From 48fc8df51d99f9d8ba251219367b3d629cc848e3 Mon Sep 17 00:00:00 2001
From: Jack Lloyd <jack@randombit.net>
Date: Tue, 5 Jun 2018 18:40:14 -0400
Subject: Address DSA/ECDSA side channel

---
 src/lib/pubkey/dsa/dsa.cpp | 38 ++++++++++++++++++++++++++++----------
 1 file changed, 28 insertions(+), 10 deletions(-)

(limited to 'src/lib/pubkey/dsa/dsa.cpp')

diff --git a/src/lib/pubkey/dsa/dsa.cpp b/src/lib/pubkey/dsa/dsa.cpp
index 172804972..7142e4788 100644
--- a/src/lib/pubkey/dsa/dsa.cpp
+++ b/src/lib/pubkey/dsa/dsa.cpp
@@ -74,7 +74,9 @@ namespace {
 class DSA_Signature_Operation final : public PK_Ops::Signature_with_EMSA
    {
    public:
-      DSA_Signature_Operation(const DSA_PrivateKey& dsa, const std::string& emsa) :
+      DSA_Signature_Operation(const DSA_PrivateKey& dsa,
+                              const std::string& emsa,
+                              RandomNumberGenerator& rng) :
          PK_Ops::Signature_with_EMSA(emsa),
          m_group(dsa.get_group()),
          m_x(dsa.get_x()),
@@ -83,6 +85,9 @@ class DSA_Signature_Operation final : public PK_Ops::Signature_with_EMSA
 #if defined(BOTAN_HAS_RFC6979_GENERATOR)
          m_rfc6979_hash = hash_for_emsa(emsa);
 #endif
+
+         m_b = BigInt::random_integer(rng, 2, dsa.group_q());
+         m_b_inv = inverse_mod(m_b, dsa.group_q());
          }
 
       size_t max_input_bits() const override { return m_group.get_q().bits(); }
@@ -96,6 +101,8 @@ class DSA_Signature_Operation final : public PK_Ops::Signature_with_EMSA
 #if defined(BOTAN_HAS_RFC6979_GENERATOR)
       std::string m_rfc6979_hash;
 #endif
+
+      BigInt m_b, m_b_inv;
    };
 
 secure_vector<uint8_t>
@@ -104,22 +111,32 @@ DSA_Signature_Operation::raw_sign(const uint8_t msg[], size_t msg_len,
    {
    const BigInt& q = m_group.get_q();
 
-   BigInt i(msg, msg_len, q.bits());
+   BigInt m(msg, msg_len, q.bits());
 
-   while(i >= q)
-      i -= q;
+   while(m >= q)
+      m -= q;
 
 #if defined(BOTAN_HAS_RFC6979_GENERATOR)
    BOTAN_UNUSED(rng);
-   const BigInt k = generate_rfc6979_nonce(m_x, q, i, m_rfc6979_hash);
+   const BigInt k = generate_rfc6979_nonce(m_x, q, m, m_rfc6979_hash);
 #else
    const BigInt k = BigInt::random_integer(rng, 1, q);
 #endif
 
-   BigInt s = inverse_mod(k, q);
+   const BigInt k_inv = inverse_mod(k, q);
+
    const BigInt r = m_mod_q.reduce(m_group.power_g_p(k));
 
-   s = m_mod_q.multiply(s, mul_add(m_x, r, i));
+   /*
+   * Blind the input message and compute x*r+m as (x*r*b + m*b)/b
+   */
+   m_b = m_mod_q.square(m_b);
+   m_b_inv = m_mod_q.square(m_b_inv);
+
+   m = m_mod_q.multiply(m_b, m);
+   const BigInt xr = m_mod_q.multiply(m_mod_q.multiply(m_x, m_b), r);
+
+   const BigInt s = m_mod_q.multiply(m_b_inv, m_mod_q.multiply(k_inv, xr + m));
 
    // With overwhelming probability, a bug rather than actual zero r/s
    if(r.is_zero() || s.is_zero())
@@ -140,7 +157,8 @@ class DSA_Verification_Operation final : public PK_Ops::Verification_with_EMSA
          m_group(dsa.get_group()),
          m_y(dsa.get_y()),
          m_mod_q(dsa.group_q())
-         {}
+         {
+         }
 
       size_t max_input_bits() const override { return m_group.get_q().bits(); }
 
@@ -193,12 +211,12 @@ DSA_PublicKey::create_verification_op(const std::string& params,
    }
 
 std::unique_ptr<PK_Ops::Signature>
-DSA_PrivateKey::create_signature_op(RandomNumberGenerator& /*rng*/,
+DSA_PrivateKey::create_signature_op(RandomNumberGenerator& rng,
                                     const std::string& params,
                                     const std::string& provider) const
    {
    if(provider == "base" || provider.empty())
-      return std::unique_ptr<PK_Ops::Signature>(new DSA_Signature_Operation(*this, params));
+      return std::unique_ptr<PK_Ops::Signature>(new DSA_Signature_Operation(*this, params, rng));
    throw Provider_Not_Found(algo_name(), provider);
    }
 
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