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Why must a public-key directory be authenticated?

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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.

A public-key directory must be authenticated to prevent impersonation and ensure the integrity of communications. In public-key cryptography, users publish their encryption keys in a directory so that others can send them private messages.1

If this directory is not authenticated, an attacker could substitute their own key for the intended recipient's key within a user's copy of the directory. Consequently, a sender (User A) could be tricked into encrypting a message with the attacker's (User C) key, believing they are communicating securely with the intended recipient (User B). Authentication ensures that the public key retrieved from the directory truly belongs to the person it claims to represent, thereby maintaining the security and reliability of the cryptosystem.12