ocb.cpp

Go to the documentation of this file.

/*
* OCB Mode
* (C) 2013,2017 Jack Lloyd
* (C) 2016 Daniel Neus, Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/

#include <botan/ocb.h>
#include <botan/block_cipher.h>
#include <botan/internal/poly_dbl.h>
#include <botan/internal/bit_ops.h>

namespace Botan {

// Has to be in Botan namespace so unique_ptr can reference it
class L_computer final
   {
   public:
      explicit L_computer(const BlockCipher& cipher) :
         m_BS(cipher.block_size()),
         m_max_blocks(cipher.parallel_bytes() / m_BS)
         {
         m_L_star.resize(m_BS);
         cipher.encrypt(m_L_star);
         m_L_dollar = poly_double(star());
         m_L.push_back(poly_double(dollar()));

         while(m_L.size() < 8)
            m_L.push_back(poly_double(m_L.back()));

         m_offset_buf.resize(m_BS * m_max_blocks);
         }

      void init(const secure_vector<uint8_t>& offset)
         {
         m_offset = offset;
         }

      bool initialized() const { return m_offset.empty() == false; }

      const secure_vector<uint8_t>& star() const { return m_L_star; }
      const secure_vector<uint8_t>& dollar() const { return m_L_dollar; }
      const secure_vector<uint8_t>& offset() const { return m_offset; }

      const secure_vector<uint8_t>& get(size_t i) const
         {
         while(m_L.size() <= i)
            m_L.push_back(poly_double(m_L.back()));

         return m_L[i];
         }

      const uint8_t*
      compute_offsets(size_t block_index, size_t blocks)
         {
         BOTAN_ASSERT(blocks <= m_max_blocks, "OCB offsets");

         uint8_t* offsets = m_offset_buf.data();

         if(block_index % 4 == 0)
            {
            const secure_vector<uint8_t>& L0 = get(0);
            const secure_vector<uint8_t>& L1 = get(1);

            while(blocks >= 4)
               {
               // ntz(4*i+1) == 0
               // ntz(4*i+2) == 1
               // ntz(4*i+3) == 0
               block_index += 4;
               const size_t ntz4 = var_ctz32(static_cast<uint32_t>(block_index));

               xor_buf(offsets, m_offset.data(), L0.data(), m_BS);
               offsets += m_BS;

               xor_buf(offsets, offsets - m_BS, L1.data(), m_BS);
               offsets += m_BS;

               xor_buf(m_offset.data(), L1.data(), m_BS);
               copy_mem(offsets, m_offset.data(), m_BS);
               offsets += m_BS;

               xor_buf(m_offset.data(), get(ntz4).data(), m_BS);
               copy_mem(offsets, m_offset.data(), m_BS);
               offsets += m_BS;

               blocks -= 4;
               }
            }

         for(size_t i = 0; i != blocks; ++i)
            { // could be done in parallel
            const size_t ntz = var_ctz32(static_cast<uint32_t>(block_index + i + 1));
            xor_buf(m_offset.data(), get(ntz).data(), m_BS);
            copy_mem(offsets, m_offset.data(), m_BS);
            offsets += m_BS;
            }

         return m_offset_buf.data();
         }

   private:
      secure_vector<uint8_t> poly_double(const secure_vector<uint8_t>& in) const
         {
         secure_vector<uint8_t> out(in.size());
         poly_double_n(out.data(), in.data(), out.size());
         return out;
         }

      const size_t m_BS, m_max_blocks;
      secure_vector<uint8_t> m_L_dollar, m_L_star;
      secure_vector<uint8_t> m_offset;
      mutable std::vector<secure_vector<uint8_t>> m_L;
      secure_vector<uint8_t> m_offset_buf;
   };

namespace {

/*
* OCB's HASH
*/
secure_vector<uint8_t> ocb_hash(const L_computer& L,
                                const BlockCipher& cipher,
                                const uint8_t ad[], size_t ad_len)
   {
   const size_t BS = cipher.block_size();
   secure_vector<uint8_t> sum(BS);
   secure_vector<uint8_t> offset(BS);

