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I've run into an expression: $x_i \in_R \mathbb{Z}_q$ – and I wonder what this means.

An example paper is here, here's example in Wikipedia.

Can anybody help me? Thanks in advance.

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If no one answers, you may consider providing more context, or even a reference. – GEdgar Sep 26 '13 at 17:41
Oh, sure. References are added. The topic is cryptography. – Andriy Makukha Sep 26 '13 at 18:15
Just a guess: $\in_R$ means chosen randomly. – RghtHndSd Sep 26 '13 at 18:16

$x \in_R S$ means $x$ is chosen uniformly randomly from $S$. This notation is common in cryptography papers. For example, this notation is defined in the beginning of section 2 of Proofs that yield nothing but their validity and a methodology of cryptographic protocol design.

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