code: func NewCryptoStore(s Store, encryptionCipher string, encryptionKey []byte) (*CryptoStore, error) {
  	code, mkh, err := createMasterKeyHash(encryptionCipher, encryptionKey)
  	if err != nil {
  		return nil, err
  	}
  	cs := &CryptoStore{
  		Store: s,
  		code:  code,
  		mkh:   mkh,
  	}
  	// On success, erase the key
  	for i := 0; i < len(encryptionKey); i++ {
  		encryptionKey[i] = 'x'
  	}
  	return cs, nil
  }
docs: // NewCryptoStore returns a CryptoStore instance with
  // given underlying store.