Whats your hardest factoring problem?

[jon virgi]

I was wondering what the hardest factoring problem you have come across. I will post a few difficult ones i have come across tommorrow. Lets see some tough factoring.

[rbayer]

How about finding prime factorizations for 128-bit numbers? That's a tough one, and would render RSA security nearly worthless if you find a way to do it quickly.