Saturday, October 15, 2016

If PRG is secure, PRG is unpredictable

Below is the definition of secure PRG.

Below is the definition of predictable PRG.

How do we proof the below theorem?

PRG is secure => PRG is unpredictable

The previous page proofed it by:

PRG is predictable => PRG is not secure

This pare proofs it in a direct way. Before proofing it, I  draw the below picture, where G is PRG, to understand the definition of secure PRG and unpredictable PRG.

We keep the picture in our mind and use simple formulas to proof it.

The fact is that G is secure:

Adv = |Pr [A (G) = 1] - Pr [A (r) = 1]| <= e (e is epsilon)

Because the statement, that is proofed by the page, is also true.

Pr [A (r) = 1] = 1/2


Pr [A (G) = 1] <= 1/2 + e

We can transfer the statement from A to B for the definition of A that is a statistical test.

Pr [B (G|1...i+1) = G|i+1] <= 1/2 + e

Therefore G is unpredictable.


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