A somewhat disturbing trend has appeared in the low-end cost-sensitive PC SIMM market. Some supposedly 9-bit SIMMs are actually 8-bit SIMMs plus a parity generator. This means that the parity checking is essentially subverted, because the parity bit is generated from the stored contents of memory at read time, rather than the stored contents when it was written to. As such, NO bit errors are detected.
So why not do a cheap trick: After the small primes check calculate a CRC checksum over the number. Then do the primality check. If it is a prime, store it together with the CRC. The CRC can be checked for every use of the number. (PGP encrypts the secret key and therefore it generates a CRC for the encrypted packet. But this CRC is generate after the primality check.) We already had some SIMM modules with bit errors. They were detected by a parity check. If cheap pc simms don't have a real parity bit, the probability of having such a bug isn't as low as 10^-40. If cheap parity-less simms are available they get sold (cheap and expensive). Hadmut BTW: Some weeks ago they found motherbords with falsified cache rams: They had just the normal DIL-ICs and the normal pins, but the plastic of the ICs didn't contain a chip. The BIOS was modified to give out a message about a good cache ram check at boot time.