DES Output Feedback Mode

DES Output Feedback Mode

The output feedback (OFB) mode of DES, illustrated in Figure 6.6, can be used for key generation as well as for stream encryption. Notice that the output of each stage of operation is a 64-bit value, of which the s leftmost bits are fed back for encryption. Successive 64-bit outputs constitute a sequence of pseudorandom numbers with good statistical properties. Again, as with the approach suggested in the preceding subsection, the use of a protected master key protects the generated session keys.

ANSI X9.17 PRNG

One of the strongest (cryptographically speaking) PRNGs is specified in ANSI X9.17. A number of applications employ this technique, including financial security applications and PGP (the latter described in Chapter 15).

Figure 7.14 illustrates the algorithm, which makes use of triple DES for encryption. The ingredients are as follows:

• Input: Two pseudorandom inputs drive the generator. One is a 64-bit representation of the current date and time, which is updated on each number generation. The other is a 64-bit seed value; this is initialized to some arbitrary value and is updated during the generation process.
• Keys: The generator makes use of three triple DES encryption modules. All three make use of the same pair of 56-bit keys, which must be kept secret and are used only for pseudorandom number generation.
• Output: The output consists of a 64-bit pseudorandom number and a 64-bit seed value.
  Define the following quantities:

  DTi	Date/time value at the beginning of ith generation stage
  Vi	Seed value at the beginning of ith generation stage
  Ri	Pseudorandom number produced by the ith generation stage
  K1, K2	DES keys used for each stage