I'm pretty familiar with using ssh-keygen to create groups that go in the /etc/ssh/moduli file for the Diffie-Hellman Group Exchange in openssh. Reading over the man page, it says "By default, each candidate will be subjected to 100 primality tests. This may be overridden using the -a option." I'm making an educated guess that the tests being performed are some non-deterministic test for primality (Miller–Rabin?), although the man page doesn't seem to say. Experimenting with that -a option shows that 4 is the lowest that ssh-keygen will accept as an argument for -a.
what I'm wondering what the chances are of getting a composite number (or a non-safe prime number, or some other problematic situation) with values like 4 or 100 for -a? Is this on the order of a 1 in 100 risk, or 1 in a million risk?