fix: remove session key auto-revoke, increase MAX_SESSION_KEYS

[newtsjamie]

Summary

• Removed auto-revoke loop from authorize_session_key() — now insert-only, producing zero nullifiers from existing notes
• Increased MAX_SESSION_KEYS from 5 to 10 — provides headroom for orphaned notes between client-side cleanup cycles

Why

In Aztec's UTXO model, the get_notes + remove(each) loop produced nullifiers. When PXE had stale state or another TX was in the mempool, the same nullifiers appeared in two TXs → "Nullifier conflict" rejection. This is fundamental to the UTXO model and cannot be fixed with sync-and-retry.

Orphaned notes (from browser refresh losing the in-memory key) are now cleaned up by the client via revoke_session_key() on a best-effort basis before each new authorization.

Companion PR

Apertrue/web#24 — TypeScript changes (stale key tracking, simplified wallet flow)

Deployment

After merge: ./build.sh → recompile + AVM transpile + copy artifact to web app. Existing devnet accounts need redeployment (new bytecode = new contract class ID).

Test plan

• nargo compile passes
• Full build.sh pipeline succeeds (requires sandbox Docker container)
• Session key authorization succeeds without nullifier conflicts
• revoke_session_key still works for individual key revocation
• is_valid_impl finds session key with up to 10 notes in storage

🤖 Generated with Claude Code

Summary by CodeRabbit

Release Notes

• New Features
  • Session key capacity increased from 5 to 10, enabling more concurrent session management.
  • Session key authorization behavior updated: existing keys are no longer automatically revoked. Manual revocation is now required for cleanup.

[coderabbitai]

📝 Walkthrough

Walkthrough

The change increases MAX_SESSION_KEYS from 5 to 10 and modifies authorize_session_key behavior from auto-revocation (removing prior session key notes before inserting new ones) to insert-only operation, requiring explicit calls to revoke_session_key for cleanup.

Changes

Cohort / File(s)	Summary
Session Key Management webauthn_account/src/main.nr	Increased MAX_SESSION_KEYS constant from 5 to 10; refactored authorize_session_key from auto-revocation pattern to insert-only behavior; updated method documentation to reflect dependency on explicit revoke_session_key() calls for session key cleanup.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related PRs

• Apertrue/web#23: Implements PXE sync and client-side retry logic to handle nullifier conflicts that result from the prior auto-revoke behavior in authorize_session_key, making this PR's shift to insert-only semantics directly relevant.

Poem

🐰 Ten keys now dance where five once stood,
No auto-clean, as one should;
Explicit revoke, a cleaner way,
The rabbit hops through change today! 🔑✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the two main changes: removing session key auto-revoke and increasing MAX_SESSION_KEYS from 5 to 10.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/session-key-remove-auto-revoke

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[coderabbitai]

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

webauthn_account/src/main.nr (1)

186-209: ⚠️ Potential issue | 🟡 Minor

Potential edge case: session key lookup may fail if orphaned notes exceed limit.

The insert-only approach is sound for avoiding nullifier conflicts, but there's a correctness edge case: if orphaned notes accumulate beyond MAX_SESSION_KEYS (10), is_valid_impl may not find a valid session key in the returned note set, causing authentication to fail unexpectedly. Similarly, revoke_session_key might miss its target.

This relies on the companion PR's client-side cleanup being robust. Consider whether the client should track note count and proactively revoke stale keys before reaching the limit.

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@webauthn_account/src/main.nr` around lines 186 - 209, authorize_session_key
currently unconditionally inserts a SessionKeyNote which can overflow the
MAX_SESSION_KEYS (10) slot window and make is_valid_impl / revoke_session_key
miss keys; update authorize_session_key to first read the existing notes at
self.storage.session_keys.at(owner), count them, and if count >=
MAX_SESSION_KEYS either (a) purge or revoke the oldest/stale notes by selecting
candidates using SessionKeyNote.expiry (and invoking revoke_session_key or the
storage removal API) before inserting, or (b) reject the new authorization with
a clear error/return so callers can retry cleanup; ensure the logic references
SessionKeyNote, MAX_SESSION_KEYS, authorize_session_key, is_valid_impl, and
revoke_session_key so behavior remains consistent.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Outside diff comments:
In `@webauthn_account/src/main.nr`:
- Around line 186-209: authorize_session_key currently unconditionally inserts a
SessionKeyNote which can overflow the MAX_SESSION_KEYS (10) slot window and make
is_valid_impl / revoke_session_key miss keys; update authorize_session_key to
first read the existing notes at self.storage.session_keys.at(owner), count
them, and if count >= MAX_SESSION_KEYS either (a) purge or revoke the
oldest/stale notes by selecting candidates using SessionKeyNote.expiry (and
invoking revoke_session_key or the storage removal API) before inserting, or (b)
reject the new authorization with a clear error/return so callers can retry
cleanup; ensure the logic references SessionKeyNote, MAX_SESSION_KEYS,
authorize_session_key, is_valid_impl, and revoke_session_key so behavior remains
consistent.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 04a81f0d-adc1-4040-8ab0-bbda4013c8e4

📥 Commits

Reviewing files that changed from the base of the PR and between 6cea3ce and 8c07137.

📒 Files selected for processing (1)

• webauthn_account/src/main.nr