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Work Factor is defined as the minimum amount of work (can be the length of the key) to determine the secret key of an cryptosystem (HAC, Menezes, Alfred J. et al).

And One time pad have unconditional security or is perfectly secure, it means it is impossible to break even with unlimited processing power and memory (when correctly implemented, of course). How these definitions can be met? Because its possible to generate all possible combinations for an cypher text encrypted with OTP, but we can't isolate the correct one, we can say that OTP have an unlimited work factor?

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The OTP is not quite a cipher per the modern definition of that, which requires ability to encrypt several messages with the same key.

Also, Work Factor is defined only for feasible tasks.

Thus there's no tension between the true facts stated in the question.


Note: when the work factor to break a cipher is (proportional to) the key length $k$, the algorithm is extremely poor. Good encryption algorithms have work factor to break them with ordinary computers roughly proportional to $2^k$, where $k$ is expressed in bits.

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  • $\begingroup$ 1st para. re 'modern'. $\endgroup$
    – Paul Uszak
    May 7, 2022 at 7:37

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