code: func newInfo(infoType string, context []byte) []byte {
  	info := append([]byte("Content-Encoding: "), []byte(infoType)...)
  	info = append(info, 0)
  	// For aes128gcm ctx is not needed.
  	if len(context) > 0 {
  		info = append(info, []byte("P-256")...)
  		info = append(info, context...)
  	}
  	return info
  }
docs: // Returns an info record. See sections 3.2 and 3.3 of
  // https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-00.
  // The context argument should match what newContext creates