Computer Security and Cryptography

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

1. A nonsecret initial seed
2. A key and
3. A feedback parameter m with 1 ≤ m ≤ 64.

The key stream {z⁽ⁱ⁾: 1 ≤ i < ∞} is defined by

where Right_m and Left_m 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.

Figure 9.7 Output feedback mode.

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 ...