Main

cPr_uniform_fiber

Section conditional_probability
Type Lemma
Proof Lines 39

Signature

Lemma cPr_uniform_fiber (cond : CondT) (v : msg * msg) : `Pr[CondRV = cond] != 0 -> v \in fiber cond -> `Pr[VarRV = v | CondRV = cond] = #|fiber cond|%:R^-1.

Description

* Uniform conditional probability over fiber. Pr[VarRV = v | CondRV = cond] = |fiber(cond)|^-1 When: 1. VarRV is uniform over msg × msg 2. VarRV is independent of InputRV 3. The conditioning event has positive probability 4. v is in the fiber of cond This is the key lemma for deriving entropy bounds in protocols where the constraint creates a fiber structure.

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