Building a Tamper-Evident Audit Log with SHA-256 Hash Chains (Zero Dependencies)

Ever wondered how blockchains detect tampering? The core mechanism is surprisingly simple: hash chains. Each record includes the hash of the previous record, creating a mathematical dependency that breaks if anything changes.

In this post, I'll walk through a complete implementation of tamper-evident logging using only vanilla JavaScript and the Web Crypto API. No npm packages, no build tools, no external dependencies.

Live Demo: veritaschain.org/regtech/demo

Source Code: All code shown below is production-ready and copy-pasteable.

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Table of Contents

1. The Problem We're Solving
2. Hash Chain Architecture
3. Implementation: SHA-256 with Web Crypto
4. Implementation: Canonical JSON
5. Implementation: Event Generation
6. Implementation: Chain Verification
7. Implementation: Tampering Detection
8. Bonus: ZIP File Generation (No Libraries)
9. Full Source Code

---

The Problem We're Solving {#the-problem}

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

// The attack
log[5].price = "1.09999";  // Changed from "1.08523"
delete log[3];              // Evidence removed
[log[7], log[8]] = [log[8], log[7]];  // Order swapped

After these changes, the log looks identical to a legitimate one. No visible corruption. No format errors. Undetectable.

We want a system where any modification — even a single character — becomes mathematically detectable.

---

Hash Chain Architecture {#architecture}

The solution is elegant:

┌──────────────┐     ┌──────────────┐     ┌──────────────┐
│   Event 0    │     │   Event 1    │     │   Event 2    │
├──────────────┤     ├──────────────┤     ├──────────────┤
│ prev: GENESIS│────▶│ prev: a3f2.. │────▶│ prev: 7bc1.. │
│ data: {...}  │     │ data: {...}  │     │ data: {...}  │
│ hash: a3f2.. │     │ hash: 7bc1.. │     │ hash: e9f0.. │
└──────────────┘     └──────────────┘     └──────────────┘

Each event stores:

• prev_hash: The hash of the previous event (or "GENESIS" for the first)
• event_hash: SHA-256 of prev_hash + canonical(event_data)

If Event 1's data changes:

1. Its hash changes from 7bc1.. to something else
2. Event 2's prev_hash no longer matches
3. Chain breaks → Tampering detected

---

Implementation: SHA-256 with Web Crypto {#sha256}

The Web Crypto API gives us native SHA-256. No libraries needed.

async function sha256(message) {
    // Encode string to Uint8Array
    const encoder = new TextEncoder();
    const data = encoder.encode(message);

    // Compute SHA-256 digest
    const hashBuffer = await crypto.subtle.digest('SHA-256', data);

    // Convert ArrayBuffer to hex string
    const hashArray = Array.from(new Uint8Array(hashBuffer));
    return hashArray.map(b => b.toString(16).padStart(2, '0')).join('');
}

// Usage
const hash = await sha256('hello world');
// Returns: "b94d27b9934d3e08a52e52d7da7dabfac484efe37a5380ee9088f7ace2efcde9"

Key points:

• crypto.subtle is available in all modern browsers (Chrome 60+, Firefox 57+, Safari 11+)
• Returns a Promise (async operation)
• Output is 64 hex characters (256 bits)

---

Implementation: Canonical JSON {#canonical-json}

Here's the gotcha: JSON.stringify() doesn't guarantee key order.

JSON.stringify({b: 2, a: 1});  // Could be '{"b":2,"a":1}' or '{"a":1,"b":2}'

Different strings → different hashes → false tampering alerts.

We need canonical JSON: deterministic stringification with sorted keys.

function canonicalStringify(obj) {
    // Primitives: use standard stringify
    if (obj === null || typeof obj !== 'object') {
        return JSON.stringify(obj);
    }

    // Arrays: preserve order, recurse on elements
    if (Array.isArray(obj)) {
        return '[' + obj.map(canonicalStringify).join(',') + ']';
    }

    // Objects: sort keys alphabetically, recurse on values
    const sortedKeys = Object.keys(obj).sort();
    const pairs = sortedKeys.map(key => 
        JSON.stringify(key) + ':' + canonicalStringify(obj[key])
    );
    return '{' + pairs.join(',') + '}';
}

// Test it
canonicalStringify({zebra: 1, apple: {banana: 2, avocado: 1}});
// Always returns: '{"apple":{"avocado":1,"banana":2},"zebra":1}'

Key points:

• Keys sorted alphabetically at every nesting level
• Arrays maintain their order (only objects get sorted)
• No whitespace (compact format)
• Recursively handles nested structures

---

Implementation: Event Generation {#event-generation}

Now let's create a chain of events:

const GENESIS_HASH = 'GENESIS';

// Compute hash for a single event
async function computeEventHash(event, prevHash) {
    // Only hash the core fields (exclude prev_hash and event_hash)
    const eventCore = {
        event_id: event.event_id,
        trace_id: event.trace_id,
        timestamp_ms: event.timestamp_ms,
        event_type: event.event_type,
        payload: event.payload
    };

    const canonical = canonicalStringify(eventCore);
    const input = prevHash + canonical;
    return await sha256(input);
}

