BLOCK CIPHER CODE

In cipher feedback mode, the ciphertext output of the previous block is encrypted before being XORed onto the plaintext block. IPsec specifies it as the only permitted mode. Authentication of the packet or message can prevent such problems if decryption is only applied to data that has passed authentication,Ĭipher block chaining is much the most widely used mode. However, loss of synchronisation is fatal if even a single bit is dropped or added, then the affected block and all that follow it will be garbled. CBC is self-recovering against bit-flipping errors.

A bit error in an encrypted block, such as might be caused by line noise, will cause the decryption of that block and the next to be garbled, but later blocks will not be affected. This is an unnecessary weakness using unique IVs is therefore standard practice.ĬBC mode makes the encryption of any block depend on all blocks previously encrypted. If the same IV is repeatedly used, then if two or more messages start with the same text, they will encrypt identically for the first block or the first few blocks. This need not be secret, but it must be different for each message and should be random. Encryption of block n is then:įor this to work for n=1, an initialisation vector (IV) must be provided to act as c 0. In cipher block chaining mode, the ciphertext output from the previous block is XORed into the plaintext before encryption.

BLOCK CIPHER CODE

If enough blocks are encrypted this way, the system becomes vulnerable to a code book attack. The disadvantage is that the same plaintext block always encrypts to the same ciphertext this gives an enemy some information. In Electronic Code Book mode, the cipher is just applied to each block of plaintext independently.

See the next section for modes which provide authentication as well. In some modes, the cipher is used only for data secrecy. The most recent NIST recommendations are in 'Recommendation for Block Cipher Modes of Operation' Īny mode can be applied with any block cipher. Modes were originally defined for the Data Encryption Standard in a US Federal Information Processing Standard (FIPS). Using the wrong mode for the task at hand may give an insecure system even if the cipher itself is secure. A block cipher mode of operation specifies how multiple block cipher operations are to be combined to accomplish some larger task such as encrypting a message or providing a pseudorandom number generator. A block cipher provides a way to encrypt blocks of plaintext to yield blocks of ciphertext.

Block cipher