> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/cryptography/hashing/42.-mac-vs-hash.md).

# 42. MAC vs Hash

## Core Difference

| Property          | Hash                | MAC                |
| ----------------- | ------------------- | ------------------ |
| Key required      | ❌ No                | ✅ Yes (secret key) |
| Authenticity      | ❌ No                | ✅ Yes              |
| Integrity         | ✅ Yes               | ✅ Yes              |
| Public verifiable | ✅ Anyone can verify | ❌ Only key holders |
| Example           | SHA-256, MD5        | HMAC-SHA256, CMAC  |

## Hash

* Deterministic: same input → same output
* One-way function
* No secret → **anyone** can compute it
* Use case: checksums, password storage (with salt), data deduplication

## MAC

* Requires a **shared secret key**
* Verifies both **integrity** AND **origin**
* Without the key, cannot forge a valid MAC
* Use case: API authentication, signed cookies, message integrity

## Naive MAC Pitfalls

### Vulnerable: `H(key || message)`

```
Hash(secret + message)  ← vulnerable to length extension attack
```

### Vulnerable: `H(message || key)`

```
Hash(message + secret)  ← vulnerable to collision attacks
```

### Secure: HMAC

```
HMAC(key, message)  ← double-hashing with key XOR padding
```

## CTF Scenarios

* **Cookie tampering**: site uses `MD5(secret + data)` → length extension attack
* **Weak MAC key**: brute-force the key with known plaintext pairs
* **Missing MAC**: forge arbitrary data if no authentication is used


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