Plot twist: The choice of 521 is adversarial specifically to exploit PuTTY without being easily noticed. (Ok I get that 2^521-1 is a mersenne prime).

maybe 2^521-1 being a mersenne prime is adversarial

only in base 10 :)

I considered making that joke, that the established common base was picked adversarially to have 521 and 512 be confusingly similar but couldn't find a non convoluted way of expressing that without implying primes change with base