ensure default serial generation fits 20 bytes

Author: BiagioFesta

rcgen/src/lib.rs

@@ -937,7 +937,8 @@ impl CertificateParams {
 			} else {
 				let hash = digest::digest(&digest::SHA256, pub_key.raw_bytes());
 				// RFC 5280 specifies at most 20 bytes for a serial number
-				let sl = &hash.as_ref()[0..20];
+				let mut sl = hash.as_ref()[0..20].to_vec();
+				sl[0] = sl[0] & 0x7f; // MSB must be 0 to ensure encoding bignum in 20 bytes
 				writer.next().write_bigint_bytes(sl, true);
 			};
 			// Write signature