# Length of a Challenge in Challenge and Response Protocol

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:

1. $$B$$ wants to access some service, that $$A$$ provides.
2. $$A$$ would like to know whether $$B$$ is authorized. Therefore, $$A$$ generates a challenge ch and sends it over to $$B$$.
3. $$B$$ is generating a response, by signing the ch with its private key and sending the signature over to $$A$$.
4. $$A$$ 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?

Bitcoin paper wallet and Bitcoincash withdrawal?

Let’s say I have a bitcoin paper wallet with a public and a private key, with an amount of X bitcoin on it. On the 25th of November, the hardfork of bitcoin gold will take place, so I will get the same amount of bitcoin gold.

The question is, how I will be able to withdraw only bitcoin gold from that paper wallet as I have only one public and private key as they are bound to a bitcoin wallet?

#### Length of a Challenge in Challenge and Response Protocol

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