Hash Chaining Explained: The Git Model for Audit Logs

Every Git commit contains a reference to its parent commit. This simple mechanism creates an unbreakable chain of history — if anyone modifies a past commit, all subsequent hashes change, making tampering immediately detectable.

LogVault applies this same principle to audit logs. Here's how it works.

The Problem: Silent Modifications

Traditional audit logs are just database rows. Anyone with write access can:

• Delete inconvenient entries
• Modify timestamps to backdate activity
• Insert fake records to cover tracks

Without cryptographic integrity, you're trusting the database — and anyone who can access it.

❌ The Compliance Gap: SOC 2 Type II requires tamper-evident audit trails. A simple DELETE FROM audit_logs query breaks this requirement entirely.

The Solution: Hash Chaining

Each audit event in LogVault contains three cryptographic fields:

interface AuditEvent {
  id: string;
  action: string;
  timestamp: string;
  // ... other fields
 
  // Cryptographic integrity
  signature: string;    // HMAC of event data
  prev_hash: string;    // Hash of previous event
  chain_hash: string;   // Hash of this event (used by next event)
}

How It Works

1. Event arrives: LogVault receives a new audit event
2. Signature created: HMAC-SHA256 signs the event data
3. Chain linked: prev_hash is set to the previous event's chain_hash
4. Hash computed: chain_hash = SHA256(prev_hash + signature + event_data)

Event 1           Event 2           Event 3
┌──────────┐     ┌──────────┐     ┌──────────┐
│ action   │     │ action   │     │ action   │
│ data     │     │ data     │     │ data     │
│          │     │          │     │          │
│ prev: ── │────▶│ prev: ── │────▶│ prev: ── │
│ hash: A1 │     │ hash: B2 │     │ hash: C3 │
└──────────┘     └──────────┘     └──────────┘

Why Tampering Is Detectable

If an attacker modifies Event 2:

1. Event 2's chain_hash changes (different input = different hash)
2. Event 3's prev_hash no longer matches Event 2's new hash
3. Chain is broken — verification fails immediately

The attacker would need to recompute every subsequent hash, but they can't because:

• They don't have the HMAC signing key
• LogVault's verification endpoints would detect the mismatch

Verification in Practice

LogVault provides three levels of verification:

1. Dashboard Verification

The chain status banner shows real-time integrity:

┌─────────────────────────────────────────────────────────┐
│ ✓ Chain Verified: 1,247 events · Last check: 2 min ago │
└─────────────────────────────────────────────────────────┘

2. SDK Verification

Programmatic verification for CI/CD pipelines:

import { LogVault } from '@logvault/sdk';
 
const logvault = new LogVault({ apiKey: 'lv_...' });
 
// Verify entire chain
const result = await logvault.verifyChain();
 
if (result.isValid) {
  console.log('✓ All events verified');
} else {
  console.error('✗ Chain broken at:', result.firstInvalidEvent);
}

from logvault import LogVault
 
client = LogVault(api_key="lv_...")
 
# Verify chain
result = client.verify_chain()
 
if result.is_valid:
    print("✓ All events verified")
else:
    print(f"✗ Chain broken at: {result.first_invalid_event}")

3. Event-Level Proof

Get cryptographic proof for a specific event:

const proof = await logvault.getEventProof('evt_abc123');
 
console.log({
  eventHash: proof.chain_hash,
  previousHash: proof.prev_hash,
  signature: proof.signature,
  position: proof.chain_position,
  totalEvents: proof.total_events,
});

Handling Legacy Events

When you first start using LogVault, existing events won't have hash chains. LogVault handles this gracefully:

ℹ️ Migration Path: New events are automatically chained. Legacy events remain accessible but show as "unverifiable" in the dashboard.

Performance Considerations

Hash computation adds minimal overhead:

• SHA-256: ~500ns per event on modern CPUs
• HMAC-SHA256: ~1μs per event
• Chain lookup: O(1) with proper indexing

At 10,000 events/second, hash chaining adds less than 10ms total latency.

Comparison: Hash Chain vs. Blockchain

You might wonder: "Why not use a blockchain?"

Hash chains provide the same tamper-evidence guarantees that compliance requires, without the overhead of consensus mechanisms or distributed storage.

✅ Right Tool for the Job: Blockchains solve decentralized trust. Audit logs need centralized trust with tamper evidence. Hash chains are the right abstraction.

Conclusion

Hash chaining transforms audit logs from "database rows we hope nobody modifies" to "cryptographically verifiable history." The same technique that makes Git reliable makes your audit trail trustworthy.

Key takeaways:

1. Every event links to its predecessor — creating an unbreakable chain
2. Tampering is mathematically detectable — not just policy-based
3. Verification is fast — milliseconds, not minutes
4. Legacy events coexist — no big-bang migration required

Ready to implement tamper-evident audit logs? Get started with LogVault — hash chaining is built in.