How is a digital signature used for authentication?

•    Suppose Alice wants to send a signed message to Bob. She creates a message digest by using a hash function on the message. The message digest serves as a "digital fingerprint" of the message; if any part of the message is modified, the hash function returns a different result. Alice then encrypts the message digest with her private key. This encrypted message digest is the digital signature for the message.

•    Alice sends both the message and the digital signature to Bob. When Bob receives them, he decrypts the signature using Alice's public key, thus revealing the message digest. To verify the message, he then hashes the message with the same hash function Alice used and compares the result to the message digest he received from Alice. If they are exactly equal, Bob can be confident that the message did indeed come from Alice and has not changed since she signed it. If the message digests are not equal, the message either originated elsewhere or was altered after it was signed.

•    Note that using a digital signature does not encrypt the message itself. If Alice wants to ensure the privacy of the message, she must also encrypt it using Bob's public key. Then only Bob can read the message by decrypting it with his private key.