Solutions For Encrypting The Final Block In A Variable Input Message In The Block Cipher Modes Of Operation

Abstract

Block cipher Modes of Operation for symmetric ciphers have been introduced to overcome the fixed size of the block ciphers. The objective of this work is to examine methods for encrypting the final block of a variable length message when block cipher modes of operation are used. Symmetric key ciphers like DES and AES encrypt data in fixed block sizes of 64 bits or 128 bits. When the Input data is not a multiple of the block size used by the encryption algorithm, the decryption algorithm cannot recover the plaintext since the plaintext input is not a complete block to the encryption algorithm. To solve this problem several approaches have been used including padding of the last block and the use of elastic block cipher design. This paper proposes including the number of bits as an additional parameter to the encryption procedure which is used to select the number of bits in the ciphe r text block during decryption. The two common approaches for encrypting the final block have their disadvantages, The padding approach when encrypted provides no information about the number of bits to be recovered during decryption. The elastic block cipher approach on the other hand performs encryption modifying the design of the encryption algorithm. The proposed approach does not modify the encryption procedure, but instead proposes to use the number of bits in the final block as a parameter to the encryption algorithm. By using the number of bits in the last block as a parameter, both the decryption module can recover the correct number of bits in the plaintext when decrypted. This method does not need any change to the internal implementation of the encryption algorithm and can correctly decrypt text and binary data when input to the encryption and decryption algorithm. We propose another hybrid method where block cipher encryption is used for all the blocks except the final one and a stream cipher encryption method is used for the final block. The use of random number generation algorithms for use as Initial Vector in the Block Cipher Modes of Operation to provide additional security to the algorithm is also discussed.