In number theory, a prime number p is a Sophie Germain prime if 2p + 1 is also prime. The number 2p + 1 associated with a Sophie Germain prime is called a safe prime. For example, 11 is a Sophie Germain prime and 2 × 11 + 1 = 23 is its associated safe prime. Sophie Germain primes and safe primes have applications in public key cryptography and primality testing. It has been conjectured that there are infinitely many Sophie Germain primes, but this remains unproven.

Sophie Germain primes are named after the French mathematician Sophie Germain, who used them in her investigations of Fermat’s Last Theorem. One attempt by Germain to prove Fermat’s Last Theorem was to let p be a prime number of the form 8k + 7 and to let n = p – 1. In this case, xⁿ + yⁿ = zⁿ is unsolvable. Germain’s proof, however, remained unfinished. Through her attempts to solve Fermat’s Last Theorem, she proved a result now known as Sophie Germain’s theorem, which states that if p is an odd prime and 2p + 1 is also prime, then p must divide x, y, or z. Otherwise, xⁿ + yⁿ ≠ zⁿ. This case where p does not divide x, y, or z is called the first case. Sophie Germain’s work was the most progress achieved on Fermat’s last theorem at that time. Later work by Kummer and others always divided the problem into first and second cases.