9.8 DES IN THE OUTPUT-FEEDBACK MODE (OFB)
DES may be used to generate a key stream to be XORed to plaintext. DES is the output feedback mode (OFB) (Fig. 9.7) and starts with
- A nonsecret initial seed
- A key and
- A feedback parameter m with 1 ≤ m ≤ 64.
The key stream {z(i): 1 ≤ i < ∞} is defined by
where Rightm and Leftm take the rightmost and leftmost m bits of w:
is XORed to plaintext to create the ciphertext.
When m = 64, the output-feedback mode mapping depicted in Fig. 9.7 is a one-to-one mapping of onto itself. The average cycle length is 263.
When m < 64, the OFB mapping is not one to one and its cycle length is O(232), an observation first made by Davies and Parkin [1982]. This means that in a large ciphertext file with m = 1, we are likely to see the same key bit used to encipher different bits of the plaintext. And why should any value of ...
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