code: func (t *envelopeTransformer) addTransformer(encKey []byte, key []byte) (value.Transformer, error) {
  	block, err := aes.NewCipher(key)
  	if err != nil {
  		return nil, err
  	}
  	transformer := t.baseTransformerFunc(block)
  	// Use base64 of encKey as the key into the cache because hashicorp/golang-lru
  	// cannot hash []uint8.
  	t.transformers.Add(base64.StdEncoding.EncodeToString(encKey), transformer)
  	return transformer, nil
  }
docs: // addTransformer inserts a new transformer to the Envelope cache of DEKs for future reads.