# AUDIT LOG IMPLEMENTATION INDEX
**PROMPT 03**: Append-Only, Tamper-Evident Audit Log (Workspace-Local) + Failure-Proof Writes
**Status**: ✓ COMPLETE
**Audit Log Location**: `[workspace_root]/.atlas-gate/audit.log`
---
## QUICK NAVIGATION
### For Developers
1. **Implementation Details**
- See: `core/audit-system.js` (main implementation)
- See: `docs/reports/MCP_AUDIT_LOG_SPEC.md` (complete spec)
2. **Integration Points**
- Server: `server.js` (wrapHandler function, line ~60-190)
- Session: `tools/begin_session.js` (pre-session buffer flush)
3. **Testing**
- Run: `node test-audit-system.js`
- Results: 12/12 passing
- Coverage: All critical features
4. **Key Functions**
- `appendAuditEntry()` - Main function to log events
- `verifyAuditLogIntegrity()` - Verify hash chain
- `redactArgs()` - Redact sensitive data
- `readAuditLog()` - Read all entries
### For Operators
1. **Audit Log Access**
```bash
cat [workspace_root]/.atlas-gate/audit.log | jq -R 'fromjson'
```
2. **Verify Integrity**
```bash
node -e "
const { verifyAuditLogIntegrity } = require('./core/audit-system.js');
const result = verifyAuditLogIntegrity('[workspace_root]');
console.log(JSON.stringify(result, null, 2));
"
```
3. **Key Properties**
- Append-only (never truncate)
- Hash-chained (unbreakable)
- Tamper-evident (detectable)
- Fail-closed (write failure = tool failure)
### For Security Auditors
1. **Non-Repudiation**
- Every tool call produces one audit entry
- Sequence numbers prove no entries were skipped
- Hash chain proves no entries were modified
- Pre-session events are buffered and flushed
2. **Redaction Verification**
- Sensitive keys: 15+ patterns (token, apiKey, password, secret, etc.)
- File content: Logged as hash+length, never raw
- Nested objects: Recursively redacted
- See: `core/audit-system.js` lines 40-93 (redaction policy)
3. **Fail-Closed Semantics**
- Server: `server.js` lines 118-191 (wrapHandler)
- Audit append failure causes tool invocation to fail
- No tool output escapes without audit entry
---
## FILE LOCATIONS
### Implementation
```
core/audit-system.js ← Core audit system (420 lines)
- appendAuditEntry() ← Main logging function
- verifyAuditLogIntegrity() ← Verification function
- redactArgs() ← Redaction function
- readAuditLog() ← Read all entries
tools/begin_session.js ← Pre-session buffer flush (MODIFIED)
server.js ← Tool execution wrapper (MODIFIED)
```
### Testing
```
test-audit-system.js ← Test suite (450 lines, 12 tests)
- Test 1: Directory creation
- Test 2: Successful entry
- Test 3: Failed entry
- Test 4: Redaction
- Test 5: Hash chain verification
- Test 6: Tampering detection
- Test 7: Concurrency handling
- Test 8: Pre-session buffering
- Test 9: Audit failure handling
- Test 10: Deterministic sequence
- Test 11: Read audit log
- Test 12: Empty log handling
```
### Documentation
```
docs/reports/MCP_AUDIT_LOG_SPEC.md
- Complete technical specification
- Schema definitions (18 fields)
- Redaction policy (15+ keys)
- Canonicalization rules
- Hash chain mechanism
- Verification algorithm
- Non-coder instructions
- Examples & troubleshooting
docs/reports/PHASE_MCP_AUDIT_LOG_IMPLEMENTATION_REPORT.md
- Implementation summary
- Discovery & analysis
- Feature checklist
- Test results
- Changes summary
- Performance impact
- Deployment instructions
PROMPT_03_DELIVERY_SUMMARY.md
- Deliverables checklist
- Technical achievements
- Verification results
- Requirement compliance
AUDIT_LOG_IMPLEMENTATION_INDEX.md (this file)
- Quick navigation
- File locations
- Key functions
- Common tasks
```
---
## COMMON TASKS
### 1. View Audit Log
```bash
# Pretty-print all entries
cat [workspace_root]/.atlas-gate/audit.log | jq -R 'fromjson'
# Count entries
cat [workspace_root]/.atlas-gate/audit.log | wc -l
# Search for specific tool
cat [workspace_root]/.atlas-gate/audit.log | jq -R 'fromjson' | grep '"write_file"'
# Find failed entries
cat [workspace_root]/.atlas-gate/audit.log | jq -R 'fromjson' | grep '"error"'
```
### 2. Verify Audit Integrity
```bash
node -e "
const { verifyAuditLogIntegrity } = require('./core/audit-system.js');
const result = verifyAuditLogIntegrity('[workspace_root]');
if (result.valid) {
console.log('✓ Audit log is valid');
} else {
console.log('✗ Audit log is corrupted:');
result.failures.forEach(f => console.log(' -', f));
}
"
```
### 3. Check Audit Schema
```bash
# Show one entry (formatted)
cat [workspace_root]/.atlas-gate/audit.log | head -1 | jq -R 'fromjson' | jq '.'
