“… Under certain conditions, a public key modulus will share a common factor with an existing modulus belonging to someone else. This may happen if both keys were generated on a system with a thoroughly-broken entropy source, or if a particular GPG implementation has been back-doored.

Submit a GPG public key to determine if any of its moduli share a common factor with with the moduli of public keys already in our database. The test is a simple application of Euclid’s GCD (Greatest Common Divisor) algorithm. You may wish to revisit your key’s page later, as more keys with potential common factors accumulate in the system…"