Some additional operations on Montgomery_Int

Needed for #1432

2 files changed, 45 insertions, 0 deletions

diff --git a/src/lib/math/numbertheory/monty.cpp b/src/lib/math/numbertheory/monty.cpp
index 503141ada..41c01abb1 100644
--- a/src/lib/math/numbertheory/monty.cpp
+++ b/src/lib/math/numbertheory/monty.cpp
@@ -27,6 +27,25 @@ Montgomery_Params::Montgomery_Params(const BigInt& p,
    m_r3 = mod_p.multiply(m_r1, m_r2);
    }
 
+Montgomery_Params::Montgomery_Params(const BigInt& p)
+   {
+
+   if(p.is_negative() || p.is_even())
+      throw Invalid_Argument("Montgomery_Params invalid modulus");
+
+   m_p = p;
+   m_p_words = m_p.sig_words();
+   m_p_dash = monty_inverse(m_p.word_at(0));
+
+   const BigInt r = BigInt::power_of_2(m_p_words * BOTAN_MP_WORD_BITS);
+
+   Modular_Reducer mod_p(p);
+
+   m_r1 = mod_p.reduce(r);
+   m_r2 = mod_p.square(m_r1);
+   m_r3 = mod_p.multiply(m_r1, m_r2);
+   }
+
 BigInt Montgomery_Params::inv_mod_p(const BigInt& x) const
    {
    return ct_inverse_mod_odd_modulus(x, p());
@@ -182,6 +201,19 @@ Montgomery_Int::Montgomery_Int(const std::shared_ptr<const Montgomery_Params> pa
       }
    }
 
+Montgomery_Int::Montgomery_Int(std::shared_ptr<const Montgomery_Params> params,
+                               const uint8_t bits[], size_t len,
+                               bool redc_needed) :
+   m_params(params),
+   m_v(bits, len)
+   {
+   if(redc_needed)
+      {
+      secure_vector<word> ws;
+      m_v = m_params->mul(m_v % m_params->p(), m_params->R2(), ws);
+      }
+   }
+
 void Montgomery_Int::fix_size()
    {
    const size_t p_words = m_params->p_words();
diff --git a/src/lib/math/numbertheory/monty.h b/src/lib/math/numbertheory/monty.h
index 137e0b967..499a4ac91 100644
--- a/src/lib/math/numbertheory/monty.h
+++ b/src/lib/math/numbertheory/monty.h
@@ -33,6 +33,13 @@ class Montgomery_Int final
                      const BigInt& v,
                      bool redc_needed = true);
 
+      /**
+      * Create a Montgomery_Int
+      */
+      Montgomery_Int(std::shared_ptr<const Montgomery_Params> params,
+                     const uint8_t bits[], size_t len,
+                     bool redc_needed = true);
+
       bool operator==(const Montgomery_Int& other) const;
       bool operator!=(const Montgomery_Int& other) const { return (m_v != other.m_v); }
 
@@ -107,6 +114,12 @@ class Montgomery_Params final
       */
       Montgomery_Params(const BigInt& p, const Modular_Reducer& mod_p);
 
+      /**
+      * Initialize a set of Montgomery reduction parameters. These values
+      * can be shared by all values in a specific Montgomery domain.
+      */
+      Montgomery_Params(const BigInt& p);
+
       const BigInt& p() const { return m_p; }
       const BigInt& R1() const { return m_r1; }
       const BigInt& R2() const { return m_r2; }