xmss_verification_operation.cpp

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/*
 * XMSS Verification Operation
 * Provides signature verification capabilities for Extended Hash-Based
 * Signatures (XMSS).
 *
 * (C) 2016,2017 Matthias Gierlings
 *
 * Botan is released under the Simplified BSD License (see license.txt)
 **/

#include <botan/internal/xmss_verification_operation.h>

namespace Botan {

XMSS_Verification_Operation::XMSS_Verification_Operation(
   const XMSS_PublicKey& public_key)
   : XMSS_Common_Ops(public_key.xmss_oid()),
     m_pub_key(public_key),
     m_msg_buf(0)
   {
   }

secure_vector<uint8_t>
XMSS_Verification_Operation::root_from_signature(const XMSS_Signature& sig,
      const secure_vector<uint8_t>& msg,
      XMSS_Address& adrs,
      const secure_vector<uint8_t>& seed)
   {
   const uint32_t next_index = static_cast<uint32_t>(sig.unused_leaf_index());
   adrs.set_type(XMSS_Address::Type::OTS_Hash_Address);
   adrs.set_ots_address(next_index);

   XMSS_WOTS_PublicKey pub_key_ots(m_pub_key.wots_parameters().oid(),
                                   msg,
                                   sig.tree().ots_signature(),
                                   adrs,
                                   seed);

   adrs.set_type(XMSS_Address::Type::LTree_Address);
   adrs.set_ltree_address(next_index);

   std::array<secure_vector<uint8_t>, 2> node;
   create_l_tree(node[0], pub_key_ots, adrs, seed);

   adrs.set_type(XMSS_Address::Type::Hash_Tree_Address);
   adrs.set_tree_index(next_index);

   for(size_t k = 0; k < m_xmss_params.tree_height(); k++)
      {
      adrs.set_tree_height(static_cast<uint32_t>(k));
      if(((next_index / (static_cast<size_t>(1) << k)) & 0x01) == 0)
         {
         adrs.set_tree_index(adrs.get_tree_index() >> 1);
         randomize_tree_hash(node[1],
                             node[0],
                             sig.tree().authentication_path()[k],
                             adrs,
                             seed);
         }
      else
         {
         adrs.set_tree_index((adrs.get_tree_index() - 1) >> 1);
         randomize_tree_hash(node[1],
                             sig.tree().authentication_path()[k],
                             node[0],
                             adrs,
                             seed);
         }
      node[0] = node[1];
      }
   return node[0];
   }

bool
XMSS_Verification_Operation::verify(const XMSS_Signature& sig,
                                    const secure_vector<uint8_t>& msg,
                                    const XMSS_PublicKey& public_key)
   {
   XMSS_Address adrs;
   secure_vector<uint8_t> index_bytes;
   XMSS_Tools::concat(index_bytes,
                      sig.unused_leaf_index(),
                      m_xmss_params.element_size());
   secure_vector<uint8_t> msg_digest =
      m_hash.h_msg(sig.randomness(),
                   public_key.root(),
                   index_bytes,
                   msg);

   secure_vector<uint8_t> node = root_from_signature(sig,
                                 msg_digest,
                                 adrs,
                                 public_key.public_seed());

   return (node == public_key.root());
   }

// FIXME: XMSS signature verification requires the "randomness" parameter out
// of the XMSS signature, which is part of the prefix that is hashed before
// msg. Since the signature is unknown till sign() is called all message
// content has to be buffered. For large messages this can be inconvenient or
// impossible.
// Possible solution: Change PK_Ops::Verification interface to take the
// signature as constructor argument, make sign a parameterless member call.
void XMSS_Verification_Operation::update(const uint8_t msg[], size_t msg_len)
   {
   std::copy(msg, msg + msg_len, std::back_inserter(m_msg_buf));
   }

bool XMSS_Verification_Operation::is_valid_signature(const uint8_t sig[],
      size_t sig_len)
   {
   try
      {
      XMSS_Signature signature(m_pub_key.xmss_parameters().oid(),
                               secure_vector<uint8_t>(sig, sig + sig_len));
      bool result = verify(signature, m_msg_buf, m_pub_key);
      m_msg_buf.clear();
      return result;
      }
   catch(...)
      {
      m_msg_buf.clear();
      return false;
      }
   }

}