Don't leak if x is zero either

1 files changed, 37 insertions, 39 deletions

diff --git a/src/lib/math/bigint/divide.cpp b/src/lib/math/bigint/divide.cpp
index ed9a12112..98df233e3 100644
--- a/src/lib/math/bigint/divide.cpp
+++ b/src/lib/math/bigint/divide.cpp
@@ -164,6 +164,8 @@ void divide(const BigInt& x, const BigInt& y_arg, BigInt& q_out, BigInt& r_out)
 
    const size_t y_words = y_arg.sig_words();
 
+   BOTAN_ASSERT_NOMSG(y_words > 0);
+
    BigInt y = y_arg;
 
    BigInt r = x;
@@ -173,62 +175,58 @@ void divide(const BigInt& x, const BigInt& y_arg, BigInt& q_out, BigInt& r_out)
    r.set_sign(BigInt::Positive);
    y.set_sign(BigInt::Positive);
 
-   if(r.is_nonzero())
-      {
-      // Calculate shifts needed to normalize y with high bit set
-      const size_t shifts = BOTAN_MP_WORD_BITS - high_bit(y.word_at(y_words-1));
-
-      y <<= shifts;
-      r <<= shifts;
-
-      // we know y has not changed size, since we only shifted up to set high bit
-      const size_t t = y_words - 1;
-      const size_t n = std::max(t, r.sig_words() - 1); // r may have changed size however
+   // Calculate shifts needed to normalize y with high bit set
+   const size_t shifts = BOTAN_MP_WORD_BITS - high_bit(y.word_at(y_words-1));
 
-      BOTAN_ASSERT_NOMSG(n >= t);
+   y <<= shifts;
+   r <<= shifts;
 
-      q.grow_to(n - t + 1);
+   // we know y has not changed size, since we only shifted up to set high bit
+   const size_t t = y_words - 1;
+   const size_t n = std::max(y_words, r.sig_words()) - 1; // r may have changed size however
 
-      word* q_words = q.mutable_data();
+   BOTAN_ASSERT_NOMSG(n >= t);
 
-      BigInt shifted_y = y << (BOTAN_MP_WORD_BITS * (n-t));
+   q.grow_to(n - t + 1);
 
-      // Set q_{n-t} to number of times r > shifted_y
-      q_words[n-t] = r.reduce_below(shifted_y, ws);
+   word* q_words = q.mutable_data();
 
-      const word y_t0  = y.word_at(t);
-      const word y_t1  = y.word_at(t-1);
+   BigInt shifted_y = y << (BOTAN_MP_WORD_BITS * (n-t));
 
-      for(size_t j = n; j != t; --j)
-         {
-         const word x_j0  = r.word_at(j);
-         const word x_j1 = r.word_at(j-1);
-         const word x_j2 = r.word_at(j-2);
+   // Set q_{n-t} to number of times r > shifted_y
+   q_words[n-t] = r.reduce_below(shifted_y, ws);
 
-         word qjt = bigint_divop(x_j0, x_j1, y_t0);
+   const word y_t0  = y.word_at(t);
+   const word y_t1  = y.word_at(t-1);
 
-         qjt = CT::Mask<word>::is_equal(x_j0, y_t0).select(MP_WORD_MAX, qjt);
+   for(size_t j = n; j != t; --j)
+      {
+      const word x_j0  = r.word_at(j);
+      const word x_j1 = r.word_at(j-1);
+      const word x_j2 = r.word_at(j-2);
 
-         // Per HAC 14.23, this operation is required at most twice
-         qjt -= division_check(qjt, y_t0, y_t1, x_j0, x_j1, x_j2);
-         qjt -= division_check(qjt, y_t0, y_t1, x_j0, x_j1, x_j2);
-         BOTAN_DEBUG_ASSERT(division_check(qjt, y_t0, y_t1, x_j0, x_j1, x_j2) == false);
+      word qjt = bigint_divop(x_j0, x_j1, y_t0);
 
-         shifted_y >>= BOTAN_MP_WORD_BITS;
-         // Now shifted_y == y << (BOTAN_MP_WORD_BITS * (j-t-1))
+      qjt = CT::Mask<word>::is_equal(x_j0, y_t0).select(MP_WORD_MAX, qjt);
 
-         r -= qjt * shifted_y;
+      // Per HAC 14.23, this operation is required at most twice
+      qjt -= division_check(qjt, y_t0, y_t1, x_j0, x_j1, x_j2);
+      qjt -= division_check(qjt, y_t0, y_t1, x_j0, x_j1, x_j2);
+      BOTAN_DEBUG_ASSERT(division_check(qjt, y_t0, y_t1, x_j0, x_j1, x_j2) == false);
 
-         // TODO this could be better
-         qjt -= r.is_negative();
-         r += static_cast<word>(r.is_negative()) * shifted_y;
+      shifted_y >>= BOTAN_MP_WORD_BITS;
+      // Now shifted_y == y << (BOTAN_MP_WORD_BITS * (j-t-1))
 
-         q_words[j-t-1] = qjt;
-         }
+      // TODO this sequence could be better
+      r -= qjt * shifted_y;
+      qjt -= r.is_negative();
+      r += static_cast<word>(r.is_negative()) * shifted_y;
 
-      r >>= shifts;
+      q_words[j-t-1] = qjt;
       }
 
+   r >>= shifts;
+
    sign_fixup(x, y_arg, q, r);
 
    r_out = r;