fix(it): tamper the decoded GCM tag, not a base64 padding bit

[cuioss-oliver]

Problem

BffCookieStatelessnessIT.thePeerRefusesACookieItCannotUnseal failed post-merge on main (341ab9e, run 30334562679):

BffCookieStatelessnessIT.thePeerRefusesACookieItCannotUnseal:138 1 expectation failed.
Expected status code <401> but was <200>.

This is a defect in the test, not in the gateway. The peer was right to answer 200.

Root cause

The test broke the sealed cookie's authentication tag by flipping the final base64url character:

char last = sealed.charAt(sealed.length() - 1);
return sealed.substring(0, sealed.length() - 1) + (last == 'A' ? 'B' : 'A');

That is not reliably a mutation. The sealed payload is version(1) || key-id(1) || nonce(12) || ciphertext || tag(16). When its byte length is not a multiple of three, the final base64url character carries 2 or 4 unused padding bits, and Base64.getUrlDecoder() discards them without validating. 'A' (0b000000) → 'B' (0b000001) flips only the lowest bit — a padding bit in that case — so the value re-decodes to the identical ciphertext and tag. The GCM tag still authenticates, the peer unseals the session correctly, and the route returns 200.

Verified directly:

len(bytes)=32 len%3=2
encoded  = AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHgA
tampered = AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHgB   (last 'A' -> 'B')
decodes to IDENTICAL bytes? true

The no-op requires the last character to be 'A' and the sealed length to be a non-multiple of 3 — roughly 4% of runs, since the ciphertext length tracks the JWT size and varies per login. That is why the suite passed on the PR head (ba3edbc) and failed on main shortly after.

Fix

Mutate a byte of the decoded 16-byte GCM tag instead:

byte[] raw = Base64.getUrlDecoder().decode(sealed);
raw[raw.length - 1] ^= 0x01;
return Base64.getUrlEncoder().withoutPadding().encodeToString(raw);

Always a real tag mutation, so the probabilistic branch is gone entirely. The Javadoc records the padding-bit trap so the original shape is not reintroduced.

Verification

• verify -Ppre-commit green
• test-compile -pl integration-tests -am green
• Integration Tests on this PR are the authoritative check — the same job that caught the failure

Not in scope

Performance Benchmark is also red on main, but it is pre-existing and unrelated: the same run-k6-passthrough-relay-benchmark checks, http_req_failed threshold crossing (exit 99) fails on PR #119 (a workflow-version bump) and on #117 / #116 before it. benchmarks/** is another plan's surface; reported, not touched here.

🤖 Generated with Claude Code

https://claude.ai/code/session_01G99PK9iS9vgHvttFZ7ueir

Summary by CodeRabbit

• Tests
  • Improved sealed-cookie tampering coverage by mutating the decoded authentication data directly.
  • Increased test reliability by avoiding false positives caused by Base64 encoding details.

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ℹ️ Recent review info

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Run ID: 2e00d8fb-8cf1-436c-a2ab-9e28c8a1d231

📥 Commits

Reviewing files that changed from the base of the PR and between 341ab9e and ff5bad7.

📒 Files selected for processing (1)

• integration-tests/src/test/java/de/cuioss/sheriff/gateway/integration/BffCookieStatelessnessIT.java

---

📝 Walkthrough

Walkthrough

The integration test’s sealed-cookie tampering helper now decodes the cookie, flips the final decoded byte, and re-encodes it as unpadded Base64URL. Its documentation and imports were updated accordingly.

Changes

Sealed-cookie tampering

Layer / File(s)	Summary
Byte-level cookie mutation integration-tests/src/test/java/de/cuioss/sheriff/gateway/integration/BffCookieStatelessnessIT.java	Adds Base64 support and changes tampering from altering the final encoded character to flipping the final decoded byte before unpadded Base64URL re-encoding.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: mutating the decoded GCM tag instead of a Base64 padding bit.

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[cuioss-review-bot]

PR Reviewer Guide 🔍

🧪 PR contains tests

🔒 No security concerns identified

⚡ No major issues detected

[cuioss-oliver]

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[cuioss-oliver]

Closing unmerged and reopening as a fresh PR so the review bots get a clean run — CodeRabbit and Sourcery both hit account rate limits when this PR was first opened, so its review history is not representative. No code change: the branch is already rebased on main (341ab9e) with no conflicts.