Unformatted text preview: igure 9.12): Ci = Pi • Si; Si = EK(Si-1) Pi = Ci • S1; Si = EK(Si-1) Figure 9.11 8-bit output-feedback mode. Si is the state, which is independent of either the plaintext or the ciphertext. One nice feature of OFB mode is that most of the work can occur offline, before the plaintext message even exists. When the message finally arrives, it can be XORed with the output of the algorithm to produce the ciphertext. Initialization Vector
The OFB shift register must also be initially loaded with an IV. It should be unique but does not have to be secret. Error Propagation
OFB mode has no error extension. A single-bit error in the ciphertext causes a single-bit error in the recovered plaintext. This can be useful in some digitized analog transmissions, like digitized voice or video, where the occasional single-bit error can be tolerated but error extension cannot. On the other hand, a loss of synchronization is fatal. If the shift registers on the encryption end and the decryption end are not identical, then the recovered plaintext will be gibberish. Any system that uses OFB mode must have a mechanism for detecting a synchronization loss and a mechanism to fill both shift registers with a new (or the same) IV to regain synchronization. Figure 9.12 n-bit OFB with an n-bit algorithm. Security Problems with OFB
An analysis of OFB mode [588,430,431,789] demonstrates that OFB should be used only when the feedback size is the same as the block size. For example, you should only use a 64-bit algorithm in 64-bit OFB mode. Even though the U.S. government authorizes other feedback sizes for DES , avoid them. OFB mode XORs a keystream with the text. This keystream will eventually repeat. It is important that it does not repeat with the same key; otherwise, there is no security. When the feedback size equals the block size, the block cipher acts as a permutation of m-bit values (where m is the block length) and the average cycle length is 2m- 1. For a 64-bit block length, this is a...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips