Some pointing devices (e.g. gaming mice) can be set to send
mouse-movement events at an extremely high sample rate like 1kHz. This
apparently translates into Windows genuinely sending WM_MOUSEMOVE
messages at that rate. So if you're using one of those mice, then
PuTTYgen's mouse-based entropy collection system will fill its buffer
almost immediately, and give you no perceptible time to actually wave
the mouse around.
I think that in that situation, there's also likely to be a strong
correlation between the contents of successive movement events,
because human-controlled mouse movements aren't fractals - the more
you zoom in on a little piece of one, the more it starts to look more
and more like something moving in a straight line at constant speed,
because the deviations from that happen on a larger time scale than
you're seeing.
So this commit adds a rate limit, not on the _collection_ of the data
(we'll still take all the bits we can get, thanks!), but on the rate
at which we increment the _counter_ for how much entropy we think
we've collected so far. That way, the user still has to spend time
wiggling the mouse back and forth in a way that varies with muscle
motions and introduces randomness.
There's no point having a separate boolean flag. All we have to do is
remember to NULL out the strbuf point state->entropy when we free the
strbuf (which is a good idea in any case!), and then we can use the
non-NULL-ness of that pointer as the indicator that we're currently
collecting entropy.
This offloads the memory management into centralised code which is
better at it than the ad-hoc code here. Now I don't have to predict in
advance how much memory the entropy will consume, and can change that
decision on the fly.
If you don't, they are permanently leaked. A user points out that this
is particularly bad in Pageant, with the new named-pipe-based IPC,
since it will spawn an input and output I/O thread per named pipe
connection, leading to two handles being leaked every time.
This gets rid of all those annoying 'win', 'ux' and 'gtk' prefixes
which made filenames annoying to type and to tab-complete. Also, as
with my other recent renaming sprees, I've taken the opportunity to
expand and clarify some of the names so that they're not such cryptic
abbreviations.
