Avoid doing a variable time division during Montgomery setup

Instead require the inputs be reduced already. For RSA-CRT use
Barrett which is const time already. For SRP6 inputs were not reduced,
use the Barrett hook available in DL_Group.

1 files changed, 2 insertions, 1 deletions

diff --git a/src/lib/misc/srp6/srp6.cpp b/src/lib/misc/srp6/srp6.cpp
index 0ec4fd2bb..825c38589 100644
--- a/src/lib/misc/srp6/srp6.cpp
+++ b/src/lib/misc/srp6/srp6.cpp
@@ -103,7 +103,8 @@ srp6_client_agree(const std::string& identifier,
 
    const BigInt x = compute_x(hash_id, identifier, password, salt);
 
-   const BigInt S = power_mod((B - (k * power_mod(g, x, p))) % p, (a + (u * x)), p);
+   const BigInt S = power_mod(group.mod_p(B - (k * power_mod(g, x, p))),
+                              group.mod_p(a + (u * x)), p);
 
    const SymmetricKey Sk(BigInt::encode_1363(S, p_bytes));