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# 9 Key Derivation

Libgcypt provides a general purpose function to derive keys from strings.

- Function:
*gpg_error_t***gcry_kdf_derive***( const void **`passphrase`, size_t`passphraselen`, int`algo`, int`subalgo`, const void *`salt`, size_t`saltlen`, unsigned long`iterations`, size_t`keysize`, void *`keybuffer`) -
Derive a key from a passphrase.

`keysize`gives the requested size of the keys in octets.`keybuffer`is a caller provided buffer filled on success with the derived key. The input passphrase is taken from`passphrase`which is an arbitrary memory buffer of`passphraselen`octets.`algo`specifies the KDF algorithm to use; see below.`subalgo`specifies an algorithm used internally by the KDF algorithms; this is usually a hash algorithm but certain KDF algorithms may use it differently.`salt`is a salt of length`saltlen`octets, as needed by most KDF algorithms.`iterations`is a positive integer parameter to most KDFs.On success 0 is returned; on failure an error code.

Currently supported KDFs (parameter

`algo`):`GCRY_KDF_SIMPLE_S2K`

The OpenPGP simple S2K algorithm (cf. RFC4880). Its use is strongly deprecated.

`salt`and`iterations`are not needed and may be passed as`NULL`

/`0`

.`GCRY_KDF_SALTED_S2K`

The OpenPGP salted S2K algorithm (cf. RFC4880). Usually not used.

`iterations`is not needed and may be passed as`0`

.`saltlen`must be given as 8.`GCRY_KDF_ITERSALTED_S2K`

The OpenPGP iterated+salted S2K algorithm (cf. RFC4880). This is the default for most OpenPGP applications.

`saltlen`must be given as 8. Note that OpenPGP defines a special encoding of the`iterations`; however this function takes the plain decoded iteration count.`GCRY_KDF_PBKDF2`

The PKCS#5 Passphrase Based Key Derivation Function number 2.

`GCRY_KDF_SCRYPT`

The SCRYPT Key Derivation Function. The subalgorithm is used to specify the CPU/memory cost parameter N, and the number of iterations is used for the parallelization parameter p. The block size is fixed at 8 in the current implementation.

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