pbkdf2.Key
// Key derives a key from the password, salt and iteration count, returning a
// []byte of length keylen that can be used as cryptographic key. The key is
// derived based on the method described as PBKDF2 with the HMAC variant using
// the supplied hash function.
//
// For example, to use a HMAC-SHA-1 based PBKDF2 key derivation function, you
// can get a derived key for e.g. AES-256 (which needs a 32-byte key) by
// doing:
//
// dk := pbkdf2.Key([]byte("some password"), salt, 4096, 32, sha1.New)
//
// Remember to get a good random salt. At least 8 bytes is recommended by the
// RFC.
//
// Using a higher iteration count will increase the cost of an exhaustive
// search but will also make derivation proportionally slower.
func Key(password []byte, salt []byte, iter int, keyLen int, h func() hash.Hash) []byte