   secure_vector<uint8_t> buf(BS);

   const size_t ad_blocks = (ad_len / BS);
   const size_t ad_remainder = (ad_len % BS);

   for(size_t i = 0; i != ad_blocks; ++i)
      {
      // this loop could run in parallel
      offset ^= L.get(var_ctz32(static_cast<uint32_t>(i+1)));
      buf = offset;
      xor_buf(buf.data(), &ad[BS*i], BS);
      cipher.encrypt(buf);
      sum ^= buf;
      }

   if(ad_remainder)
      {
      offset ^= L.star();
      buf = offset;
      xor_buf(buf.data(), &ad[BS*ad_blocks], ad_remainder);
      buf[ad_remainder] ^= 0x80;
      cipher.encrypt(buf);
      sum ^= buf;
      }

   return sum;
   }

}

OCB_Mode::OCB_Mode(BlockCipher* cipher, size_t tag_size) :
   m_cipher(cipher),
   m_checksum(m_cipher->parallel_bytes()),
   m_ad_hash(m_cipher->block_size()),
   m_tag_size(tag_size),
   m_block_size(m_cipher->block_size()),
   m_par_blocks(m_cipher->parallel_bytes() / m_block_size)
   {
   const size_t BS = block_size();

   /*
   * draft-krovetz-ocb-wide-d1 specifies OCB for several other block
   * sizes but only 128, 192, 256 and 512 bit are currently supported
   * by this implementation.
   */
   BOTAN_ARG_CHECK(BS == 16 || BS == 24 || BS == 32 || BS == 64,
                   "Invalid block size for OCB");

   BOTAN_ARG_CHECK(m_tag_size % 4 == 0 &&
                   m_tag_size >= 8 && m_tag_size <= BS &&
                   m_tag_size <= 32,
                   "Invalid OCB tag length");
   }

OCB_Mode::~OCB_Mode() { /* for unique_ptr destructor */ }

void OCB_Mode::clear()
   {
   m_cipher->clear();
   m_L.reset(); // add clear here?
   reset();
   }

void OCB_Mode::reset()
   {
   m_block_index = 0;
   zeroise(m_ad_hash);
   zeroise(m_checksum);
   m_last_nonce.clear();
   m_stretch.clear();
   }

bool OCB_Mode::valid_nonce_length(size_t length) const
   {
   if(length == 0)
      return false;
   if(block_size() == 16)
      return length < 16;
   else
      return length < (block_size() - 1);
   }

std::string OCB_Mode::name() const
   {
   return m_cipher->name() + "/OCB"; // include tag size?
   }

size_t OCB_Mode::update_granularity() const
   {
   return (m_par_blocks * block_size());
   }

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

void OCB_Mode::key_schedule(const uint8_t key[], size_t length)
   {
   m_cipher->set_key(key, length);
   m_L.reset(new L_computer(*m_cipher));
   }

void OCB_Mode::set_associated_data(const uint8_t ad[], size_t ad_len)
   {
   verify_key_set(m_L != nullptr);
   m_ad_hash = ocb_hash(*m_L, *m_cipher, ad, ad_len);
   }

secure_vector<uint8_t>
OCB_Mode::update_nonce(const uint8_t nonce[], size_t nonce_len)
   {
   const size_t BS = block_size();

   BOTAN_ASSERT(BS == 16 || BS == 24 || BS == 32 || BS == 64,
                "OCB block size is supported");

   const size_t MASKLEN = (BS == 16 ? 6 : ((BS == 24) ? 7 : 8));

   const uint8_t BOTTOM_MASK =
      static_cast<uint8_t>((static_cast<uint16_t>(1) << MASKLEN) - 1);

   secure_vector<uint8_t> nonce_buf(BS);

   copy_mem(&nonce_buf[BS - nonce_len], nonce, nonce_len);
   nonce_buf[0] = static_cast<uint8_t>(((tag_size()*8) % (BS*8)) << (BS <= 16 ? 1 : 0));

   nonce_buf[BS - nonce_len - 1] ^= 1;

   const uint8_t bottom = nonce_buf[BS-1] & BOTTOM_MASK;
   nonce_buf[BS-1] &= ~BOTTOM_MASK;

   const bool need_new_stretch = (m_last_nonce != nonce_buf);

   if(need_new_stretch)
      {
      m_last_nonce = nonce_buf;

      m_cipher->encrypt(nonce_buf);

      /*
      The loop bounds (BS vs BS/2) are derived from the relation
      between the block size and the MASKLEN. Using the terminology
      of draft-krovetz-ocb-wide, we have to derive enough bits in
      ShiftedKtop to read up to BLOCKLEN+bottom bits from Stretch.

