Generally speaking, the goal of cryptography is the design of systems that withstand any malicious attempt to subvert them. The archetypical problem in cryptography is that of secret communication: two parties want to communicate with each other, and keep the conversation private, i.e., no one, other than the two legitimate parties, should be able to get any information about the messages being exchanged. This secrecy goal can be achieved if the two parties share a common random key that is known only to them. Then, in order to privately send a message, one can encode it using the key, and send the enciphered message to the other party over a public communication network. The receiver uses the shared key to invert the encoding procedure, and recover the original message. The original message, the enciphered message and the encoding and decoding processes are usually called cleartext, ciphertext, encryption and decryption.An encryption scheme is secure if recovering (any partial information about) the cleartext from the ciphertext without knowing the secret key is a computationally infeasible task. So, an adversary intercepting the ciphertext won’t learn anything about the message, other than the fact that a message was sent, and possibly the length of the message.
Unable to display preview. Download preview PDF.