// Generate a chain of events
async function generateEventChain(eventDataArray) {
    const chain = [];
    let prevHash = GENESIS_HASH;

    for (const data of eventDataArray) {
        const event = {
            event_id: generateEventId(),
            trace_id: data.trace_id,
            timestamp_ms: Date.now(),
            event_type: data.type,
            payload: data.payload,
            prev_hash: prevHash,
            event_hash: ''  // Will be computed
        };

        event.event_hash = await computeEventHash(event, prevHash);
        chain.push(event);
        prevHash = event.event_hash;
    }

    return chain;
}

// UUIDv7-like event ID (time-sortable)
function generateEventId() {
    const timestamp = Date.now();
    const hex = timestamp.toString(16).padStart(12, '0');
    const random = Array.from({ length: 4 }, () => 
        Math.floor(Math.random() * 65536).toString(16).padStart(4, '0')
    ).join('');
    return `${hex.slice(0, 8)}-${hex.slice(8, 12)}-7${random.slice(0, 3)}-${random.slice(3, 7)}-${random.slice(7, 19)}`;
}

Why UUIDv7-like IDs?

• First 48 bits are timestamp → lexicographic sort = chronological sort
• 7 indicates version 7 format
• Remaining bits are random for uniqueness

---

Implementation: Chain Verification {#verification}

The verification algorithm checks two things for each event:

1. Linkage: Does prev_hash match the previous event's event_hash?
2. Integrity: Does event_hash match a fresh computation from the data?

async function verifyChain(events) {
    const results = {
        valid: true,
        checks: [],
        firstFailure: null
    };

    for (let i = 0; i < events.length; i++) {
        const event = events[i];

        // Check 1: prev_hash linkage
        const expectedPrevHash = i === 0 ? GENESIS_HASH : events[i - 1].event_hash;
        const linkageValid = event.prev_hash === expectedPrevHash;

        results.checks.push({
            index: i,
            check: 'prev_hash linkage',
            pass: linkageValid,
            detail: linkageValid 
                ? `Links to ${i === 0 ? 'GENESIS' : 'Event #' + (i-1)}`
                : `Expected ${expectedPrevHash.slice(0,16)}..., got ${event.prev_hash.slice(0,16)}...`
        });

        if (!linkageValid && !results.firstFailure) {
            results.valid = false;
            results.firstFailure = { index: i, reason: 'prev_hash mismatch' };
        }

        // Check 2: event_hash integrity
        const recomputed = await computeEventHash(event, event.prev_hash);
        const integrityValid = event.event_hash === recomputed;

        results.checks.push({
            index: i,
            check: 'event_hash integrity',
            pass: integrityValid,
            detail: integrityValid
                ? `Hash verified: ${event.event_hash.slice(0,24)}...`
                : `Content modified - hash mismatch`
        });

        if (!integrityValid && !results.firstFailure) {
            results.valid = false;
            results.firstFailure = { 
                index: i, 
                reason: 'event_hash mismatch',
                expected: recomputed,
                actual: event.event_hash
            };
        }
    }

    return results;
}

Verification complexity: O(n) — one SHA-256 computation per event.

---

Implementation: Tampering Detection {#tampering}

Let's implement three attack scenarios:

1. Payload Modification (Post-hoc Edit)

function tamperModify(events, targetIndex, field, newValue) {
    const tampered = JSON.parse(JSON.stringify(events));  // Deep clone

    // Modify the field
    tampered[targetIndex].payload[field] = newValue;

    // Note: We're NOT recalculating the hash
    // This simulates an attacker who changes data but can't update hashes
    // (because they don't know the future events' data)

    return tampered;
}

// Example: Change execution price
const tampered = tamperModify(originalEvents, 3, 'price', '1.09999');
const result = await verifyChain(tampered);
// result.valid = false
// result.firstFailure = { index: 3, reason: 'event_hash mismatch' }

2. Event Deletion (Omission)

function tamperDelete(events, targetIndex) {
    const tampered = JSON.parse(JSON.stringify(events));
    tampered.splice(targetIndex, 1);  // Remove event
    return tampered;
}

// Example: Delete ACK event
const tampered = tamperDelete(originalEvents, 2);
const result = await verifyChain(tampered);
// result.valid = false
// result.firstFailure = { index: 2, reason: 'prev_hash mismatch' }
// (Event #3's prev_hash points to deleted Event #2's hash)

3. Event Reordering (Sequence Tampering)

function tamperSwap(events, index1, index2) {
    const tampered = JSON.parse(JSON.stringify(events));
    [tampered[index1], tampered[index2]] = [tampered[index2], tampered[index1]];
    return tampered;
}

// Example: Swap events 2 and 3
const tampered = tamperSwap(originalEvents, 2, 3);
const result = await verifyChain(tampered);
// result.valid = false
// (Both events now have incorrect prev_hash values)

