# Key Derivation Functions

Key derivation functions are used to turn some amount of shared secret material into uniform random keys suitable for use with symmetric algorithms. An example of an input which is useful for a KDF is a shared secret created using Diffie-Hellman key agreement.

class `KDF`
  secure_vector<uint8_t> `derive_key`(size_t key_len, const std::vector<uint8_t> &secret, const std::string &salt = "") const
  secure_vector<uint8_t> `derive_key`(size_t key_len, const std::vector<uint8_t> &secret, const std::vector<uint8_t> &salt) const
  secure_vector<uint8_t> `derive_key`(size_t key_len, const std::vector<uint8_t> &secret, const uint8_t *salt, size_t salt_len) const
  secure_vector<uint8_t> `derive_key`(size_t key_len, const uint8_t *secret, size_t secret_len, const std::string &salt) const

    All variations on the same theme. Deterministically creates a uniform random value from secret and salt. Typically salt is a label or identifier, such as a session id.

You can create a `KDF` using

KDF *`get_kdf`(const std::string &algo_spec)

## Available KDFs

Botan includes many different KDFs simply because different protocols and standards have created subtly different approaches to this problem. For new code, use HKDF which is conservative, well studied, widely implemented and NIST approved.

### HKDF

Defined in RFC 5869, HKDF uses HMAC to process inputs. Also available are variants HKDF-Extract and HKDF-Expand. HKDF is the combined Extract+Expand operation. Use the combined HKDF unless you need compatibility with some other system.

Available if `BOTAN_HAS_HKDF` is defined.

### KDF2

KDF2 comes from IEEE 1363. It uses a hash function.

Available if `BOTAN_HAS_KDF2` is defined.

### KDF1-18033

KDF1 from ISO 18033-2. Very similar to (but incompatible with) KDF2.

Available if `BOTAN_HAS_KDF1_18033` is defined.

### KDF1

KDF1 from IEEE 1363. It can only produce an output at most the length of the hash function used.

Available if `BOTAN_HAS_KDF1` is defined.

### X9.42 PRF

A KDF from ANSI X9.42. Sometimes used for Diffie-Hellman.

Available if `BOTAN_HAS_X942_PRF` is defined.

### SP800-108

KDFs from NIST SP 800-108. Variants include “SP800-108-Counter”, “SP800-108-Feedback” and “SP800-108-Pipeline”.

Available if `BOTAN_HAS_SP800_108` is defined.

### SP800-56A

KDF from NIST SP 800-56A.

Available if `BOTAN_HAS_SP800_56A` is defined.

### SP800-56C

KDF from NIST SP 800-56C.

Available if `BOTAN_HAS_SP800_56C` is defined.