Here is the paragraph I can't quite get my head around.
Alice sends a message to Bob and wants him to be sure the message has come from her. • She publishes her public key. • She computes a message digest • She encrypts the digest using her private key • She sends the message and digest to Bob • Bob decrypts the digest using Alice's public key • Bob Creates a message digest from the message using the same algorithm as Alice • Compares this with the decrypted digest received from Alice • If they match we have established two things – The message must have come from Alice - Authenticity – The message has not been altered - Integrity
I understand private and public keys and how they work, also I know that you could add Bobs public key to the entire message at the start for confidentiality. But what I can't get my head around is why Bob had to create the digest, I thought the fact that he could decrypt the digest itself wouldn't that give him the original message? What am I missing. thank you in advance