Identification with Zero Knowledge Protocols

Modern cryptography is based on the secrecy of the key. In Secret-Key Cryptosystems, two parties have to meet and agree on a common secret key before any communication can happen. In Public-key Cryptosystems, each party has a pair of keys, one published in a database available to everybody and the other kept secret. This scheme eliminates the need for a preliminary secure interaction between the two parties. The strength of this scheme rests on the limited computational resources available to each user, legitimate or malicious. The main idea in any public key cryptosystem is a difficult computational problem. The security is based on the fact that the private key can be computed from the public key only by solving this difficult problem. With the public key, a user could encrypt messages, and another could decrypt them with the private key. The owner of the private key would be the only one who could decrypt the messages, but anyone knowing the public key could send them in privacy. The idea of proving knowledge of some assertion without revealing any information about the assertion itself is very attractive. Zero-Knowledge protocols allow this kind of scenario.