tailieunhanh - Using Modern Symmetric Key Ciphers

USE OF MODERN BLOCK CIPHERS Symmetric-key encipherment can be done using modern block ciphers. Modes of operation have been devised to encipher text of any size employing either DES or AES. Error Propagation A single bit error in transmission can create errors in several in the corresponding block. | Le Trong Ngoc letrongngoc@ Using Modern Symmetric-Key Ciphers 1 USING MODERN CIPHERS 1 USE OF MODERN BLOCK CIPHERS 2 USE OF MODERN STREAM CIPHERS 1 USE OF MODERN BLOCK CIPHERS Symmetric-key encipherment can be done using modern block ciphers. Modes of operation have been devised to encipher text of any size employing either DES or AES. 1 USE OF MODERN BLOCK CIPHERS Electronic codebook (ECB) mode Error Propagation A single bit error in transmission can create errors in several in the corresponding block. However, the error does not have any effect on the other blocks. 1 USE OF MODERN BLOCK CIPHERS Ciphertext Stealing A technique called ciphertext stealing (CTS) can make it possible to use ECB mode without padding. In this technique the last two plaintext blocks, PN−1 and PN , are encrypted differently and out of order, as shown below, assuming that PN−1 has n bits and PN has m bits, where m ≤ n . 1 USE OF MODERN BLOCK CIPHERS Cipher block chaining (CBC) mode Error . | Le Trong Ngoc letrongngoc@ Using Modern Symmetric-Key Ciphers 1 USING MODERN CIPHERS 1 USE OF MODERN BLOCK CIPHERS 2 USE OF MODERN STREAM CIPHERS 1 USE OF MODERN BLOCK CIPHERS Symmetric-key encipherment can be done using modern block ciphers. Modes of operation have been devised to encipher text of any size employing either DES or AES. 1 USE OF MODERN BLOCK CIPHERS Electronic codebook (ECB) mode Error Propagation A single bit error in transmission can create errors in several in the corresponding block. However, the error does not have any effect on the other blocks. 1 USE OF MODERN BLOCK CIPHERS Ciphertext Stealing A technique called ciphertext stealing (CTS) can make it possible to use ECB mode without padding. In this technique the last two plaintext blocks, PN−1 and PN , are encrypted differently and out of order, as shown below, assuming that PN−1 has n bits and PN has m bits, where m ≤ n . 1 USE OF MODERN BLOCK CIPHERS Cipher block chaining (CBC) mode Error Propagation In CBC mode, a single bit error in ciphertext block Cj during transmission may create error in most bits in plaintext block Pj during decryption. 1 USE OF MODERN BLOCK CIPHERS Ciphertext Stealing The ciphertext stealing technique described for ECB mode can also be applied to CBC mode, as shown below. The head function is the same as described in ECB mode; the pad function inserts 0’s. 1 USE OF MODERN BLOCK CIPHERS Encryption in cipher feedback (CFB) mode In CFB mode, encipherment and decipherment use the encryption function of the underlying block cipher. 1 USE OF MODERN BLOCK CIPHERS CFB as a Stream Cipher 1 USE OF MODERN BLOCK CIPHERS Encryption in output feedback (OFB) mode 1 USE OF MODERN BLOCK CIPHERS OFB as a Stream Cipher 1 USE OF MODERN BLOCK CIPHERS In the counter (CTR) mode, there is no feedback. The pseudorandomness in the key stream is achieved using a counter. 1 USE OF MODERN BLOCK CIPHERS Counter (CTR) mode as a stream cipher 1 USE OF MODERN BLOCK CIPHERS .