A bug in the Miller-Rabin primality test resulted in only a single random base being used instead of a sequence of such bases. This increased the probability that a non-prime would be accepted by is_prime or that a randomly generated prime might actually be composite. The probability of a random 1024 bit number being incorrectly classed as prime with a single base is around 2^-40. Introduced in 1.8.3, fixed in 1.10.8 and 1.11.9 Original bug report: https://bugzilla.novell.com/show_bug.cgi?id=974521