# Security of Pohlig-Hellman exponentation cipher?

I am looking into implementing Pohlig-Hellman exponentation cipher and I would like to know how secure that algorithm is? I am guessing it's security relates greatly to the prime number used in it. Does anyone have some reference that might be helpful in determining the minimal prime number one should use in order to ensure the algorithm is safe enough for various applications?

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Looking at the requirements for the hardness of the discrete logarithm problem (DLP) would be a good start. Indeed, $p$ needs to be chosen with care, a large random prime simply won't do. Size is only half of the story. As in $\log{(p)}$, I mean. –  Thomas Mar 2 '13 at 22:58

You need a large random prime modulus where the discrete log is hard. Read about how to choose a prime so the discrete log is hard. Also, you want $p-1$ to have as few small factors as possible. Therefore, the short version is, I suggest you choose a large random 2048-bit prime $p$ such that $(p-1)/2$ is prime.