Bodo Moeller (from the OpenSSL core team, bodo AT openssl DOT org) has this to say when I asked him about this:
"The main (or maybe only) reason this is not enabled automatically is that the OpenSSL configure script is not clever enough to detect whether the compiler supports __uint128_t. If you can use this code, it is superior to the default in multiple regards (speed, and also security as the new implementations are secure against timing attacks)." -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
