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libsec: fix mkbigint(), asn.1 uses two's compement signed representation

quick fix is to bias the rounding so the msb will always
be zero. should write proper conversion code to actually
deal with signed mpints... also for asn1mpint()... -- cinap

--- a/sys/src/libsec/port/x509.c

+++ b/sys/src/libsec/port/x509.c

@@ -2461,15 +2461,12 @@

 mkbigint(mpint *p)

 {

 	Elem e;

-	uchar *buf;

-	int buflen;

 	e.tag.class = Universal;

 	e.tag.num = INTEGER;

 	e.val.tag = VBigInt;

-	buflen = mptobe(p, nil, 0, &buf);

-	e.val.u.bigintval = makebytes(buf, buflen);

-	free(buf);

+	e.val.u.bigintval = newbytes((mpsignif(p)+8)/8);

+	mptober(p, e.val.u.bigintval->data, e.val.u.bigintval->len);

 	return e;

 }