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kira avatar Crypto

Find a prime $p$ vulnerable to pohlig-Hellman

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kira

I need to find a prime number $p$ with the following constraints:

  • $p$ is at least $1000$ bits long
  • $p-1$ is a smooth number with the largest factor below $1000$
  • any factor of $p-1$ can be present multiple times

Does this number exist? and if yes, does there is an algorithm to find it?

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fgrieu avatar
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fgrieu
Welcome to crypto-SE. This looks like homework, thus I'll only give a (strong) hint: propose a simple algorithm that constructs randomly-seeded $r$ with the characteristics asked for $p-1$ (including size), ignoring for now the requirement that $p$ is prime; then derive using the [Prime Number Theorem](https://en.wikipedia.org/wiki/Prime_number_theorem) a plausible lower bound for the probability that $p=r+1$ is prime, and from that a rough plausible higher bound of the expected cost of the now obvious probabilistic algorithm. It's smart to answer your own question.
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