Block Ciphers – Cipher Block Chaining (CBC) and Electronic Code Book (ECB)

Block Ciphers

Unlike block ciphers which are used to encrypt messages of predetermined block length, block cipher mode can encrypt a message which exceeds the fixed block size. The concept of block cipher mode is based on the block cipher. These encryption modes would prevent an intruder from reading the information but the intruder may make changes in the information which is equally unsafe. It is also unsecure as it gives the intruder an information of how, when and where the information is passed and also the information about the length of the message.

Cipher Block Chaining (CBC):

It is the frequently used block cipher mode. The encryption method follows a chain system, where the encryption of each block depends on its previous blocks. In this method each block of plaintext that is to be encrypted is XORed with the preceding block that has been encrypted (cipher text). Initially the first plaintext block to be encrypted uses an n-bit initialization vector (IV). Every message that has to be encrypted uses a different IV.

The succeeding text blocks are encrypted using its preceding encrypted block, and hence this ensures that even though the unencrypted message has the same matter, the encrypted form would be different. This method also provides high security by ensuring that no block could be decrypted without referring to its previous block. A corruption in a block would lead to the corruption of the entire message as the entire message encryption and decryption depends on its preceding block. 

Electronic Code Book (ECB):

To overcomes the problem of dependency of each block upon its previous block as in CBC. Hence every encrypted block is independent which makes decryption easier. The encryption method in ECB remains the same as in CBC, except for the chaining mechanism. Hence the task of encryption and decryption becomes easy. The only drawback of EBC is that two blocks of message which are identical would have the same encrypted data. Also ECB does not accept smaller blocks of data for encryption.

This Paper is written & submitted by Deepak V.

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