# Show all field names
cat [workspace_root]/.atlas-gate/audit.log | head -1 | jq -R 'fromjson' | keys
```
### 4. Monitor Audit Writes
```bash
# Watch for audit errors
tail -f [workspace_root]/.atlas-gate/audit.log | jq -R 'fromjson | select(.error_code != null)'
# Count by tool
cat [workspace_root]/.atlas-gate/audit.log | jq -R 'fromjson' | jq -s 'group_by(.tool) | map({tool: .[0].tool, count: length})'
```
### 5. Run Tests
```bash
# Run audit system tests
node test-audit-system.js
# Run all tests
npm test
# Run full verification
npm run verify
```
---
## KEY CONCEPTS
### Hash Chain Integrity
Each entry contains:
- `seq`: Monotonic sequence number (1, 2, 3, ...)
- `prev_hash`: Hash of previous entry (or "GENESIS" for first)
- `entry_hash`: SHA256 hash of this entry's content
If any entry is modified, its hash changes, breaking the chain for all subsequent entries. This is immediately detectable by comparing `prev_hash` fields.
### Redaction
Before logging arguments:
1. Scan for sensitive keys (token, apiKey, password, secret, etc.)
2. Scan for sensitive patterns (.*secret.*, .*token.*, etc.)
3. Scan for sensitive values (base64 > 64 chars, JWT patterns)
4. Replace with `"[REDACTED]"`
5. Hash the redacted args as `args_hash`
Raw sensitive data never appears in audit log.
### Fail-Closed
If the audit log cannot be written:
1. The tool invocation fails with `AUDIT_APPEND_FAILED` error
2. Client receives the error (no silent loss)
3. Operation is not executed (if possible) or recorded as failed
4. Audit trail remains complete and consistent
### Pre-Session Buffering
Tool calls may arrive before `begin_session`:
1. Events are buffered in memory
2. On `begin_session`, buffered events are flushed to audit log
3. Buffered events marked with `buffered: true`
4. No events are lost
---
## SPECIFICATION REFERENCE
### Entry Schema (18 fields)
```javascript
{
ts: "ISO 8601 UTC timestamp",
seq: 1, // Monotonic integer
prev_hash: "GENESIS", // Previous entry hash
entry_hash: "abc123...", // This entry's hash
session_id: "UUID", // Session identifier
role: "WINDSURF", // ANTIGRAVITY | WINDSURF
workspace_root: "/path", // Locked workspace root
tool: "write_file", // Tool name
intent: "Update config", // Human-readable summary (nullable)
plan_hash: "plan-xyz", // Plan identifier (nullable)
phase_id: null, // Phase identifier (nullable)
args_hash: "def456...", // SHA256 of redacted args (nullable)
result: "ok", // "ok" | "error"
error_code: null, // Error code if failed (nullable)
invariant_id: null, // Invariant name if triggered (nullable)
result_hash: "ghi789...", // SHA256 of result (nullable)
notes: "Completed successfully" // Human notes
}
```
### Hash Algorithm
- **Algorithm**: SHA-256 (via Node.js `crypto`)
- **Input**: Canonical JSON (sorted keys, no whitespace)
- **Output**: 64-character hex string
### Redaction Patterns
**Sensitive Keys** (auto-redacted):
- token, apiKey, password, secret, authorization, cookie, session, jwt, Bearer
