COMPUTER PROTECTION 17 CRYPTOGRAPHY Answer these questions. a. How often do you change your passwords? Do you use the same for every site? b. Do you use an antivirus software or a firewall regularly? What about your classmates? to craft: creare drawback: svantaggio to scramble: mescolare, codificare Caesar Cipher Caesar Cipher is one of the earliest systems of encryption. It is a type of substitution cipher in which each letter in the plaintext is shifted of a certain number of places down the alphabet. For example, with a shift of 1, A would be replaced by B, B by C and so on. Caesar used this method to communicate with his generals. Unfortunately, the system can be easily broken even by hand. Cryptography is the study and practice of hiding information so that only those for whom it is intended can read and process it. In today s computercentric world, 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, 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. 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. There are two types of cryptography according to the type of key: public key or asymmetrical cryptography 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. private/secret key or symmetrical cryptography 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. Digital signatures are another 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. 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 Symmetric-key encryption can be either stream of the information cannot deny at a later ciphers, i.e. the bytes of the stage his or her intentions in the creation message one at a time, or or transmission of the information block ciphers, i.e. a number authentication the sender and receiver of bits are encrypted as a can confirm each other s identity and the single unit. origin/destination of the information. A modern Caesar Cipher medallion 262 Module 6 FLOWCHARTING