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rsagen: prefer 65537 as the default exponent when elen == 0, otherwise pick randomly

--- a/sys/src/cmd/auth/rsagen.c

+++ b/sys/src/cmd/auth/rsagen.c

@@ -42,7 +42,7 @@

 	do{

 		if(key)

 			rsaprivfree(key);

-		key = rsagen(bits, 6, 0);

+		key = rsagen(bits, 0, 0);

 	}while(mpsignif(key->pub.n) != bits);

 	s = smprint("key proto=rsa %s%ssize=%d ek=%B !dk=%B n=%B !p=%B !q=%B !kp=%B !kq=%B !c2=%B\n",

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

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

@@ -26,9 +26,13 @@

 	// find an e relatively prime to phi

 	t1 = mpnew(0);

 	t2 = mpnew(0);

-	mprand(elen, genrandom, e);

-	if(mpcmp(e,mptwo) <= 0)

-		itomp(3, e);

+	if(elen == 0)

+		itomp(65537, e);

+	else {

+		mprand(elen, genrandom, e);

+		if(mpcmp(e,mptwo) <= 0)

+			itomp(3, e);

+	}

 	// See Menezes et al. p.291 "8.8 Note (selecting primes)" for discussion

 	// of the merits of various choices of primes and exponents.  e=3 is a

 	// common and recommended exponent, but doesn't necessarily work here