1 files changed, 119 insertions, 0 deletions
diff --git a/comm/third_party/botan/src/lib/math/numbertheory/reducer.cpp b/comm/third_party/botan/src/lib/math/numbertheory/reducer.cpp
new file mode 100644
index 0000000000..deb3874d3e
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+++ b/comm/third_party/botan/src/lib/math/numbertheory/reducer.cpp
@@ -0,0 +1,119 @@
+/*
+* Modular Reducer
+* (C) 1999-2011,2018 Jack Lloyd
+*
+* Botan is released under the Simplified BSD License (see license.txt)
+*/
+
+#include <botan/reducer.h>
+#include <botan/internal/ct_utils.h>
+#include <botan/internal/mp_core.h>
+#include <botan/divide.h>
+
+namespace Botan {
+
+/*
+* Modular_Reducer Constructor
+*/
+Modular_Reducer::Modular_Reducer(const BigInt& mod)
+   {
+   if(mod < 0)
+      throw Invalid_Argument("Modular_Reducer: modulus must be positive");
+
+   // Left uninitialized if mod == 0
+   m_mod_words = 0;
+
+   if(mod > 0)
+      {
+      m_modulus = mod;
+      m_mod_words = m_modulus.sig_words();
+
+      // Compute mu = floor(2^{2k} / m)
+      m_mu.set_bit(2 * BOTAN_MP_WORD_BITS * m_mod_words);
+      m_mu = ct_divide(m_mu, m_modulus);
+      }
+   }
+
+BigInt Modular_Reducer::reduce(const BigInt& x) const
+   {
+   BigInt r;
+   secure_vector<word> ws;
+   reduce(r, x, ws);
+   return r;
+   }
+
+namespace {
+
+/*
+* Like if(cnd) x.rev_sub(...) but in const time
+*/
+void cnd_rev_sub(bool cnd, BigInt& x, const word y[], size_t y_sw, secure_vector<word>& ws)
+   {
+   if(x.sign() != BigInt::Positive)
+      throw Invalid_State("BigInt::sub_rev requires this is positive");
+
+   const size_t x_sw = x.sig_words();
+
+   const size_t max_words = std::max(x_sw, y_sw);
+   ws.resize(std::max(x_sw, y_sw));
+   clear_mem(ws.data(), ws.size());
+   x.grow_to(max_words);
+
+   const int32_t relative_size = bigint_sub_abs(ws.data(), x.data(), x_sw, y, y_sw);
+
+   x.cond_flip_sign((relative_size > 0) && cnd);
+   bigint_cnd_swap(cnd, x.mutable_data(), ws.data(), max_words);
+   }
+
+}
+
+void Modular_Reducer::reduce(BigInt& t1, const BigInt& x, secure_vector<word>& ws) const
+   {
+   if(&t1 == &x)
+      throw Invalid_State("Modular_Reducer arguments cannot alias");
+   if(m_mod_words == 0)
+      throw Invalid_State("Modular_Reducer: Never initalized");
+
+   const size_t x_sw = x.sig_words();
+
+   if(x_sw > 2*m_mod_words)
+      {
+      // too big, fall back to slow boat division
+      t1 = ct_modulo(x, m_modulus);
+      return;
+      }
+
+   t1 = x;
+   t1.set_sign(BigInt::Positive);
+   t1 >>= (BOTAN_MP_WORD_BITS * (m_mod_words - 1));
+
+   t1.mul(m_mu, ws);
+   t1 >>= (BOTAN_MP_WORD_BITS * (m_mod_words + 1));
+
+   // TODO add masked mul to avoid computing high bits
+   t1.mul(m_modulus, ws);
+   t1.mask_bits(BOTAN_MP_WORD_BITS * (m_mod_words + 1));
+
+   t1.rev_sub(x.data(), std::min(x_sw, m_mod_words + 1), ws);
+
+   /*
+   * If t1 < 0 then we must add b^(k+1) where b = 2^w. To avoid a
+   * side channel perform the addition unconditionally, with ws set
+   * to either b^(k+1) or else 0.
+   */
+   const word t1_neg = t1.is_negative();
+
+   if(ws.size() < m_mod_words + 2)
+      ws.resize(m_mod_words + 2);
+   clear_mem(ws.data(), ws.size());
+   ws[m_mod_words + 1] = t1_neg;
+
+   t1.add(ws.data(), m_mod_words + 2, BigInt::Positive);
+
+   // Per HAC this step requires at most 2 subtractions
+   t1.ct_reduce_below(m_modulus, ws, 2);
+
+   cnd_rev_sub(t1.is_nonzero() && x.is_negative(), t1, m_modulus.data(), m_modulus.size(), ws);
+   }
+
+}