I'm not quite sure what you're asking here but note that 521 != 512, and the former is not a typo.

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

A zero bit is not a cryptographically random bit.

Here it is the difference between a bit that is part of a cryptographically-safe random number and just happens to be zero, but had equal chance of being one, and a bit that is zero every time because of the way the numbers are being generated.