# hash_hkdf

(PHP 7 >= 7.1.2)

hash_hkdf — Generate a HKDF key derivation of a supplied key input

### Description

hash_hkdf ( string `$algo` , string `$ikm` [, int `$length` = 0 [, string `$info` = '' [, string `$salt` = '' ]]] ) : string

### Parameters

`algo`

  Name of selected hashing algorithm (i.e. "sha256", "sha512", "haval160,4", etc..) See hash_algos() for a list of supported algorithms.

  > Note:

  > Non-cryptographic hash functions are not allowed.

`ikm`

  Input keying material (raw binary). Cannot be empty.

`length`

  Desired output length in bytes. Cannot be greater than 255 times the chosen hash function size.

  If `length` is 0, the output length will default to the chosen hash function size.

`info`

  Application/context-specific info string.

`salt`

  Salt to use during derivation.

  While optional, adding random salt significantly improves the strength of HKDF.

### Return Values

Returns a string containing a raw binary representation of the derived key (also known as output keying material - OKM); or `FALSE` on failure.

### Errors/Exceptions

An `E_WARNING` will be raised if `ikm` is empty, `algo` is unknown/non-cryptographic, `length` is less than 0 or too large (greater than 255 times the size of the hash function).

### Examples

Example #1 hash_hkdf() example

`<?php
// Generate a random key, and salt to strengthen it during derivation.
$inputKey = random_bytes(32);
$salt = random_bytes(16);
// Derive a pair of separate keys, using the same input created above.
$encryptionKey = hash_hkdf('sha256', $inputKey, 32, 'aes-256-encryption', $salt);
$authenticationKey = hash_hkdf('sha256', $inputKey, 32, 'sha-256-authentication', $salt);
var_dump($encryptionKey !== $authenticationKey); // bool(true)
?>`

The above example produces a pair of separate keys, suitable for creation of an encrypt-then-HMAC construct, using AES-256 and SHA-256 for encryption and authentication respectively.

### See Also

- hash_pbkdf2() - Generate a PBKDF2 key derivation of a supplied password
- » RFC 5869
- » userland implementation