# Property of a cryptographic function

What property of a cryptographic function is described below?

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

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Self-invertibility. –  Arturo Magidin Mar 8 '11 at 2:26

$F$ 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.