> 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/41.-hmac.md).

# 41. HMAC (Hash-based Message Authentication Code)

## What is HMAC?

HMAC is a specific construction for creating a Message Authentication Code (MAC) using a cryptographic hash function combined with a secret key.

**Formula:**

```
HMAC(K, m) = H((K' ⊕ opad) || H((K' ⊕ ipad) || m))
```

Where:

* `H` = hash function (MD5, SHA-1, SHA-256, etc.)
* `K` = secret key
* `K'` = key padded/truncated to block size
* `opad` = outer padding (0x5c repeated)
* `ipad` = inner padding (0x36 repeated)
* `||` = concatenation

## Why HMAC?

* Resistant to **length extension attacks** (unlike plain `H(key || message)`)
* Provides both **integrity** and **authenticity**
* Security depends on the underlying hash function

## Common HMAC Variants

| Variant     | Output Size |
| ----------- | ----------- |
| HMAC-MD5    | 128 bits    |
| HMAC-SHA1   | 160 bits    |
| HMAC-SHA256 | 256 bits    |
| HMAC-SHA512 | 512 bits    |

## Python Example

```python
import hmac
import hashlib

key = b"supersecretkey"
message = b"hello world"

mac = hmac.new(key, message, hashlib.sha256).hexdigest()
print(mac)
```

## CTF Context

* Often used to sign tokens or cookies
* Look for `HMAC-SHA256` in JWT headers
* If the key is weak or guessable → brute-force attack
* If implementation uses `==` for comparison → timing attack


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