I am reading the Wikipedia page for Interactive proof systems and am having trouble understand the notation in the definition of soundness, many of which is left unspecified.

Given a formal language of strings $L$, a verifier $\mathcal{V}$ for this language satisfies the soundness property if for every prover $(\tilde{\mathcal{P}})$ and every $y \notin \mathcal{L}$,

$$ \operatorname{Pr}[(\bot,(\text{accept})) \leftarrow (\tilde{\mathcal{P}})(y) \leftrightarrow (\mathcal{V})(y)] < \epsilon $$

for some small $\epsilon \ll 1$. What is meant by the use of $\leftarrow$ and $\leftrightarrow$, as well as $(\bot,(\text{accept}))$? I would appreciate if someone could answer this or point me to a reference that uses (and defines) the same notation.


1 Answer 1


This notation is pretty common for interactive protocols (but it can vary from paper to paper). I believe:

  • $\tilde{\mathcal{P}}(y)\leftrightarrow\mathcal{V}(y)$ denotes that $\tilde{\mathcal{P}}$ and $\mathcal{V}$ are interactive machines involved in an interactive protocol with common input the statement $y$ (an alternative is to use $\langle\tilde{\mathcal{P}},\mathcal{V}\rangle(y)$)
  • $\leftarrow$ usually denotes the randomised output that results from this interaction (the verifier and the prover could use random coins)
  • $\bot$ denotes that the (malicious) prover $\tilde{\mathcal{P}}$ has no (private) output at the end of the protocol (think of zero-knowledge protocols, e.g.)
  • $\text{accept}$ denotes that the verifier accepts at the end of the protocol (again, think of zero-knowledge protocols)

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