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How can superencryption provide both privacy and authentication for a message?

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A Vigenere cipher table displays the alphabet in both the top row and left column, with rows shifted to create a grid for encryption and decryption. Highlighted letters illustrate how to use the table. An explanation and example using the words cipher, key, and plaintext appears to the right. — cryptology In encrypting plaintext, the cipher letter is found at the intersection of the column headed by the plaintext letter and the row indexed by the key letter. To decrypt ciphertext, the plaintext letter is found at the head of the column determined by the intersection of the diagonal containing the cipher letter and the row containing the key letter.

In the context of cryptology, superencryption—often referred to as superencipherment—is the process of applying multiple layers of encryption to a message. In an asymmetric (public-key) cryptosystem, this technique allows a sender to achieve both privacy and authentication simultaneously.12

To secure a message for a recipient (B) while ensuring the recipient can verify the sender (A), the following protocol is used:

Authentication: A first encrypts the message using their own secret key. Because this cipher can only be decrypted by anyone holding A's public key, it serves as a signature that confirms the message originated from someone possessing A's secret key.2
Privacy: A then superencrypts that resulting cipher using B's public key. This ensures that only B, who holds the corresponding secret key, can decrypt the outer layer to access the inner cipher.2

By combining these steps, the message remains private (only B can open the outer layer) and authenticated (only A could have created the inner layer), providing a robust paradigm for secure communication.2