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What property of a cryptographic function is described below?

B = F(A) A = F(B)

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    Self-invertibility.2011-03-08

2 Answers 2

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$F$ is an involution. You don't want this property for a block cipher since the function doesn't look "random". On the other hand, a stream cipher works exactly that way, but then it's a bit misleading to think of it as a function $F$: the function is missing an important parameter, namely time.

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    That's quite probable; in what way?2011-03-08
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Most mechanical cipher machines use a reciprocal cipher, so the machine doesn't need a separate "encode mode" and "decode mode". A reciprocal cipher is an involution.