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I'm currently looking at a compsci text containing the following definition:

Let $f : \{0,1\}^k \rightarrow \{0,1\}^k$ and $b : \{0,1\}^k \rightarrow \{0,1\}$ be computable in poly($k$) time. We say that $f$ is a one-way function with hard-core bit $b$ if, for all polynomial-time randomized algorithms $A$ and all constants $c$, $$ \underset{x\in\{0,1\}^k}{\mathbb{E}} \text{Pr}[A(f(x))=b(x)] = \frac{1}{2}+o(k^{-c}) $$

I find the left-hand side quite disorienting. If I were to unwrap the notation I'd arrive at something like this:

$$ \sum_{x\in\{0,1\}^k}\text{Pr}\big[\{x\in\{0,1\}^k\big|A(f(x))=b(x)\}\big]\cdot2^{-k} $$

The $x$ is bound twice?

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The randomness in the $\Pr$ is over the behavior of $A$, while the randomness in the expectation is over the choice of $x$.

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  • $\begingroup$ Right, that makes sense now that I think about it. $\endgroup$ – Sebastian Oberhoff Mar 13 '18 at 18:26

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