in msg_type.FIELDS: # unknown field, skip it
	if wtype == WIRE_TYPE_INT:
	load_uvarint(reader)
	elif wtype == WIRE_TYPE_LENGTH:
	ivalue = load_uvarint(reader)
	if ivalue > MAX_FIELD_SIZE:
	raise ValueError(f"Unknown field {ftag} too large ({ivalue} bytes)")
	reader.readinto(bytearray(ivalue))
	else:
	raise ValueError
	continue

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 53ac49c and 68406c3.

📒 Files selected for processing (2)

• core/src/trezor/wire/thp/interface_context.py
• tests/device_tests/thp/test_basic.py

🚧 Files skipped from review as they are similar to previous changes (1)

• core/src/trezor/wire/thp/interface_context.py

---

Walkthrough

The PR adjusts the THP codec_v1 error response serialization: the failure payload length/type bytes for an InvalidProtocol Failure are changed from 0x14 to 0x02 in the encoded response. No public API signatures were modified and the handler's control flow (construct buffer, memcpy bytes, write packet) remains the same. Tests were updated to use a local expected Failure instance and to assert encoded output matches exactly, preventing trailing bytes.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 1 | ❌ 2

❌ Failed checks (1 warning, 1 inconclusive)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	❓ Inconclusive	The description explains the problem (trailing bytes sent to host) and the technical context, but lacks structured sections matching the template (no Development status, PR setup sections, or QA notes despite PR being open).	Clarify the PR development status (Draft vs. Final) and add a 'Notes for QA' section if testable, or confirm 'Done (no QA)' if not testable, to complete the template structure.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and specifically describes the main change: correcting the Failure_InvalidProtocol constant message size in the THP codec.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch romanz/thp-invalid

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[github-actions]

en main(all)

model	device_test	click_test	persistence_test
T2T1	test(all) main(all)	test(all) main(all)	test(all) main(all)
T3B1	test(all) main(all)	test(all) main(all)	test(all) main(all)
T3T1	test(all) main(all)	test(all) main(all)	test(all) main(all)
T3W1	test(all) main(all)	test(all) main(all)	test(all) main(all)

Latest CI run: 22625299399

[mmilata]

Seems to parse correctly by trezorlib in main. Does this fix interoperability with some other parser?

[romanz]

Yes - @szymonlesisz has found this issue when using a new parser (which didn't skip the trailing zero bytes).

[mmilata]

LGTM, protoscope seems to agree. Will fix this in rust/trezor-thp.

[romanz]

Added a check in tests/device_tests/thp/test_basic.py::test_v1:
https://github.com/trezor/trezor-firmware/compare/53ac49c2f79f4bcb1e32c38d1c109032f710c56a..68406c3b4b9e83407db247718576ee8d7f0313f3