Public-key encryption can be applied in a unique way to ensure message authenticity rather than confidentiality. Consider a scenario where Bob wants to send a message to Alice. While the secrecy of the message isn't vital, Bob wants Alice to be certain that the message is genuinely from him. To achieve this, Bob encrypts the message using his private key. When Alice receives the encrypted message, she can decrypt it with Bob's public key. This confirms that Bob is indeed the originator of the message since only he possesses the private key required to encrypt it. Consequently, the entire encrypted message acts as a digital signature, ensuring both authenticity and integrity, as no one else could have created a ciphertext decryptable by Bob’s public key.
Although this method validates both the author and the message content, it involves encrypting the entire message, which can demand significant storage space. Documents must be stored in plaintext for regular use and in ciphertext for verification in dispute situations. A more storage-efficient approach is to encrypt a small data block derived from the document, known as an authenticator. This block should be such that modifying the document without also altering the authenticator is infeasible. By encrypting the authenticator with the sender’s private key, it acts as a signature verifying origin, content, and sequence. Secure hash codes, like SHA-1, can function as authenticators. This concept is illustrated in Figure 3.2b.
It's crucial to note that this encryption method does not secure the message from eavesdropping. While the message is protected against alterations, it isn't protected from being intercepted. Particularly with signatures derived from parts of the message, the remainder is transmitted openly. Even when the entire message is encrypted, confidentiality is not ensured, as anyone can use the sender's public key to decrypt the message.
0 Comments