Add BigInt::ct_cond_add

Also make low_zero_bits constant time.

2 files changed, 15 insertions, 1 deletions

diff --git a/src/lib/math/bigint/bigint.cpp b/src/lib/math/bigint/bigint.cpp
index 3ada94dce..fac82defa 100644
--- a/src/lib/math/bigint/bigint.cpp
+++ b/src/lib/math/bigint/bigint.cpp
@@ -453,6 +453,15 @@ void BigInt::binary_decode(const uint8_t buf[], size_t length)
    m_data.swap(reg);
    }
 
+void BigInt::ct_cond_add(bool predicate, const BigInt& value)
+   {
+   this->grow_to(1 + value.sig_words());
+
+   bigint_cnd_add(static_cast<word>(predicate),
+                  this->mutable_data(), this->size(),
+                  value.data(), value.sig_words());
+   }
+
 void BigInt::ct_cond_swap(bool predicate, BigInt& other)
    {
    const size_t max_words = std::max(size(), other.size());
diff --git a/src/lib/math/bigint/bigint.h b/src/lib/math/bigint/bigint.h
index ac0a57038..9fda4ceb0 100644
--- a/src/lib/math/bigint/bigint.h
+++ b/src/lib/math/bigint/bigint.h
@@ -721,6 +721,11 @@ class BOTAN_PUBLIC_API(2,0) BigInt final
      void ct_cond_swap(bool predicate, BigInt& other);
 
      /**
+     * If predicate is true add value to *this
+     */
+     void ct_cond_add(bool predicate, const BigInt& value);
+
+     /**
      * If predicate is true flip the sign of *this
      */
      void cond_flip_sign(bool predicate);
@@ -982,7 +987,7 @@ class BOTAN_PUBLIC_API(2,0) BigInt final
                  {
                  const word mask = (static_cast<word>(1) << (n % BOTAN_MP_WORD_BITS)) - 1;
                  const size_t len = size() - (top_word + 1);
-                 if (len > 0)
+                 if(len > 0)
                     {
                     clear_mem(&m_reg[top_word+1], len);
                     }