ccna at aiub

ccna at aiub.(false) Note: The real implementation of the XOR operation is beyond the scope of this course.BlockCipher Mode )ECB mode encrypts each 64-bit block of plaintext consecutively using the same 56-bit key. If two identical blocks of plaintext are encrypted using the same key, the resulting blocks of ciphertext are identical. Therefore, an attacker is able to identify similar or identical traffic passing through a channel. The attacker is then able to create a directory of messages without knowing the meaning of the traffic and to play them back later with a view to gaining unauthorized access to the system. For example, an attacker could unknowingly capture a login sequence protected by DES.ECB with administrator privileges and then replay it. Since this risk is not what we need, the CBC model was invented to eliminate this risk.202 Chapter 7 Cryptosystems In CBC mode, each 64-bit block of plaintext is subjected to a per-bit anomaly with the previous block of ciphertext and then encrypted using a DES key. The encryption of each block is dependent on the previous block. Encryption of the same 64-bit plaintext block can produce different ciphertext blocks.CBC mode can help protect against some attacks, but not against sophisticated cryptanalysis or extended brute-force attacks. Stream Cipher Mode To encrypt or decrypt more than 64 bits of data, DES uses two common stream cipher modes.Cipherfeedback (CFB), similar to CBC, can encrypt any number of bits, including single bits or individual characters.Outputfeedback (OFB), which generates a keystream block that is subsequently heterodyned with the plaintext block to obtain the ciphertext.In stream cipher mode, the previous ciphertext and encryption key are used to generate a pseudo-random stream of bits, which only the encryption key can generate. To encrypt the data, the data and the pseudo-random stream perform an iso-or operation by bit (or sometimes by byte) to get the ciphertext. The decryption process is the same. The receiver uses the secret key to generate the same pseudo-random stream, and the ciphertext is heterodyned with the pseudo-random stream to obtain the plaintext.7.3.2.3 DES SummaryHere are a few things to consider when protecting DES-encrypted data. Change the key frequently to help protect against brute force attacks.