6.2 COMPUTER PROTECTION CRYPTOGRAPHY Keys are usually made up of 64 binary digits of which 56 are randomly generated and used directly by the algorithm, while the other 8 bits are used for error detection. Symmetric-key encryption can be either stream ciphers, i.e. the bytes of the message one at a time, or block ciphers, i.e. a number of bits are encrypted as a single unit. What is cryptography? Cryptography is the study and practice of hiding information so that only those for whom it is intended can read and process it. Cryptography is most often associated with scrambling an ordinary text, called plaintext or cleartext, into ciphertext through a process called encryption, and then back again through decryption. Therefore, encryption is the process of transforming information using an algorithm or cipher to make it unreadable to anyone except those possessing special knowledge, which is usually referred to as a key. The result of the process is encrypted data. Decryption is the opposite process, which takes place when the same key is used to convert the data back into its original form. The system is secure because the mathematical algorithm is not easily reversible . Types of cryptography There are two types of cryptography, which are classified according to the type of key used: to craft: creare drawback: svantaggio to scramble: mescolare, codificare public key or asymmetrical cryptography is a system that uses a pair of related keys: a recipient s public key, which is widely distributed, and a private or secret key which is known only to the recipient. Any person can encrypt a message using the public key of the recipient, but only the holder of the paired private key can decrypt the message encrypted with the public key. Digital signatures are an example of public key cryptography: in fact, a message signed with the sender s private key can be verified by anyone who has access to the sender s public key; private/secret key or symmetrical cryptography is a system that uses the same key for both encryption and decryption. The keys may be identical or there may be a simple transformation. However, the fact that both parties have access to the secret key is one of the main drawbacks of the system. Objectives of cryptography Modern cryptography has four objectives: confidentiality: the information cannot be understood by people for whom it was not intended; integrity: the information cannot be altered in storage or in transit between the sender and the intended receiver without the alteration being detected; non-repudiation: the creator or sender of the information cannot deny at a later stage their intentions in the creation or transmission of the information; authentication: the sender and receiver can confirm each other s identity and the origin/ destination of the information. 272 protecting computers