(This is only a concern for RSA keys for other key types, primes are either not secret or not involved.) If you use PuTTYgen to generate an RSA key on a computer that is potentially susceptible to timing- or cache-based side-channel attacks, such as a shared computer, the probable primes method is designed to resist such attacks, whereas the proven primes methods are not. There in one way in which PuTTYgen’s proven primes method is not strictly better than its probable primes method.
This takes more effort, but it eliminates that theoretical risk in the probabilistic method. The other methods cause PuTTYgen to use numbers that it is sure are prime, because it generates the output number together with a proof of its primality.
So, in practice, nobody worries about it very much.
#Executable key cracker software#
There is in theory a possibility that it might accidentally generate a number that isn’t prime, but the software does enough checking to make that probability vanishingly small (less than 1 in 2^80, or 1 in 10^24). The probable primes method sounds unsafe, but it’s the most commonly used prime-generation strategy.
If you don’t care about this, it’s entirely sensible to leave it on the default setting. The prime-generation method does not affect compatibility: a key generated with any of these methods will still work with all the same SSH servers. (The other key types don’t require generating prime numbers at all.) On the Key menu, you can also optionally change the method for generating the prime numbers used in the generated key.
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