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Say that one uses a known LFG (2 known lags $a$ & $b$, known inner state length $N$, known modulo $m$) to hide some text with a basic à la Vigenère cipher: $C_i=(P_i + {PRNG}_i) \bmod 26$.

Say that the adversary intercepts some ciphered text (not the LFG output), could they use it to perform a correlation attack to get the initial inner state of the LFG? And if so, how much ciphertext would be needed?

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    $\begingroup$ Hi Peyre and welcome. Please try and format your question to your best abilities (check the edit by clicking edited xxx ago). That will make us take the question more seriously and others to take this site more seriously :) $\endgroup$
    – Maarten Bodewes
    Jun 6, 2015 at 14:22

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