# Primes in arithmetic progressions for some

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probability is acceptably small for most practical purposes. Primes in arithmetic progressions For some applications, one needs a prime number of a given bit-length k , but with additional special properties. One convenient property is that p - 1 should be divisible by a prime q of given length . So we want an algorithm takes as input k and , with ‘ < k , and outputs p and q such that p is a k -bit prime, q is an -bit prime, and p 1 (mod q ). One way to generate p and q is as follows: 68

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Step 1: Generate an -bit prime q , using an algorithm such as the “generate and test” algorithm discussed above. Step 2: Choose m at random from the interval I = { x Z : (2 k - 1 - 1) /q < x < (2 k - 1) /q } , set p = mq + 1 (which is a k -bit integer), and test if p is prime; if not, repeat this step; otherwise, output p and q . For what values of k and will this algorithm perform reasonably well? If we view as fixed and let k tend to infinity, then Dirichlet’s theorem on primes in arithmetic progressions tells us that for any -bit prime q , the probability that m chosen at random from I yields a prime is Θ(1 /k ). However, suppose we want to let both k and tend to infinity. Clearly, if k = + 1, for a given q of length , there is only one possible value for p , namely p = 2 q + 1. So if 2 q + 1 is not prime, the above algorithm will never terminate. But suppose that k and both tend to infinity, but we restrict so that it is not too big relative to k . For example, we may require that ‘ < k/ 3. In this case, it turns out that there is strong mathematical evidence (namely, the Generalized Riemann Hypothesis) that the probability that m chosen at random from I yields a prime is Θ(1 /k ). Thus, in this case it is reasonable to conjecture, and it is born out in practice, that Step 2 of the above algorithm terminates on average after Θ( k ) iterations. Sophie Germain primes Sometimes, one wants a prime p of a given length to satisfy a stronger property; namely, that p = 2 q + 1, where q is prime. Mathemeticians call the prime q in this case a “Sophie Germain” prime, while cryptographers call the prime p in this case a “strong” or “safe” prime. It is not known whether there exist an infinite number of strong primes. However, it is conjec- tured, and supported by experiment, that the probability that a random k -bit number is a strong prime is Θ(1 /k 2 ). The intuition is that a random number p of length k is prime with probability 1 /k , and for a random prime p , it does not seem unreasonable to believe that q = ( p - 1) / 2 is also prime with roughly the same probability. If we believe this conjecture, then a reasonable way to generate strong primes is the same “generate and test” procedure we used above; namely, generate a random k -bit number p , and test if both p and q = ( p - 1) / 2 are prime; if so, output p ; otherwise, repeat.
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