Proving Identity Using a Private Key
Alice wishes to send a message to Bob. Alice has a private key and a public key (both are actually functions involving several digits, but the term key will suffice). Alice can send a message to Bob and prove her identity as follows.
Epvt(Alice){Message}
By encrypting the message using her private key, which only she possesses and which is protected on her computer, Alice can send a message that can be verified as coming from her. When Bob receives the message, he performs the following.
Dpub(Alice){Message}
If the result of the decryption operation is a meaningful message, then we can be assured that the message came from Alice. To be more exact, we can be assured that the message was encrypted using Alice’s private key. To be assured that the message came from Alice, we also have to trust that Alice’s key management system has protected her key from unauthorized use, and has validated her identity before allowing her to use the key.