Cross Block Chaining (XBC) Submission To NIST For A General Block Cipher Mode Of Operation

Cross Block Chaining (XBC) is designed to be similar to the classic standard modes of operation, specifically CBC, except that it operates using two IVs that “collect ” data through the course of the encryption process in a way that resembles CFB and OFB operation. This has two main net effects. Firstly, for very short messages, the keys can be split to prevent decryption by a single person, effectively creating a two-man rule [1]. Secondly, because the mode crosses itself, the entire previous message is necessary to decrypt the remaining blocks, which insures extra security in that the entire message must be decrypted from the beginning with no errors. This does, however, mean that any error will propagate through the entire message. Two versions of this algorithm are presented with what is a minute difference in operations. Each version has different implications and will be discussed inline. An interesting feature for both versions is that technically only one vector can be necessary to decrypt the majority of a longer message. While this would break any sort of two-man rule for longer messages, it still creates another layer of security through obfuscation by creating another mode of operation which would need to be tested against in an attack. Further, with two positions that a compromised vector could occupy, this increases the challenge of any attack. It is also possible that the same vector can be used for both IVs, which is another option to consider. 1.1 Algorithm XBC-1 The first version of XBC is quite similar to CBC, but with an additional IV that is XORed with the entry data into the block cipher, and this in turn is fed into the output of the block cipher of the next round. As can be seen in Figure 1, this creates the crossing between rounds of the block cipher that the algorithm is named after. 2 Figure 1: The block diagram of XBC-1 operation. The grey blocks are the underlying block cipher used which is not covered in this paper, as any block cipher may be used. The pseudo code description for XBC-1 is as follows