ccna at a glance

ccna at a glance.The outgoing data generated by a block cipher is usually larger than the input data, because the ciphertext must be a multiple of the block size. For example, DES encryption uses a 56-bit key to encrypt a block of data into a 64-bit chunk. o To achieve this, the block algorithm takes the data one block at a time, e.g., 8 bytes per block, until the entire block size is reached. If the input data is less than a full chunk, the algorithm adds false data or spaces until the 64 bits are completely used up.Common block ciphers include DES, which uses a 64-bit block size, AES, which uses a 128-bit block size, and RSA, which has a variable block size. stream cipherUnlike block ciphers, stream ciphers encrypt only one byte of plaintext at a time or -. bit at a time. Think of a stream cipher as a block cipher with a block size of I bits. With a stream cipher, the transformation of these smaller plaintext units varies depending on when they occur during the encryption process. Stream ciphers can be much faster than block ciphers and generally do not increase the message size, since they can encrypt an arbitrary number of bits.The Virginia cipher is an example of a stream cipher. Such ciphers are periodic because the key length is limited and the key is repeated again if the key length is shorter than the message length. Common stream ciphers include the A5 and RC4 ciphers, where A5 is used to encrypt GSM cellular communications. DES can also be used in stream cipher mode.7.3.1.5 Choosing an encryption algorithmChoosing an encryption algorithm is one of the most important decisions a security practitioner will make in building a cryptosystem. Two main criteria should be considered when selecting an encryption algorithm for an enterprise organization. The algorithm is trusted by the cryptographic community. Most new algorithms are quickly cracked, so preference should be given to algorithms that have successfully resisted attacks for several years. Inventors and advocates often exaggerate the benefits of new algorithms. Algorithms can adequately resist brute-force attacks. A good cryptographic algorithm should be designed to resist common cryptographic attacks. The best way to break into data protected by an algorithm is to try to decrypt the data using all possible keys. The time required for this type of attack depends on the number of possible keys, but it is usually a long period of time. With a key of suitable length, this type of attack is often considered infeasible. If the algorithm is considered trustworthy and no shortcut can break it, the attacker must search the key space to guess the correct key. The algorithm must allow the key length to meet an organization's confidentiality needs. For example, DES does not provide sufficient protection for most modern needs because its keys are short.7.3 Confidentiality 201Other criteria to consider are as follows.The algorithm supports variable key yields and long key lengths as well as scalability. Variable key length and scalability are also properties that a good encryption algorithm should have. The longer the encryption key, the longer it will take for an attacker to break it. For example, a 16-bit key has 65536 possibilities, while a 56-bit key has 7.2x]016 possibilities. Scalability provides flexible key lengths, allowing administrators to choose the desired encryption strength and speed.