ABSTRACT
Public-key cryptosystems are invariably block ciphers, while secret-key cryptosystems can be block ciphers or stream ciphers. Thus, hybrid cryptography achieves the efficiency of secret-key cryptography, but it can be used in a situation where Alice and Bob do not have a previously determined secret key. Cryptographic hash functions are usually required to satisfy additional security properties. It should be emphasized that hash functions cannot be used for encryption, for two fundamental reasons. Cryptographic tools such as cryptosystems, signature schemes, hash functions, etc., can be used on their own to achieve specific security objectives. A fundamental goal for a cryptosystem, signature scheme, etc., is for it to be “secure.” Finally, unconditional security means that the cryptosystem cannot be broken, even with unlimited computational resources, because there is not enough information available to the adversary for them to be able to do this.
