I feel kind of divided about using the Cryptocat as an example. On the one hand, yes, off-by-one errors (and similar simple mistakes) are really easy to make if

I feel kind of divided about using the Cryptocat as an example. On the one hand, yes, off-by-one errors (and similar simple mistakes) are really easy to make if you you're writing something with low feedback. But the Cryptocat example is frustrating because the code is so bad. Not in a "poorly written" sense, but in a "going about things in a completely insane way" sense. The code generates a random number between 0 and 1 not by dividing a 53 bit random number by 2^53, but by generating 16 decimal digits, with the rationale that 2^53 ≈ 10^16. If the code hadn't been trying to do the wrong thing in the first place, there wouldn't have been an opportunity for that off-by-one error. My point isn't that you'll be protected from simple classes of errors as long as you're coding sanely, but rather that that is the lesson you teach if you point to Cryptocat as your example. Crypto is hard, but you can't make that point very well by pointing to code written by someone who doesn't know the correct (and incredibly standard) solutions to common non-crypto problems.