xts.cpp

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/*
* XTS Mode
* (C) 2009,2013,2026 Jack Lloyd
* (C) 2016 Daniel Neus, Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/internal/xts.h>
 
#include <botan/exceptn.h>
#include <botan/mem_ops.h>
#include <botan/internal/fmt.h>
#include <botan/internal/poly_dbl.h>
 
#if defined(BOTAN_HAS_MODE_XTS_AVX512_CLMUL)
   #include <botan/internal/cpuid.h>
#endif
 
namespace Botan {
 
XTS_Mode::XTS_Mode(std::unique_ptr<BlockCipher> cipher) :
      m_cipher(std::move(cipher)),
      m_cipher_block_size(m_cipher->block_size()),
      m_cipher_parallelism(m_cipher->parallel_bytes()),
      m_tweak_blocks(m_cipher_parallelism / m_cipher_block_size) {
   if(!poly_double_supported_size(m_cipher_block_size)) {
      throw Invalid_Argument(fmt("Cannot use {} with XTS", m_cipher->name()));
   }
 
   m_tweak_cipher = m_cipher->new_object();
}
 
void XTS_Mode::clear() {
   m_cipher->clear();
   m_tweak_cipher->clear();
   reset();
}
 
size_t XTS_Mode::update_granularity() const {
   return m_cipher_block_size;
}
 
size_t XTS_Mode::ideal_granularity() const {
   return m_cipher_parallelism;
}
 
void XTS_Mode::reset() {
   m_tweak.clear();
}
 
std::string XTS_Mode::name() const {
   return cipher().name() + "/XTS";
}
 
size_t XTS_Mode::minimum_final_size() const {
   return cipher_block_size();
}
 
Key_Length_Specification XTS_Mode::key_spec() const {
   return cipher().key_spec().multiple(2);
}
 
size_t XTS_Mode::default_nonce_length() const {
   return cipher_block_size();
}
 
bool XTS_Mode::valid_nonce_length(size_t n) const {
   return n <= cipher_block_size();
}
 
bool XTS_Mode::has_keying_material() const {
   return m_cipher->has_keying_material() && m_tweak_cipher->has_keying_material();
}
 
void XTS_Mode::key_schedule(std::span<const uint8_t> key) {
   const size_t key_half = key.size() / 2;
 
   if(key.size() % 2 == 1 || !m_cipher->valid_keylength(key_half)) {
      throw Invalid_Key_Length(name(), key.size());
   }
 
   m_cipher->set_key(key.first(key_half));
   m_tweak_cipher->set_key(key.last(key_half));
 
   // Drop the tweak: it was computed under the previous tweak-cipher key
   // and the previous nonce; the user must call start_msg again before
   // any further processing.
   reset();
}
 
void XTS_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_tweak.resize(m_cipher_parallelism);
   clear_mem(m_tweak.data(), m_tweak.size());
   copy_mem(m_tweak.data(), nonce, nonce_len);
   m_tweak_cipher->encrypt(m_tweak.data());
 
   // Just repeated doubling from first, remaining contents are junk...
   xts_compute_tweak_block(m_tweak.data(), m_tweak_cipher->block_size(), tweak_blocks());
}
 
//static
void XTS_Mode::update_tweak_block(uint8_t tweak[], size_t BS, size_t blocks_in_tweak) {
#if defined(BOTAN_HAS_MODE_XTS_AVX512_CLMUL)
   if(BS == 16 && blocks_in_tweak % 8 == 0 && CPUID::has(CPUID::Feature::AVX512_CLMUL)) {
      return update_tweak_block_avx512_clmul(tweak, BS, blocks_in_tweak);
   }
#endif
 
   /*
   * If we don't have a fast method available, just set the first tweak block to
   * the doubling of the last tweak block, and recompute all the rest via
   * successive doublings.
   */
   poly_double_n_le(tweak, &tweak[(blocks_in_tweak - 1) * BS], BS);
   xts_compute_tweak_block(tweak, BS, blocks_in_tweak);
}
 
void XTS_Mode::update_tweak(size_t consumed) {
   const size_t BS = m_tweak_cipher->block_size();
   const size_t blocks_in_tweak = tweak_blocks();
 
   BOTAN_ASSERT_NOMSG(consumed > 0 && consumed <= blocks_in_tweak);
 
   if(consumed == blocks_in_tweak) {
      // Update all in parallel
      update_tweak_block(m_tweak.data(), BS, blocks_in_tweak);
   } else {
      /*
      The last remaining tweaks can just be shifted over
 
      This could be a lot better though! We can copy all of the remaining tweaks
      and just recompute the last few
      */
      copy_mem(m_tweak.data(), &m_tweak[(consumed * BS)], BS);
      xts_compute_tweak_block(m_tweak.data(), BS, blocks_in_tweak);
   }
}
 
size_t XTS_Encryption::output_length(size_t input_length) const {
   return input_length;
}
 
size_t XTS_Encryption::process_msg(uint8_t buf[], size_t sz) {
   BOTAN_STATE_CHECK(tweak_set());
   const size_t BS = cipher_block_size();
 
