Suppose only public-key cryptography can be used to encrypt the communications b

Question

Suppose only public-key cryptography can be used to encrypt the communications between Alice and Bob. Alice’s public key is eA, private key is dA; Bob’s public key is eB, private key is dB. Now Bob wants to send a message m to Alice. Describe which keys should be used to encrypt the message and decrypt the ciphertext.

Suppose only secret-key cryptography can be used to protect the communications between Alice and Bob. Suppose Alice and Bob already have a shared secret key k. Now Bob wants to send a message m to Alice. Which components should be included in the message to protect the integrity of the message?

Explanation / Answer

In public-key cryptography, if bob wants to send message m to Alice, Bob will encrypt that message using its private key dB and share that encrypted message to Alice, which will then decrypt that message using Bob's public key eB.

In secret-key cryptography, maintaining integrity is one of the most important property, so that no one can change the message on the fly. So, to maintain integrity, a hash of a message is also generated and send with the message. For more security purpose, a passphrase or salt can be generate and then added to the message before hasing. This salt or passphrase is common or shared between sender and reciever.

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