audit-logging

# Audit Logging

## Overview

Compliance audit logs must answer: **who** did **what** to **which resource**, **when**, from **where**, and with **what result**. They must also be tamper-evident — an auditor must be able to verify logs haven't been modified.

**Regulatory retention requirements:**
| Standard | Retention |
|----------|-----------|
| HIPAA | 6 years |
| PCI DSS | 12 months online + 12 months archive |
| SOC 2 (typical) | 1 year |
| GDPR | Defined by purpose (often 1-3 years) |

## Required Log Fields

```typescript
interface AuditLogEntry {
  // Core required fields
  id: string;             // UUID — unique event identifier
  timestamp: string;      // ISO 8601 UTC — when the event occurred
  actor_id: string;       // User/service that performed the action
  actor_type: 'user' | 'service' | 'admin' | 'system';
  action: string;         // What happened: "read" | "write" | "delete" | "login" | "export"
  resource_type: string;  // "patient_record" | "invoice" | "user_account"
  resource_id: string;    // ID of the affected resource
  result: 'success' | 'failure' | 'denied';
  
  // Context fields
  ip_address: string;     // Source IP
  user_agent?: string;    // Browser/client identifier
  session_id?: string;    // Session identifier
  
  // Optional fields
  changed_fields?: string[];    // For write operations: which fields changed
  old_values?: Record<string, unknown>;  // Previous values (be careful with PII)
  reason?: string;              // Clinical justification (HIPAA), business reason
  
  // Tamper-evidence
  prev_hash?: string;     // Hash of previous log entry (hash chaining)
  hash: string;           // SHA-256 of this entry
}
```

## Hash Chain Implementation

Hash chaining makes log tampering detectable: each entry includes a hash of the previous entry. Modifying any entry breaks the chain.

```python
import json
import hashlib
import uuid
from datetime import datetime, timezone
from typing import Optional

class AuditLogger:
    def __init__(self, db_connection):
        self.db = db_connection
        self._last_hash: Optional[str] = None
    
    def _compute_hash(self, entry: dict, prev_hash: Optional[str]) -> str:
        """Compute SHA-256 of the log entry for tamper-evidence."""
        hashable = {
            "id": entry["id"],
            "timestamp": entry["timestamp"],
            "actor_id": entry["actor_id"],
            "action": entry["action"],
            "resource_type": entry["resource_type"],
            "resource_id": entry["resource_id"],
            "result": entry["result"],
            "prev_hash": prev_hash or "GENESIS"
        }
        content = json.dumps(hashable, sort_keys=True)
        return hashlib.sha256(content.encode()).hexdigest()
    
    async def log(
        self,
        actor_id: str,
        actor_type: str,
        action: str,
        resource_type: str,
        resource_id: str,
        result: str,
        ip_address: str,
        **kwargs
    ) -> dict:
        """Create and persist a tamper-evident audit log entry."""
        # Get last hash for chaining
        prev_hash = self._last_hash or await self._get_last_hash()
        
        entry = {
            "id": str(uuid.uuid4()),
            "timestamp": datetime.now(timezone.utc).isoformat(),
            "actor_id": actor_id,
            "actor_type": actor_type,
            "action": action,
            "resource_type": resource_type,
            "resource_id": resource_id,
            "result": result,
            "ip_address": ip_address,
            "prev_hash": prev_hash or "GENESIS",
            **kwargs
        }
        
        entry["hash"] = self._compute_hash(entry, prev_hash)
        self._last_hash = entry["hash"]
        
        # Persist to append-only storage
        await self.db.audit_logs.insert_one(entry)
        return entry
    
    async def verify_chain(self, limit: int = 1000) -> bool:
        """Verify the hash chain integrity."""
        entries = await self.db.audit_logs.find(
            sort=[("timestamp", 1)], 
            limit=limit
        )
        
        prev_hash = "GENESIS"
        for i, entry in enumerate(entries):
            expected_hash = self._compute_hash(entry, prev_hash)
            if entry["hash"] != expected_hash:
                print(f"❌ Chain broken at entry {i}: id={entry['id']}")
                return False
            if entry["prev_hash"] != prev_hash:
                print(f"❌ prev_hash mismatch at entry {i}")
                return False
            prev_hash = entry["hash"]
        
        print(f"✅ Chain verified: {len(entries)} entries intact")
        return True
    
    async def _get_last_hash(self) -> Optional[str]:
        last = await self.db.audit_logs.find_one(sort=[("timestamp", -1)])
        return last["hash"] if last else None
```

## Express.js Audit Middleware

```javascript
const { v4: uuidv4 } = require('uuid');
const crypto = require('crypto');

class AuditLogger {
  constructor(db) {
    this.db = db;
  }
  
  async log({ actorId, actorType = 'user', action, resourceType, resourceId, 
               result, ipAddress, sessionId, reason, changedFields }) {
    const prevHash = await this.getLastHash();
    
    const entry = {
      id: uuidv4(),
      timestamp: new Date().toISOString(),
      actor_id: actorId,
      actor_type: actorType,
      action,
      resource_type: resourceType,
      resource_id: resourceId,
      result,
      ip_address: ipAddress,
      session_id: sessionId,
      reason,
      changed_fields: changedFields,
      prev_hash: prevHash || 'GENESIS',
    };
    
    entry.hash = this.computeHash(entry);
    await this.db.auditLogs.create(entry);
    return entry;
  }
  
  computeHash(entry) {
    const hashable = JSON.stringify({
      id: entry.id,
      timestamp: entry.timestamp,
      actor_id: entry.actor_id,
      action: entry.action,
      resource_type: entry.resource_type,
      resource_id: entry.resource_id,
      result