Add EC_Group::inverse_mod_order

Centralizing this logic allows curve specific implementations such
as using a precomputed ladder for exponentiating by p - 2

GH #1479

6 files changed, 21 insertions, 6 deletions

diff --git a/src/lib/pubkey/ec_group/ec_group.cpp b/src/lib/pubkey/ec_group/ec_group.cpp
index fc512b733..ac23aa151 100644
--- a/src/lib/pubkey/ec_group/ec_group.cpp
+++ b/src/lib/pubkey/ec_group/ec_group.cpp
@@ -89,6 +89,11 @@ class EC_Group_Data final
          return m_mod_order.multiply(x, y);
          }
 
+      BigInt inverse_mod_order(const BigInt& x) const
+         {
+         return inverse_mod(x, m_order);
+         }
+
       PointGFp blinded_base_point_multiply(const BigInt& k,
                                            RandomNumberGenerator& rng,
                                            std::vector<BigInt>& ws) const
@@ -477,6 +482,11 @@ BigInt EC_Group::multiply_mod_order(const BigInt& x, const BigInt& y) const
    return data().multiply_mod_order(x, y);
    }
 
+BigInt EC_Group::inverse_mod_order(const BigInt& x) const
+   {
+   return data().inverse_mod_order(x);
+   }
+
 const OID& EC_Group::get_curve_oid() const
    {
    return data().oid();
diff --git a/src/lib/pubkey/ec_group/ec_group.h b/src/lib/pubkey/ec_group/ec_group.h
index 8bb1a3044..f273108d2 100644
--- a/src/lib/pubkey/ec_group/ec_group.h
+++ b/src/lib/pubkey/ec_group/ec_group.h
@@ -199,6 +199,11 @@ class BOTAN_PUBLIC_API(2,0) EC_Group final
       BigInt mod_order(const BigInt& x) const;
 
       /*
+      * Return inverse of x modulo the order
+      */
+      BigInt inverse_mod_order(const BigInt& x) const;
+
+      /*
       * Reduce (x*y) modulo the order
       */
       BigInt multiply_mod_order(const BigInt& x, const BigInt& y) const;
diff --git a/src/lib/pubkey/ecdh/ecdh.cpp b/src/lib/pubkey/ecdh/ecdh.cpp
index adadb2703..59f245a00 100644
--- a/src/lib/pubkey/ecdh/ecdh.cpp
+++ b/src/lib/pubkey/ecdh/ecdh.cpp
@@ -31,7 +31,7 @@ class ECDH_KA_Operation final : public PK_Ops::Key_Agreement_with_KDF
          m_group(key.domain()),
          m_rng(rng)
          {
-         m_l_times_priv = inverse_mod(m_group.get_cofactor(), m_group.get_order()) * key.private_value();
+         m_l_times_priv = m_group.inverse_mod_order(m_group.get_cofactor()) * key.private_value();
          }
 
       secure_vector<uint8_t> raw_agree(const uint8_t w[], size_t w_len) override
diff --git a/src/lib/pubkey/ecdsa/ecdsa.cpp b/src/lib/pubkey/ecdsa/ecdsa.cpp
index 03f5e57ab..6e104f164 100644
--- a/src/lib/pubkey/ecdsa/ecdsa.cpp
+++ b/src/lib/pubkey/ecdsa/ecdsa.cpp
@@ -89,7 +89,7 @@ ECDSA_Signature_Operation::raw_sign(const uint8_t msg[], size_t msg_len,
    const BigInt k = m_group.random_scalar(rng);
 #endif
 
-   const BigInt k_inv = inverse_mod(k, m_group.get_order());
+   const BigInt k_inv = m_group.inverse_mod_order(k);
    const BigInt r = m_group.mod_order(
       m_group.blinded_base_point_multiply_x(k, rng, m_ws));
 
@@ -142,7 +142,7 @@ bool ECDSA_Verification_Operation::verify(const uint8_t msg[], size_t msg_len,
    if(r <= 0 || r >= m_group.get_order() || s <= 0 || s >= m_group.get_order())
       return false;
 
-   const BigInt w = inverse_mod(s, m_group.get_order());
+   const BigInt w = m_group.inverse_mod_order(s);
 
    const BigInt u1 = m_group.multiply_mod_order(e, w);
    const BigInt u2 = m_group.multiply_mod_order(r, w);
diff --git a/src/lib/pubkey/ecgdsa/ecgdsa.cpp b/src/lib/pubkey/ecgdsa/ecgdsa.cpp
index 192d999a8..61b7ae055 100644
--- a/src/lib/pubkey/ecgdsa/ecgdsa.cpp
+++ b/src/lib/pubkey/ecgdsa/ecgdsa.cpp
@@ -116,7 +116,7 @@ bool ECGDSA_Verification_Operation::verify(const uint8_t msg[], size_t msg_len,
    if(r <= 0 || r >= m_group.get_order() || s <= 0 || s >= m_group.get_order())
       return false;
 
-   const BigInt w = inverse_mod(r, m_group.get_order());
+   const BigInt w = m_group.inverse_mod_order(r);
 
    const BigInt u1 = m_group.multiply_mod_order(e, w);
    const BigInt u2 = m_group.multiply_mod_order(s, w);
diff --git a/src/lib/pubkey/sm2/sm2.cpp b/src/lib/pubkey/sm2/sm2.cpp
index 95fe28f14..1096ea99f 100644
--- a/src/lib/pubkey/sm2/sm2.cpp
+++ b/src/lib/pubkey/sm2/sm2.cpp
@@ -31,7 +31,7 @@ SM2_Signature_PrivateKey::SM2_Signature_PrivateKey(const AlgorithmIdentifier& al
                                                    const secure_vector<uint8_t>& key_bits) :
    EC_PrivateKey(alg_id, key_bits)
    {
-   m_da_inv = inverse_mod(m_private_key + 1, domain().get_order());
+   m_da_inv = domain().inverse_mod_order(m_private_key + 1);
    }
 
 SM2_Signature_PrivateKey::SM2_Signature_PrivateKey(RandomNumberGenerator& rng,
@@ -39,7 +39,7 @@ SM2_Signature_PrivateKey::SM2_Signature_PrivateKey(RandomNumberGenerator& rng,
                                                    const BigInt& x) :
    EC_PrivateKey(rng, domain, x)
    {
-   m_da_inv = inverse_mod(m_private_key + 1, domain.get_order());
+   m_da_inv = domain.inverse_mod_order(m_private_key + 1);
    }
 
 std::vector<uint8_t> sm2_compute_za(HashFunction& hash,