code: func deriveKey(passphrase string, salt []byte) [keyLength]byte {
  	secretKeyBytes := argon2.IDKey([]byte(passphrase), salt, 4, 64*1024, 4, 32)
  	var secretKey [keyLength]byte
  	copy(secretKey[:], secretKeyBytes)
  	return secretKey
  }
docs: // parameters chosen as per https://godoc.org/golang.org/x/crypto/argon2#IDKey