use Knuth shuffles

firmware/pinmatrix.c

@@ -49,20 +49,12 @@ void pinmatrix_draw(const char *text)
 
 void pinmatrix_start(const char *text)
 {
-	int i, j, k;
-	char t;
-
+	int i;
 	for (i = 0; i < 9; i++) {
 		pinmatrix_perm[i] = '1' + i;
 	}
 	pinmatrix_perm[9] = 0;
-	for (i = 0; i < 10000; i++) {
-		j = random32() % 9;
-		k = random32() % 9;
-		t = pinmatrix_perm[j];
-		pinmatrix_perm[j] = pinmatrix_perm[k];
-		pinmatrix_perm[k] = t;
-	}
+	random_permute(pinmatrix_perm, 9);
 	pinmatrix_draw(text);
 }
 

firmware/recovery.c

@@ -40,7 +40,7 @@ void next_word(void) {
 	word_pos = word_order[word_index];
 	if (word_pos == 0) {
 		const char **wl = mnemonic_wordlist();
-		strlcpy(fake_word, wl[random32() & 0x7FF], sizeof(fake_word));
+		strlcpy(fake_word, wl[random_uniform(2048)], sizeof(fake_word));
 		layoutDialogSwipe(DIALOG_ICON_INFO, NULL, NULL, NULL, "Please enter the word", NULL, fake_word, NULL, "on your computer", NULL);
 	} else {
 		fake_word[0] = 0;
@@ -89,21 +89,14 @@ void recovery_init(uint32_t _word_count, bool passphrase_protection, bool pin_pr
 	storage_setLanguage(language);
 	storage_setLabel(label);
 
-	uint32_t i, j, k;
-	char t;
+	uint32_t i;
 	for (i = 0; i < word_count; i++) {
 		word_order[i] = i + 1;
 	}
 	for (i = word_count; i < 24; i++) {
 		word_order[i] = 0;
 	}
-	for (i = 0; i < 10000; i++) {
-		j = random32() % 24;
-		k = random32() % 24;
-		t = word_order[j];
-		word_order[j] = word_order[k];
-		word_order[k] = t;
-	}
+	random_permute(word_order, 24);
 	awaiting_word = true;
 	word_index = 0;
 	next_word();

rng.c

@@ -35,6 +35,13 @@ uint32_t random32(void)
 	return new;
 }
 
+uint32_t random_uniform(uint32_t n)
+{
+	uint32_t x, max = 0xFFFFFFFF - (0xFFFFFFFF % n);
+	while ((x = random32()) >= max);
+	return x / (max / n);
+}
+
 void random_buffer(uint8_t *buf, size_t len)
 {
 	size_t i;
@@ -46,3 +53,15 @@ void random_buffer(uint8_t *buf, size_t len)
 		buf[i] = (r >> ((i % 4) * 8)) & 0xFF;
 	}
 }
+
+void random_permute(char *str, size_t len)
+{
+	int i, j;
+	char t;
+	for (i = len - 1; i >= 1; i--) {
+		j = random_uniform(i + 1);
+		t = str[j];
+		str[j] = str[i];
+		str[i] = t;
+	}
+}

rng.h

@@ -24,6 +24,8 @@
 #include <stdlib.h>
 
 uint32_t random32(void);
+uint32_t random_uniform(uint32_t n);
 void random_buffer(uint8_t *buf, size_t len);
+void random_permute(char *buf, size_t len);
 
 #endif