Randomise the rekey interval a little. Previously, the chacha20

instance would be rekeyed every 1.6MB. This makes it happen at a
random point somewhere in the 1-2MB range.

Feedback deraadt@ visa@, ok tb@ visa@

lib/libc/crypt/arc4random.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: arc4random.c,v 1.56 2022/02/28 21:56:29 dtucker Exp $	*/
+/*	$OpenBSD: arc4random.c,v 1.57 2022/07/31 05:10:36 djm Exp $	*/
 
 /*
  * Copyright (c) 1996, David Mazieres <dm@uun.org>
@@ -49,6 +49,8 @@
 #define BLOCKSZ	64
 #define RSBUFSZ	(16*BLOCKSZ)
 
+#define REKEY_BASE	(1024*1024) /* NB. should be a power of 2 */
+
 /* Marked MAP_INHERIT_ZERO, so zero'd out in fork children. */
 static struct _rs {
 	size_t		rs_have;	/* valid bytes at end of rs_buf */
@@ -86,6 +88,7 @@ static void
 _rs_stir(void)
 {
 	u_char rnd[KEYSZ + IVSZ];
+	uint32_t rekey_fuzz = 0;
 
 	if (getentropy(rnd, sizeof rnd) == -1)
 		_getentropy_fail();
@@ -100,7 +103,10 @@ _rs_stir(void)
 	rs->rs_have = 0;
 	memset(rsx->rs_buf, 0, sizeof(rsx->rs_buf));
 
-	rs->rs_count = 1600000;
+	/* rekey interval should not be predictable */
+	chacha_encrypt_bytes(&rsx->rs_chacha, (uint8_t *)&rekey_fuzz,
+	     (uint8_t *)&rekey_fuzz, sizeof(rekey_fuzz));
+	rs->rs_count = REKEY_BASE + (rekey_fuzz % REKEY_BASE);
 }
 
 static inline void