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Windows PuTTYgen: rate-limit entropy counter increment.

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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.
This commit is contained in:
Simon Tatham 2022-01-08 12:09:36 +00:00
parent e7a695103f
commit 5ad601ffcd
1 changed files with 25 additions and 2 deletions
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27
windows/puttygen.c
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@ -634,6 +634,7 @@ struct MainDlgState {
bool key_exists; bool key_exists;
int entropy_got, entropy_required; int entropy_got, entropy_required;
strbuf *entropy; strbuf *entropy;
ULONG entropy_prev_msgtime;
int key_bits, curve_bits; int key_bits, curve_bits;
bool ssh2; bool ssh2;
keytype keytype; keytype keytype;
@ -651,6 +652,23 @@ struct MainDlgState {
HMENU filemenu, keymenu, cvtmenu; HMENU filemenu, keymenu, cvtmenu;
}; };
/*
* Rate limit for incrementing the entropy_got counter.
*
* Some pointing devices (e.g. gaming mice) can be set to send
* mouse-movement events at an extremely high sample rate like 1kHz.
* In that situation, there's likely to be a strong correlation
* between the contents of successive movement events, so you have to
* regard the mouse movements as containing less entropy each.
*
* A reasonably simple approach to this is to continue to buffer all
* mouse data, but limit the rate at which we increment the counter
* for how much entropy we think we've collected. 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.
*/
#define ENTROPY_RATE_LIMIT 10 /* in units of GetMessageTime(), i.e. ms */
static void hidemany(HWND hwnd, const int *ids, bool hideit) static void hidemany(HWND hwnd, const int *ids, bool hideit)
{ {
while (*ids) { while (*ids) {
@ -1406,9 +1424,13 @@ static INT_PTR CALLBACK MainDlgProc(HWND hwnd, UINT msg,
case WM_MOUSEMOVE: case WM_MOUSEMOVE:
state = (struct MainDlgState *) GetWindowLongPtr(hwnd, GWLP_USERDATA); state = (struct MainDlgState *) GetWindowLongPtr(hwnd, GWLP_USERDATA);
if (state->entropy && state->entropy_got < state->entropy_required) { if (state->entropy && state->entropy_got < state->entropy_required) {
ULONG msgtime = GetMessageTime();
put_uint32(state->entropy, lParam); put_uint32(state->entropy, lParam);
put_uint32(state->entropy, GetMessageTime()); put_uint32(state->entropy, msgtime);
state->entropy_got += 2; if (msgtime - state->entropy_prev_msgtime > ENTROPY_RATE_LIMIT) {
state->entropy_got += 2;
state->entropy_prev_msgtime = msgtime;
}
SendDlgItemMessage(hwnd, IDC_PROGRESS, PBM_SETPOS, SendDlgItemMessage(hwnd, IDC_PROGRESS, PBM_SETPOS,
state->entropy_got, 0); state->entropy_got, 0);
if (state->entropy_got >= state->entropy_required) { if (state->entropy_got >= state->entropy_required) {
@ -1634,6 +1656,7 @@ static INT_PTR CALLBACK MainDlgProc(HWND hwnd, UINT msg,
state->entropy_got = 0; state->entropy_got = 0;
state->entropy = strbuf_new_nm(); state->entropy = strbuf_new_nm();
state->entropy_prev_msgtime = GetMessageTime();
SendDlgItemMessage(hwnd, IDC_PROGRESS, PBM_SETRANGE, 0, SendDlgItemMessage(hwnd, IDC_PROGRESS, PBM_SETRANGE, 0,
MAKELPARAM(0, state->entropy_required)); MAKELPARAM(0, state->entropy_required));