A permutation block (P-box) in a modern block cipher is an example of a keyless transposition cipher. What does this statement mean? (See Figure 31.8 in the text.)

Figure 31.8

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2 3. 2 Straight permutation Compression permutation Expansion permutation Transposition п bits n bits b7 b6 bs b4 b3 b2bi bo A predefined Key (n bits) function Shift left (3 bits) b4 b3 b2 b1 bo b7 b6 b5 m bits n bits Substitution Exclusive-OR Shift b7 b6 bsb4 b3b2bi bo b7 b6 bs b4 b3 b2 bi bo b7 bg bs b4 b3|b2b| bo bbb bob7 be bs b4 b7 b6 bs b4 b3 b2 b1 bo b7 b6 bs b4 b3 b2b bo Swap Split Combine