code: func appendHeader(salt, serverPublicKey, ciphertext []byte) []byte {
  	var result []byte
  	// A push service is not required to support more than 4096 octets of
  	// payload body so the record size can be at most 4096.
  	rsbuf := make([]byte, 4)
  	binary.BigEndian.PutUint32(rsbuf, uint32(maxPayloadRecordSize))
  	idlen := uint8(len(serverPublicKey))
  
  	result = append(result, salt...)
  	result = append(result, rsbuf...)
  	result = append(result, byte(idlen))
  	result = append(result, serverPublicKey...)
  	result = append(result, ciphertext...)
  
  	return result
  }
docs: // Returns an 86 octet header. See section 2.1 of:
  // https://tools.ietf.org/html/draft-ietf-httpbis-encryption-encoding-06#section-2.1