Lets consider a challenge and response protocol with the following setup.
Two entities $A$ and $B$. $A$ is generating the challenge and $B$ is generating the response using a digital signature scheme. Lets suppose, that $A$ already has the certificate of $B$. So the procedure is as follows:
- $B$ wants to access some service, that $A$ provides.
- $A$ would like to know whether $B$ is authorized. Therefore, $A$ generates a challenge ch and sends it over to $B$.
- $B$ is generating a response, by signing the ch with its private key and sending the signature over to $A$.
- $A$ verifies the received response using the public key inside the available certificate.
This is just a common challenge and response procedure.
Question: How long shall the challenge be? If the challenge is to small, it repeats in a short amount of time. Are there any common methods that are "good-practice" in order to determine a suitable length?
Does this have anything to do with the security level of 128-bit that shall be satisfied in any case?