big problem of game server

std::string EncryptGamePacket(string str)
{
std::string encrypted_string;
std::vector<std::string> buffer;

buffer = split(str, ' ');
encrypted_string += buffer[0].length();

for (int i = 0 ; i < str.length(); i++)
{
if (i == buffer[0].length())
{
int size = str.length() - buffer[0].length();
encrypted_string += size;
}

encrypted_string += str[i] ^ 0xFF;
}

return encrypted_string += 0xFF;
}

Success Example (length:127 message):
Server sent Message:
11111111111111111111111111111111111111111111111111 11111111111111111111111111111111111111111111111111 111111111111111111111111111

Client receive Message:
11111111111111111111111111111111111111111111111111 11111111111111111111111111111111111111111111111111 111111111111111111111111111

Incorrect Example (length:128 message):
Server sent Message:
11111111111111111111111111111111111111111111111111 11111111111111111111111111111111111111111111111111 1111111111111111111111111111

Client receive Message:
88888888888888888888888888888888888888888888888888 88888888888888888888888888888888888888888888888888 8888888888888888888888888888