code: func newKeyInfo(subscriptionPublicKey, serverPublicKey []byte) []byte {
  	info := make([]byte, len(keyInfoPrefix)+len(subscriptionPublicKey)+len(serverPublicKey))
  	n := copy(info, keyInfoPrefix)
  	n += copy(info[n:], subscriptionPublicKey)
  	copy(info[n:], serverPublicKey)
  	return info
  }
docs: // Returns a keying material. See sections of
  // https://tools.ietf.org/html/draft-ietf-webpush-encryption-07#section-3.3