eax.cpp

/*
* EAX Mode Encryption
* (C) 1999-2007 Jack Lloyd
* (C) 2016 Daniel Neus, Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#include <botan/eax.h>
#include <botan/cmac.h>
#include <botan/ctr.h>

namespace Botan {

namespace {

/*
* EAX MAC-based PRF
*/
secure_vector<uint8_t> eax_prf(uint8_t tag, size_t block_size,
                           MessageAuthenticationCode& mac,
                           const uint8_t in[], size_t length)
   {
   for(size_t i = 0; i != block_size - 1; ++i)
      {
      mac.update(0);
      }
   mac.update(tag);
   mac.update(in, length);
   return mac.final();
   }

}

/*
* EAX_Mode Constructor
*/
EAX_Mode::EAX_Mode(BlockCipher* cipher, size_t tag_size) :
   m_tag_size(tag_size),
   m_cipher(cipher),
   m_ctr(new CTR_BE(m_cipher->clone())),
   m_cmac(new CMAC(m_cipher->clone()))
   {
   if(m_tag_size < 8 || m_tag_size > m_cmac->output_length())
      throw Invalid_Argument(name() + ": Bad tag size " + std::to_string(tag_size));
   }

void EAX_Mode::clear()
   {
   m_cipher->clear();
   m_ctr->clear();
   m_cmac->clear();
   reset();
   }

void EAX_Mode::reset()
   {
   m_ad_mac.clear();
   m_nonce_mac.clear();

   // Clear out any data added to the CMAC calculation
   try {
      m_cmac->final();
   }
   catch(Key_Not_Set&) {}
   }

std::string EAX_Mode::name() const
   {
   return (m_cipher->name() + "/EAX");
   }

size_t EAX_Mode::update_granularity() const
   {
   /*
   * For EAX this actually can be as low as 1 but that causes problems
   * for applications which use update_granularity as the buffer size.
   */
   return m_cipher->parallel_bytes();
   }

Key_Length_Specification EAX_Mode::key_spec() const
   {
   return m_cipher->key_spec();
   }

/*
* Set the EAX key
*/
void EAX_Mode::key_schedule(const uint8_t key[], size_t length)
   {
   /*
   * These could share the key schedule, which is one nice part of EAX,
   * but it's much easier to ignore that here...
   */
   m_ctr->set_key(key, length);
   m_cmac->set_key(key, length);
   }

/*
* Set the EAX associated data
*/
void EAX_Mode::set_associated_data(const uint8_t ad[], size_t length)
   {
   if(m_nonce_mac.empty() == false)
      throw Invalid_State("Cannot set AD for EAX while processing a message");
   m_ad_mac = eax_prf(1, block_size(), *m_cmac, ad, length);
   }

void EAX_Mode::start_msg(const uint8_t nonce[], size_t nonce_len)
   {
   if(!valid_nonce_length(nonce_len))
      throw Invalid_IV_Length(name(), nonce_len);

   m_nonce_mac = eax_prf(0, block_size(), *m_cmac, nonce, nonce_len);

   m_ctr->set_iv(m_nonce_mac.data(), m_nonce_mac.size());

   for(size_t i = 0; i != block_size() - 1; ++i)
      m_cmac->update(0);
   m_cmac->update(2);
   }

size_t EAX_Encryption::process(uint8_t buf[], size_t sz)
   {
   BOTAN_STATE_CHECK(m_nonce_mac.size() > 0);
   m_ctr->cipher(buf, buf, sz);
   m_cmac->update(buf, sz);
   return sz;
   }

void EAX_Encryption::finish(secure_vector<uint8_t>& buffer, size_t offset)
   {
   BOTAN_ASSERT_NOMSG(m_nonce_mac.empty() == false);
   update(buffer, offset);

   secure_vector<uint8_t> data_mac = m_cmac->final();
   xor_buf(data_mac, m_nonce_mac, data_mac.size());

   if(m_ad_mac.empty())
      {
      m_ad_mac = eax_prf(1, block_size(), *m_cmac, nullptr, 0);
      }

   xor_buf(data_mac, m_ad_mac, data_mac.size());

   buffer += std::make_pair(data_mac.data(), tag_size());
   }

size_t EAX_Decryption::process(uint8_t buf[], size_t sz)
   {
   BOTAN_STATE_CHECK(m_nonce_mac.size() > 0);
   m_cmac->update(buf, sz);
   m_ctr->cipher(buf, buf, sz);
   return sz;
   }

void EAX_Decryption::finish(secure_vector<uint8_t>& buffer, size_t offset)
   {
   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
   const size_t sz = buffer.size() - offset;
   uint8_t* buf = buffer.data() + offset;

   BOTAN_ASSERT(sz >= tag_size(), "Have the tag as part of final input");

   const size_t remaining = sz - tag_size();

   if(remaining)
      {
      m_cmac->update(buf, remaining);
      m_ctr->cipher(buf, buf, remaining);
      }

   const uint8_t* included_tag = &buf[remaining];

   secure_vector<uint8_t> mac = m_cmac->final();
   mac ^= m_nonce_mac;

   if(m_ad_mac.empty())
      {
      m_ad_mac = eax_prf(1, block_size(), *m_cmac, nullptr, 0);
      }

   mac ^= m_ad_mac;

   if(!constant_time_compare(mac.data(), included_tag, tag_size()))
      throw Invalid_Authentication_Tag("EAX tag check failed");

   buffer.resize(offset + remaining);

   m_nonce_mac.clear();
   }

}