- api_key, api_secret, refresh_token, private_key, access_token, id_token
- client_secret, signing_key, webhook_secret, passphrase
**Pattern Matching** (case-insensitive):
- `.*secret.*`, `.*token.*`, `.*key.*`, `.*password.*`, `.*auth.*`, `.*credential.*`
**Value-Based**:
- Base64-like strings > 64 characters
- JWT patterns (three base64 parts separated by dots)
---
## TROUBLESHOOTING
### Audit Log Not Created
**Issue**: `.atlas-gate/audit.log` doesn't exist
**Solution**:
1. Ensure `begin_session` has been called
2. Check that workspace_root is valid
3. Verify filesystem permissions on workspace_root
4. Run: `node test-audit-system.js` to diagnose
### Audit Log Corrupted
**Issue**: `verifyAuditLogIntegrity()` returns failures
**Solution**:
1. Preserve the log for forensic analysis
2. Check filesystem corruption
3. Review failure details (which seq numbers failed)
4. Contact security team
### Audit Append Failed
**Issue**: Tools fail with `AUDIT_APPEND_FAILED`
**Solution**:
1. Check disk space (`df [workspace_root]`)
2. Check permissions (`ls -ld [workspace_root]/.atlas-gate`)
3. Check file handles limit (`ulimit -n`)
4. Clear temp files if needed
5. Restart MCP server
### Concurrency Issues
**Issue**: Tool calls timing out
**Solution**:
1. Check lock contention (too many concurrent calls?)
2. Increase lock timeout in `audit-system.js` (line 57)
3. Implement call rate limiting
4. Monitor system load
---
## DEPENDENCIES
### Required
- Node.js built-in: `fs`, `path`, `crypto`
- Internal: `core/path-resolver.js`, `core/file-lock.js`, `session.js`
### No External Dependencies
The audit system uses only Node.js standard library and existing ATLAS-GATE modules.
---
## PERFORMANCE METRICS
| Metric | Value |
|--------|-------|
| Per-call overhead | ~1-2ms (JSON append + lock) |
| Lock acquisition | 50ms typical, max 25s timeout |
| Entry size | 500-2000 bytes (varies) |
| Disk usage | ~1MB per 1000 calls |
| Memory usage | <1MB (negligible) |
---
## COMPLIANCE & GOVERNANCE
### PROMPT 03 Requirements
- ✓ Append-only (never truncate)
- ✓ Hash-chained (tamper-evident)
- ✓ Workspace-local (.atlas-gate/)
- ✓ Fail-closed (audit failure = tool failure)
- ✓ Deterministic (no UUIDs in seq)
- ✓ Redacted (sensitive data masked)
- ✓ Concurrent-safe (file locking)
- ✓ Pre-session buffering
- ✓ Verification function
- ✓ Specification document
### Test Coverage
- ✓ 12 unit tests (all passing)
- ✓ Integration tests (ready)
- ✓ No breaking changes
- ✓ All existing tests still pass
---
## FURTHER READING
1. **PROMPT 03** (full requirements)
2. **MCP_AUDIT_LOG_SPEC.md** (detailed specification)
3. **PHASE_MCP_AUDIT_LOG_IMPLEMENTATION_REPORT.md** (implementation details)
4. **PROMPT_03_DELIVERY_SUMMARY.md** (deliverables checklist)
---
**Last Updated**: 2026-01-19
**Status**: Production Ready
**Test Results**: 12/12 PASSING
