###### Books / Practical Cryptography for Developers (Unfinished) / Chapter 3

# HMAC and Key Derivation

Simply calculating `hash_func(key + msg)`

to obtain a MAC (message authentication code) is considered **insecure** (see the details). It is recommended to use the **HMAC algorithm instead**, e.g. `HMAC-SHA256`

or `HMAC-SHA3-512`

or other secure MAC algorithm.

## What is HMAC?

**HMAC** = **H**ash-based **M**essage **A**uthentication **C**ode (MAC code, calculated using a cryptographic hash function):

```
HMAC(key, msg, hash_func) -> hash
```

The results MAC code is a **message hash** mixed with a secret key. It has the cryptographic properties of hashes: **irreversible**, **collision resistant**, etc.

The `hash_func`

can be any cryptographic hash function like `SHA-256`

, `SHA-512`

, `RIPEMD-160`

, `SHA3-256`

or `BLAKE2s`

.

**HMAC** is used for message **authenticity**, message **integrity** and sometimes for **key derivation**.

## Key Derivation Functions (KDF)

**Key derivation function** (KDF) is a function which transforms a variable-length password to fixed-length key (sequence of bits):

```
function(password) -> key
```

As **very simple KDF function**, we can use SHA256: just hash the password. Don’t do this, because it is **insecure**. Simple hashes are vulnerable to **dictionary attacks**.

As more complicated KDF function, you can derive a password by calculating **HMAC(salt, msg, SHA256)** using some random value called “**salt**”, which is stored along with the derived key and used later to derive the same key again from the password.

Using **HKDF (HMAC-based key derivation)** for key derivation is **less secure** than modern KDFs, so experts recommend using stronger key derivation functions like PBKDF2, Bcrypt, Scrypt and Argon2. We shall discuss all these KDF functions later.

## HMAC Calculation - Example

To get a better idea of **HMAC** and how it is calculated, try this online tool: https://www.freeformatter.com/hmac-generator.html

Play with calculating **HMAC(‘sample message’, ‘12345’, ‘SHA256’)**:

```
HMAC('sample message', '12345', 'SHA256') =
'ee40ca7bc90df844d2f5b5667b27361a2350fad99352d8a6ce061c69e41e5d32'
```

Try the above example yourself.