code: func (t *envelopeTransformer) getTransformer(encKey []byte) value.Transformer {
  	_transformer, found := t.transformers.Get(base64.StdEncoding.EncodeToString(encKey))
  	if found {
  		return _transformer.(value.Transformer)
  	}
  	return nil
  }
docs: // getTransformer fetches the transformer corresponding to encKey from cache, if it exists.