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kernel: better nonce partitioning for chacha random number generator

leave the block counter to chacha_encrypt() and increment the 96 bit
iv instead.

--- a/sys/src/9/port/random.c

+++ b/sys/src/9/port/random.c

@@ -89,7 +89,6 @@

 randomread(void *p, ulong n)

 {

 	Chachastate c;

-	ulong b;

 	if(n == 0)

 		return 0;

@@ -97,12 +96,12 @@

 	if(hwrandbuf != nil)

 		(*hwrandbuf)(p, n);

-	/* copy chacha state and advance block counter */

+	/* copy chacha state and increment iv */

 	qlock(rs);

 	c = *rs;

-	b = rs->input[12];

-	rs->input[12] += (n + ChachaBsize-1)/ChachaBsize;

-	if(rs->input[12] < b) rs->input[13]++;

+	if(++rs->input[13] == 0)

+		if(++rs->input[14] == 0)

+			++rs->input[15];

 	qunlock(rs);

 	/* encrypt the buffer, can fault */