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gtkask.c: use dedicated PRNG for initial area choice.

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At Coverity's urging, I put in an instance of the Fortuna PRNG in
place of the use of rand() to select which drawing area to light up
next on a keypress. But Coverity turns out to be unhappy that I'm
still using rand() to select the _initial_ area, before the user
enters any input at all.

It's even harder to imagine this being a genuine information leak than
the previous complaint. But I don't really see a reason _not_ to
switch it over, now it's been pointed out.
This commit is contained in:
Simon Tatham 2019-07-23 18:40:09 +01:00
parent 37bff968eb
commit 8872a97ebd
1 changed files with 7 additions and 4 deletions
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11
unix/gtkask.c
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@ -78,7 +78,7 @@ static void cleanup_keypress_prng(void)
{ {
prng_free(keypress_prng); prng_free(keypress_prng);
} }
static int choose_new_area(int prev_area) static uint64_t keypress_prng_value(void)
{ {
/* /*
* Don't actually put the passphrase keystrokes themselves into * Don't actually put the passphrase keystrokes themselves into
@ -89,8 +89,11 @@ static int choose_new_area(int prev_area)
noise_ultralight(NOISE_SOURCE_KEY, 0); noise_ultralight(NOISE_SOURCE_KEY, 0);
uint8_t data[8]; uint8_t data[8];
prng_read(keypress_prng, data, 8); prng_read(keypress_prng, data, 8);
uint64_t randval = GET_64BIT_MSB_FIRST(data); return GET_64BIT_MSB_FIRST(data);
int reduced = randval % (N_DRAWING_AREAS - 1); }
static int choose_new_area(int prev_area)
{
int reduced = keypress_prng_value() % (N_DRAWING_AREAS - 1);
return (prev_area + 1 + reduced) % N_DRAWING_AREAS; return (prev_area + 1 + reduced) % N_DRAWING_AREAS;
} }
@ -353,7 +356,7 @@ static gboolean try_grab_keyboard(gpointer vctx)
* And repaint the key-acknowledgment drawing areas as not greyed * And repaint the key-acknowledgment drawing areas as not greyed
* out. * out.
*/ */
ctx->active_area = rand() % N_DRAWING_AREAS; ctx->active_area = keypress_prng_value() % N_DRAWING_AREAS;
for (i = 0; i < N_DRAWING_AREAS; i++) { for (i = 0; i < N_DRAWING_AREAS; i++) {
ctx->drawingareas[i].state = ctx->drawingareas[i].state =
(i == ctx->active_area ? CURRENT : NOT_CURRENT); (i == ctx->active_area ? CURRENT : NOT_CURRENT);