I'm looking for a way to sign a 624 bits message on 256bits (sig length = 256 bits) with an asymmetric keys system?
I'm trying to sign a qrcode so a client can be sure that this qrcode has been emitted by the server.
It should be very hard to generate a qr code without the private key. (It should take more than a month)
The client has a public key.
Is it possible to obtain a "secure" (Meaning I can't sign my own qr code under a month without knowing the private key) priv/pub key signature system on such a restricted space?