Private / Public key algorithms

[Hans M�ller]

Hello   Does it exist a private / public key algorithm that can generate smaller messages than 512 bits.   I need this for a challenge / response algorithm. The challenge code will be the encrypted with a private key (on a web server) and the encrypted challenge code will be the response code.   The user will enter the response code (in the software) and the software will decrypt the response code with the public key.   The problem is that haven found any public / private algorithms that generate smaller encrypted messages than 512 bits. And the user must be able to enter the response code in the software and this demands that the generated message (response code) must be less that 128 bits.   Best regards Hans M�ller