Make Montgomery reduction constant time.

It was already close, but the carry loop would break early and
selecting which value to copy out was indexed on the borrow bit. Have
the carry loop run through, and add a const-time conditional copy
operation and use that to copy the output.

Convert ct_utils to CT namespace. Templatize the utils, which I was
hesitant to do initially but is pretty useful when dealing with
arbitrary word sizes.

Remove the poison macros, replace with inline funcs which reads
cleaner at the call site.

1 files changed, 5 insertions, 5 deletions

diff --git a/src/lib/pubkey/curve25519/donna.cpp b/src/lib/pubkey/curve25519/donna.cpp
index ab9363761..78966f745 100644
--- a/src/lib/pubkey/curve25519/donna.cpp
+++ b/src/lib/pubkey/curve25519/donna.cpp
@@ -420,8 +420,8 @@ crecip(felem out, const felem z) {
 int
 curve25519_donna(u8 *mypublic, const u8 *secret, const u8 *basepoint) {
 
-  BOTAN_CONST_TIME_POISON(secret, 32);
-  BOTAN_CONST_TIME_POISON(basepoint, 32);
+  CT::poison(secret, 32);
+  CT::poison(basepoint, 32);
 
   limb bp[5], x[5], z[5], zmone[5];
   uint8_t e[32];
@@ -438,9 +438,9 @@ curve25519_donna(u8 *mypublic, const u8 *secret, const u8 *basepoint) {
   fmul(z, x, zmone);
   fcontract(mypublic, z);
 
-  BOTAN_CONST_TIME_UNPOISON(secret, 32);
-  BOTAN_CONST_TIME_UNPOISON(basepoint, 32);
-  BOTAN_CONST_TIME_UNPOISON(mypublic, 32);
+  CT::unpoison(secret, 32);
+  CT::unpoison(basepoint, 32);
+  CT::unpoison(mypublic, 32);
   return 0;
 }