---

Bonus: ZIP File Generation (No Libraries) {#zip}

For the evidence pack feature, I needed ZIP generation without JSZip or similar. Here's a pure JS implementation:

async function createZip(files) {
    const encoder = new TextEncoder();
    let offset = 0;
    const localHeaders = [];
    const centralDirectory = [];

    // Build local file headers and content
    for (const file of files) {
        const content = encoder.encode(file.content);
        const nameBytes = encoder.encode(file.name);

        // Local file header (30 bytes + filename)
        const localHeader = new Uint8Array(30 + nameBytes.length);
        const view = new DataView(localHeader.buffer);

        view.setUint32(0, 0x04034b50, true);  // Local file signature
        view.setUint16(4, 20, true);           // Version needed
        view.setUint16(6, 0, true);            // Flags
        view.setUint16(8, 0, true);            // Compression (none)
        view.setUint16(10, 0, true);           // Mod time
        view.setUint16(12, 0, true);           // Mod date
        view.setUint32(14, crc32(content), true);  // CRC-32
        view.setUint32(18, content.length, true);  // Compressed size
        view.setUint32(22, content.length, true);  // Uncompressed size
        view.setUint16(26, nameBytes.length, true);  // Filename length
        view.setUint16(28, 0, true);           // Extra field length
        localHeader.set(nameBytes, 30);

        localHeaders.push({ header: localHeader, content, name: nameBytes, offset });
        offset += localHeader.length + content.length;
    }

    // Build central directory
    const cdStart = offset;
    for (const item of localHeaders) {
        const cdHeader = new Uint8Array(46 + item.name.length);
        const view = new DataView(cdHeader.buffer);

        view.setUint32(0, 0x02014b50, true);   // Central directory signature
        view.setUint16(4, 20, true);            // Version made by
        view.setUint16(6, 20, true);            // Version needed
        // ... (set remaining fields)
        view.setUint32(42, item.offset, true);  // Offset to local header
        cdHeader.set(item.name, 46);

        centralDirectory.push(cdHeader);
        offset += cdHeader.length;
    }

    // End of central directory
    const eocd = new Uint8Array(22);
    const eocdView = new DataView(eocd.buffer);
    eocdView.setUint32(0, 0x06054b50, true);  // EOCD signature
    eocdView.setUint16(8, files.length, true);  // Entries
    eocdView.setUint32(12, offset - cdStart, true);  // CD size
    eocdView.setUint32(16, cdStart, true);  // CD offset

    // Combine all parts
    const result = new Uint8Array(offset + 22);
    let pos = 0;
    for (const item of localHeaders) {
        result.set(item.header, pos);
        pos += item.header.length;
        result.set(item.content, pos);
        pos += item.content.length;
    }
    for (const cd of centralDirectory) {
        result.set(cd, pos);
        pos += cd.length;
    }
    result.set(eocd, pos);

    return result;
}

// CRC-32 implementation
function crc32(data) {
    const table = new Uint32Array(256);
    for (let i = 0; i < 256; i++) {
        let c = i;
        for (let j = 0; j < 8; j++) {
            c = (c & 1) ? (0xEDB88320 ^ (c >>> 1)) : (c >>> 1);
        }
        table[i] = c;
    }

    let crc = 0xFFFFFFFF;
    for (let i = 0; i < data.length; i++) {
        crc = table[(crc ^ data[i]) & 0xFF] ^ (crc >>> 8);
    }
    return (crc ^ 0xFFFFFFFF) >>> 0;
}

Why bother? Zero dependencies means the demo works offline, loads instantly, and has no supply chain risk.

---

Full Source Code {#full-source}

The complete implementation is ~500 lines of vanilla JS:

• Demo: veritaschain.org/regtech/demo
• GitHub: github.com/veritaschain

File structure:

├── index.html   (80 lines)
├── style.css    (300 lines)
└── app.js       (500 lines)

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Key Takeaways

1. Hash chains are simple. The core concept is ~50 lines of code.

2. Canonical JSON is essential. Without deterministic serialization, you get false positives.

3. Web Crypto API is underrated. SHA-256, AES, ECDSA — all available natively.

4. Zero dependencies is achievable. Even ZIP generation can be done in pure JS.

5. Tamper-evidence ≠ tamper-proof. Anyone can still modify data. But now it's detectable.

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What's Next?

This demo covers the basics. A production system would add:

• Digital signatures (Ed25519) for non-repudiation
• Merkle trees for efficient batch verification
• External anchoring (timestamp authority or blockchain)
• PTP time synchronization for microsecond precision

The VeritasChain Protocol (VCP) specification covers all of these. If you're building audit infrastructure for trading systems, check out the IETF draft.

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Try It Yourself

🔗 veritaschain.org/regtech/demo

Generate events. Apply tampering. Watch verification fail. Download the evidence pack.

All in your browser. No setup required.

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Questions? Find me at [e-mail] or open an issue on GitHub.

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Tags: #javascript #cryptography #webdev #security #sha256 #auditlog #webapi #vanillajs