emsa_x931.cpp

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/*
* EMSA_X931
* (C) 1999-2007 Jack Lloyd
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/internal/emsa_x931.h>
 
#include <botan/exceptn.h>
#include <botan/mem_ops.h>
#include <botan/internal/fmt.h>
#include <botan/internal/hash_id.h>
#include <botan/internal/stl_util.h>
 
namespace Botan {
 
namespace {
 
std::vector<uint8_t> emsa2_encoding(std::span<const uint8_t> msg,
                                    size_t output_bits,
                                    std::span<const uint8_t> empty_hash,
                                    uint8_t hash_id) {
   const size_t HASH_SIZE = empty_hash.size();
 
   const size_t output_length = (output_bits + 1) / 8;
 
   if(msg.size() != HASH_SIZE) {
      throw Encoding_Error("EMSA_X931::encoding_of: Bad input length");
   }
   if(output_length < HASH_SIZE + 4) {
      throw Encoding_Error("EMSA_X931::encoding_of: Output length is too small");
   }
 
   const bool empty_input = constant_time_compare(msg, empty_hash);
 
   std::vector<uint8_t> output(output_length);
   BufferStuffer stuffer(output);
 
   stuffer.append(empty_input ? 0x4B : 0x6B);
   stuffer.append(0xBB, stuffer.remaining_capacity() - (1 + msg.size() + 2));
   stuffer.append(0xBA);
   stuffer.append(msg);
   stuffer.append(hash_id);
   stuffer.append(0xCC);
   BOTAN_ASSERT_NOMSG(stuffer.full());
 
   return output;
}
 
}  // namespace
 
std::string EMSA_X931::name() const {
   return fmt("X9.31({})", m_hash->name());
}
 
void EMSA_X931::update(const uint8_t input[], size_t length) {
   m_hash->update(input, length);
}
 
std::vector<uint8_t> EMSA_X931::raw_data() {
   return m_hash->final_stdvec();
}
 
/*
* EMSA_X931 Encode Operation
*/
std::vector<uint8_t> EMSA_X931::encoding_of(std::span<const uint8_t> msg,
                                            size_t output_bits,
                                            RandomNumberGenerator& /*rng*/) {
   return emsa2_encoding(msg, output_bits, m_empty_hash, m_hash_id);
}
 
/*
* EMSA_X931 Verify Operation
*/
bool EMSA_X931::verify(std::span<const uint8_t> coded, std::span<const uint8_t> raw, size_t key_bits) {
   try {
      const auto emsa2 = emsa2_encoding(raw, key_bits, m_empty_hash, m_hash_id);
      return constant_time_compare(coded, emsa2);
   } catch(...) {
      return false;
   }
}
 
/*
* EMSA_X931 Constructor
*/
EMSA_X931::EMSA_X931(std::unique_ptr<HashFunction> hash) : m_hash(std::move(hash)) {
   m_empty_hash = m_hash->final_stdvec();
 
   m_hash_id = ieee1363_hash_id(m_hash->name());
 
   if(!m_hash_id) {
      throw Encoding_Error("EMSA_X931 no hash identifier for " + m_hash->name());
   }
}
 
}  // namespace Botan