29-09-2012, 02:43 PM
Basic Uses of Public Key Encryption and Digital Signatures
ABSTRACT
The RSA Algorithm is only one implementation of the more general concept of public key cryptography, which permits two parties who have never met and who can only communicate on an insecure channel to nonetheless send secure and verifiable messages to each other. The Internet as currently structured is an insecure communications channel with an obvious use for such technologies. Indeed, the greatest expected growth for public key techniques is in Internet-related communications.
With public key techniques, each user has two different keys, one made available to the public and the other kept secret.[4] One of the keys is used to encrypt a message, and the other is used to decrypt the message. If Alice wants to send a secret message to Bob, for example, she looks up Bob's public key and uses it to encrypt the message. Because Bob's public key cannot undo the encryption process, no one who intercepts the message can read it. Only Bob, who possesses the secret key corresponding to his public key, can read the message. Alice never has to meet Bob out of the hearing of others to exchange keys or passwords; this is a substantial improvement over older encryption methods in which an exchange of private keys was necessary.
This system can also be used as a means for Bob to be sure a message comes from Alice. If Alice wants to sign a message, she can encrypt it with her private key.[5] When Bob receives an encrypted message which purports to be from Alice, he can obtain Alice's public key and decrypt the message. If a readable message emerges, Bob can have confidence that the message came from Alice, because Alice's public key would only properly unlock a message which was locked with her private key (known only to Alice).[6]
Of course, digitally signing the message does not make the content of the message private, because anyone with Alice's public key can read a message she encrypted with her private key. Alice can send a private, signed message to Bob, however, by first encrypting the message with Bob's public key (so only Bob can read it with his private key) and then encrypting the message a second time with her private key, forming her signature.