                 +----------+---------+-------+---------+
                 | BLOCKLEN | RESIDUE | SHIFT | MASKLEN |
                 +----------+---------+-------+---------+
                 |       32 |     141 |    17 |    4    |
                 |       64 |      27 |    25 |    5    |
                 |       96 |    1601 |    33 |    6    |
                 |      128 |     135 |     8 |    6    |
                 |      192 |     135 |    40 |    7    |
                 |      256 |    1061 |     1 |    8    |
                 |      384 |    4109 |    80 |    8    |
                 |      512 |     293 |   176 |    8    |
                 |     1024 |  524355 |   352 |    9    |
                 +----------+---------+-------+---------+
      */
      if(BS == 16)
         {
         for(size_t i = 0; i != BS / 2; ++i)
            nonce_buf.push_back(nonce_buf[i] ^ nonce_buf[i+1]);
         }
      else if(BS == 24)
         {
         for(size_t i = 0; i != 16; ++i)
            nonce_buf.push_back(nonce_buf[i] ^ nonce_buf[i+5]);
         }
      else if(BS == 32)
         {
         for(size_t i = 0; i != BS; ++i)
            nonce_buf.push_back(nonce_buf[i] ^ (nonce_buf[i] << 1) ^ (nonce_buf[i+1] >> 7));
         }
      else if(BS == 64)
         {
         for(size_t i = 0; i != BS / 2; ++i)
            nonce_buf.push_back(nonce_buf[i] ^ nonce_buf[i+22]);
         }

      m_stretch = nonce_buf;
      }

   // now set the offset from stretch and bottom
   const size_t shift_bytes = bottom / 8;
   const size_t shift_bits  = bottom % 8;

   BOTAN_ASSERT(m_stretch.size() >= BS + shift_bytes + 1, "Size ok");

   secure_vector<uint8_t> offset(BS);
   for(size_t i = 0; i != BS; ++i)
      {
      offset[i]  = (m_stretch[i+shift_bytes] << shift_bits);
      offset[i] |= (m_stretch[i+shift_bytes+1] >> (8-shift_bits));
      }

   return offset;
   }

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

   verify_key_set(m_L != nullptr);

   m_L->init(update_nonce(nonce, nonce_len));
   zeroise(m_checksum);
   m_block_index = 0;
   }

void OCB_Encryption::encrypt(uint8_t buffer[], size_t blocks)
   {
   verify_key_set(m_L != nullptr);
   BOTAN_STATE_CHECK(m_L->initialized());

   const size_t BS = block_size();

   while(blocks)
      {
      const size_t proc_blocks = std::min(blocks, par_blocks());
      const size_t proc_bytes = proc_blocks * BS;

      const uint8_t* offsets = m_L->compute_offsets(m_block_index, proc_blocks);

      xor_buf(m_checksum.data(), buffer, proc_bytes);

      m_cipher->encrypt_n_xex(buffer, offsets, proc_blocks);

      buffer += proc_bytes;
      blocks -= proc_blocks;
      m_block_index += proc_blocks;
      }
   }

size_t OCB_Encryption::process(uint8_t buf[], size_t sz)
   {
   BOTAN_ASSERT(sz % update_granularity() == 0, "Invalid OCB input size");
   encrypt(buf, sz / block_size());
   return sz;
   }

void OCB_Encryption::finish(secure_vector<uint8_t>& buffer, size_t offset)
   {
   verify_key_set(m_L != nullptr);

   const size_t BS = block_size();

