Reduce the base in the fixed window exponentiator

Otherwise we can end up calling the Barrett reducer with an input that
is more than the square of the modulus, which will make it fall back
to the (slow) const time division.

This only affected even moduli, and only when the base was larger than
the modulus.

OSS-Fuzz 11750

1 files changed, 1 insertions, 1 deletions

diff --git a/src/lib/math/numbertheory/powm_fw.cpp b/src/lib/math/numbertheory/powm_fw.cpp
index c326ffa44..c6ff169a1 100644
--- a/src/lib/math/numbertheory/powm_fw.cpp
+++ b/src/lib/math/numbertheory/powm_fw.cpp
@@ -27,7 +27,7 @@ void Fixed_Window_Exponentiator::set_base(const BigInt& base)
 
    m_g.resize(static_cast<size_t>(1) << m_window_bits);
    m_g[0] = 1;
-   m_g[1] = base;
+   m_g[1] = m_reducer.reduce(base);
 
    for(size_t i = 2; i != m_g.size(); ++i)
       m_g[i] = m_reducer.multiply(m_g[i-1], m_g[1]);