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I have placed a bet that I can create a public key such that my adversary will not be able to crack (decrypt) it for at least one week. For my primes $p$ and $q$, I chose very large numbers that are $101$ digits long, each. I also tried to make them to be as distinct as possible.

Now what I am wondering is whether to choose a big encryption exponent or a small one. Since I have began my study of Cryptography about a month ago, I am not too familiar with choosing a solid encryption exponent. I heard that choosing the digits $3, 5, 17, 257$ or $65537$ would give me a good security but I have not read about it. I was thinking of going with $257$ because intuition tells me that there exists a inverse relation between the two: when choosing large primes for $p$ and $q$, then pick a small encryption exponent and vice versa. But I don't know if this makes complete sense in some regards.

Any input is appreciated.

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1 Answer 1

The primes $p$ and $q$ for RSA should be big, $p\pm1$ and $q\pm1$ should not have a simple factorization, $\frac pq$ should not be too close to a simple fraction and I think there are a few more caveats.

Then pick (public) $e$ and (private) $d$ with $ed\equiv 1\pmod {(p-1)(q-1)}$, which can be done by picking odd $e$ randomly in the full range and compute $d$ via Euclidean algorithm. I don't think that any choice of $e$ is better or worse than another. Of course $d$ should not be guessable, but if $e$ is perfectly random, so is $d$.

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