   BOTAN_ARG_CHECK(sz % BS == 0, "Input is not full blocks");
   size_t blocks = sz / BS;
 
   const size_t blocks_in_tweak = tweak_blocks();
 
   while(blocks > 0) {
      const size_t to_proc = std::min(blocks, blocks_in_tweak);
      const size_t proc_bytes = to_proc * BS;
 
      xor_buf(buf, tweak(), proc_bytes);
      cipher().encrypt_n(buf, buf, to_proc);
      xor_buf(buf, tweak(), proc_bytes);
 
      buf += proc_bytes;
      blocks -= to_proc;
 
      update_tweak(to_proc);
   }
 
   return sz;
}
 
void XTS_Encryption::finish_msg(secure_vector<uint8_t>& buffer, size_t offset) {
   BOTAN_STATE_CHECK(tweak_set());
   BOTAN_ARG_CHECK(buffer.size() >= offset, "Offset is out of range");
   const size_t sz = buffer.size() - offset;
   uint8_t* buf = buffer.data() + offset;
 
   BOTAN_ARG_CHECK(sz >= minimum_final_size(), "missing sufficient final input in XTS encrypt");
 
   const size_t BS = cipher_block_size();
 
   if(sz % BS == 0) {
      update(buffer, offset);
   } else {
      // steal ciphertext
      const size_t full_blocks = ((sz / BS) - 1) * BS;
      const size_t final_bytes = sz - full_blocks;
      BOTAN_ASSERT(final_bytes > BS && final_bytes < 2 * BS, "Left over size in expected range");
 
      secure_vector<uint8_t> last(buf + full_blocks, buf + full_blocks + final_bytes);
      buffer.resize(full_blocks + offset);
      update(buffer, offset);
 
      xor_buf(last, tweak(), BS);
      cipher().encrypt(last);
      xor_buf(last, tweak(), BS);
 
      for(size_t i = 0; i != final_bytes - BS; ++i) {
         last[i] ^= last[i + BS];
         last[i + BS] ^= last[i];
         last[i] ^= last[i + BS];
      }
 
      xor_buf(last, tweak() + BS, BS);
      cipher().encrypt(last);
      xor_buf(last, tweak() + BS, BS);
 
      buffer += last;
   }
}
 
size_t XTS_Decryption::output_length(size_t input_length) const {
   return input_length;
}
 
size_t XTS_Decryption::process_msg(uint8_t buf[], size_t sz) {
   BOTAN_STATE_CHECK(tweak_set());
   const size_t BS = cipher_block_size();
 
   BOTAN_ARG_CHECK(sz % BS == 0, "Input is not full blocks");
   size_t blocks = sz / BS;
 
   const size_t blocks_in_tweak = tweak_blocks();
 
   while(blocks > 0) {
      const size_t to_proc = std::min(blocks, blocks_in_tweak);
      const size_t proc_bytes = to_proc * BS;
 
      xor_buf(buf, tweak(), proc_bytes);
      cipher().decrypt_n(buf, buf, to_proc);
      xor_buf(buf, tweak(), proc_bytes);
 
      buf += proc_bytes;
      blocks -= to_proc;
 
      update_tweak(to_proc);
   }
 
   return sz;
}
 
void XTS_Decryption::finish_msg(secure_vector<uint8_t>& buffer, size_t offset) {
   BOTAN_STATE_CHECK(tweak_set());
   BOTAN_ARG_CHECK(buffer.size() >= offset, "Offset is out of range");
   const size_t sz = buffer.size() - offset;
   uint8_t* buf = buffer.data() + offset;
 
   BOTAN_ARG_CHECK(sz >= minimum_final_size(), "missing sufficient final input in XTS decrypt");
 
   const size_t BS = cipher_block_size();
 
   if(sz % BS == 0) {
      update(buffer, offset);
   } else {
      // steal ciphertext
      const size_t full_blocks = ((sz / BS) - 1) * BS;
      const size_t final_bytes = sz - full_blocks;
      BOTAN_ASSERT(final_bytes > BS && final_bytes < 2 * BS, "Left over size in expected range");
 
      secure_vector<uint8_t> last(buf + full_blocks, buf + full_blocks + final_bytes);
      buffer.resize(full_blocks + offset);
      update(buffer, offset);
 
      xor_buf(last, tweak() + BS, BS);
      cipher().decrypt(last);
      xor_buf(last, tweak() + BS, BS);
 
      for(size_t i = 0; i != final_bytes - BS; ++i) {
         last[i] ^= last[i + BS];
         last[i + BS] ^= last[i];
         last[i] ^= last[i + BS];
      }
 
      xor_buf(last, tweak(), BS);
      cipher().decrypt(last);
      xor_buf(last, tweak(), BS);
 
      buffer += last;
   }
}
 
}  // namespace Botan