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libsec: fix wrong tls1.0 prf regression

The change 775a4bea4386c12067057de0e56dd8baa34f43ec
"libsec: various changes to tls"
...
4. simply prf code...

... broke the TLS1.0 prf function, missing the fact
that the prf ouput for sha1 and md5 need to be
xored together.

--- a/sys/src/libsec/port/tlshand.c

+++ b/sys/src/libsec/port/tlshand.c

@@ -2342,6 +2342,7 @@

 	auth_freerpc(rpc);

 }

+// buf ^= prf

 static void

 tlsP(uchar *buf, int nbuf, uchar *key, int nkey, uchar *label, int nlabel, uchar *seed, int nseed,

 	DigestState* (*x)(uchar*, ulong, uchar*, ulong, uchar*, DigestState*), int xlen)

@@ -2348,7 +2349,7 @@

 {

 	uchar ai[SHA2_256dlen], tmp[SHA2_256dlen];

 	DigestState *s;

-	int n;

+	int n, i;

 	assert(xlen <= sizeof(ai) && xlen <= sizeof(tmp));

 	// generate a1

@@ -2362,7 +2363,8 @@

 		n = xlen;

 		if(n > nbuf)

 			n = nbuf;

-		memmove(buf, tmp, n);

+		for(i = 0; i < n; i++)

+			buf[i] ^= tmp[i];

 		buf += n;

 		nbuf -= n;

 		x(ai, xlen, key, nkey, tmp, nil);

@@ -2370,6 +2372,7 @@

 	}

 }

+

 // fill buf with md5(args)^sha1(args)

 static void

 tls10PRF(uchar *buf, int nbuf, uchar *key, int nkey, char *label, uchar *seed, int nseed)

@@ -2377,6 +2380,7 @@

 	int nlabel = strlen(label);

 	int n = (nkey + 1) >> 1;

+	memset(buf, 0, nbuf);

 	tlsP(buf, nbuf, key, n, (uchar*)label, nlabel, seed, nseed,

 		hmac_md5, MD5dlen);

 	tlsP(buf, nbuf, key+nkey-n, n, (uchar*)label, nlabel, seed, nseed,

@@ -2386,6 +2390,7 @@

 static void

 tls12PRF(uchar *buf, int nbuf, uchar *key, int nkey, char *label, uchar *seed, int nseed)

 {

+	memset(buf, 0, nbuf);

 	tlsP(buf, nbuf, key, nkey, (uchar*)label, strlen(label), seed, nseed,

 		hmac_sha2_256, SHA2_256dlen);

 }