   BOTAN_ASSERT(buffer.size() >= offset, "Offset is sane");
   const size_t sz = buffer.size() - offset;
   uint8_t* buf = buffer.data() + offset;

   secure_vector<uint8_t> mac(BS);

   if(sz)
      {
      const size_t final_full_blocks = sz / BS;
      const size_t remainder_bytes = sz - (final_full_blocks * BS);

      encrypt(buf, final_full_blocks);
      mac = m_L->offset();

      if(remainder_bytes)
         {
         BOTAN_ASSERT(remainder_bytes < BS, "Only a partial block left");
         uint8_t* remainder = &buf[sz - remainder_bytes];

         xor_buf(m_checksum.data(), remainder, remainder_bytes);
         m_checksum[remainder_bytes] ^= 0x80;

         // Offset_*
         mac ^= m_L->star();

         secure_vector<uint8_t> pad(BS);
         m_cipher->encrypt(mac, pad);
         xor_buf(remainder, pad.data(), remainder_bytes);
         }
      }
   else
      {
      mac = m_L->offset();
      }

   // now compute the tag

   // fold checksum
   for(size_t i = 0; i != m_checksum.size(); i += BS)
      {
      xor_buf(mac.data(), m_checksum.data() + i, BS);
      }

   xor_buf(mac.data(), m_L->dollar().data(), BS);
   m_cipher->encrypt(mac);
   xor_buf(mac.data(), m_ad_hash.data(), BS);

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

   zeroise(m_checksum);
   m_block_index = 0;
   }

void OCB_Decryption::decrypt(uint8_t buffer[], size_t blocks)
   {
   verify_key_set(m_L != nullptr);
   BOTAN_STATE_CHECK(m_L->initialized());

   const size_t BS = block_size();

   while(blocks)
      {
      const size_t proc_blocks = std::min(blocks, par_blocks());
      const size_t proc_bytes = proc_blocks * BS;

      const uint8_t* offsets = m_L->compute_offsets(m_block_index, proc_blocks);

      m_cipher->decrypt_n_xex(buffer, offsets, proc_blocks);

      xor_buf(m_checksum.data(), buffer, proc_bytes);

      buffer += proc_bytes;
      blocks -= proc_blocks;
      m_block_index += proc_blocks;
      }
   }

size_t OCB_Decryption::process(uint8_t buf[], size_t sz)
   {
   BOTAN_ASSERT(sz % update_granularity() == 0, "Invalid OCB input size");
   decrypt(buf, sz / block_size());
   return sz;
   }

void OCB_Decryption::finish(secure_vector<uint8_t>& buffer, size_t offset)
   {
   verify_key_set(m_L != nullptr);

   const size_t BS = block_size();

   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(), "We have the tag");

   const size_t remaining = sz - tag_size();

   secure_vector<uint8_t> mac(BS);

   if(remaining)
      {
      const size_t final_full_blocks = remaining / BS;
      const size_t final_bytes = remaining - (final_full_blocks * BS);

      decrypt(buf, final_full_blocks);
      mac ^= m_L->offset();

      if(final_bytes)
         {
         BOTAN_ASSERT(final_bytes < BS, "Only a partial block left");

         uint8_t* remainder = &buf[remaining - final_bytes];

         mac ^= m_L->star();
         secure_vector<uint8_t> pad(BS);
         m_cipher->encrypt(mac, pad); // P_*
         xor_buf(remainder, pad.data(), final_bytes);

         xor_buf(m_checksum.data(), remainder, final_bytes);
         m_checksum[final_bytes] ^= 0x80;
         }
      }
   else
      mac = m_L->offset();

   // compute the mac

   // fold checksum
   for(size_t i = 0; i != m_checksum.size(); i += BS)
      {
      xor_buf(mac.data(), m_checksum.data() + i, BS);
      }

   mac ^= m_L->dollar();
   m_cipher->encrypt(mac);
   mac ^= m_ad_hash;

   // reset state
   zeroise(m_checksum);
   m_block_index = 0;

   // compare mac
   const uint8_t* included_tag = &buf[remaining];

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

   // remove tag from end of message
   buffer.resize(remaining + offset);
   }

}