If usera sends a secure message to userb using an asymmetric cryptographic algorithm, and userb sends a reply, what key is used if usera wants another user, userc to read the reply message

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The answer is User Cs uses the Public Key.

Explanation:

Public-key cryptography refers to a cryptographic system that uses pairs of keys:  one that is a private keys,which are seen only by the use and public keys, which may be used widely by different users.

The generation of such keys depends on algorithms that are cryptographic   based on mathematical problems to provide function that is one way. A strong  security only needs keeping the private key private; the public key can be openly distributed among users without compromising security.

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