cfb.cpp

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/*
* CFB Mode
* (C) 1999-2007,2013,2017 Jack Lloyd
* (C) 2016 Daniel Neus, Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/internal/cfb.h>
 
#include <botan/exceptn.h>
#include <botan/mem_ops.h>
#include <botan/internal/fmt.h>
 
namespace Botan {
 
CFB_Mode::CFB_Mode(std::unique_ptr<BlockCipher> cipher, size_t feedback_bits) :
      m_cipher(std::move(cipher)),
      m_block_size(m_cipher->block_size()),
      m_feedback_bytes(feedback_bits != 0 ? feedback_bits / 8 : m_block_size) {
   if(feedback_bits % 8 != 0 || feedback() > m_block_size) {
      throw Invalid_Argument(fmt("{} does not support feedback bits of {}", name(), feedback_bits));
   }
}
 
void CFB_Mode::clear() {
   m_cipher->clear();
   m_keystream.clear();
   reset();
}
 
void CFB_Mode::reset() {
   m_state.clear();
   zeroise(m_keystream);
   m_keystream_pos = 0;
}
 
std::string CFB_Mode::name() const {
   if(feedback() == cipher().block_size()) {
      return fmt("{}/CFB", cipher().name());
   } else {
      return fmt("{}/CFB({})", cipher().name(), feedback() * 8);
   }
}
 
size_t CFB_Mode::output_length(size_t input_length) const {
   return input_length;
}
 
size_t CFB_Mode::update_granularity() const {
   return feedback();
}
 
size_t CFB_Mode::ideal_granularity() const {
   // Multiplier here is arbitrary
   return 16 * feedback();
}
 
size_t CFB_Mode::minimum_final_size() const {
   return 0;
}
 
Key_Length_Specification CFB_Mode::key_spec() const {
   return cipher().key_spec();
}
 
size_t CFB_Mode::default_nonce_length() const {
   return block_size();
}
 
bool CFB_Mode::valid_nonce_length(size_t n) const {
   return (n == 0 || n == block_size());
}
 
bool CFB_Mode::has_keying_material() const {
   return m_cipher->has_keying_material();
}
 
void CFB_Mode::key_schedule(std::span<const uint8_t> key) {
   m_cipher->set_key(key);
   m_keystream.resize(m_cipher->block_size());
 
   // Drop the IV/feedback register and keystream from any prior message;
   // they were computed under the previous key.
   reset();
}
 
void CFB_Mode::start_msg(const uint8_t nonce[], size_t nonce_len) {
   if(!valid_nonce_length(nonce_len)) {
      throw Invalid_IV_Length(name(), nonce_len);
   }
 
   assert_key_material_set();
 
   if(nonce_len == 0) {
      if(m_state.empty()) {
         throw Invalid_State("CFB requires a non-empty initial nonce");
      }
      // No reason to encrypt state->keystream_buf, because no change
   } else {
      m_state.assign(nonce, nonce + nonce_len);
      cipher().encrypt(m_state, m_keystream);
      m_keystream_pos = 0;
   }
}
 
void CFB_Mode::shift_register() {
   const size_t shift = feedback();
   const size_t carryover = block_size() - shift;
 
   if(carryover > 0) {
      copy_mem(m_state.data(), &m_state[shift], carryover);
   }
   copy_mem(&m_state[carryover], m_keystream.data(), shift);
   cipher().encrypt(m_state, m_keystream);
   m_keystream_pos = 0;
}
 
size_t CFB_Encryption::process_msg(uint8_t buf[], size_t sz) {
   assert_key_material_set();
   BOTAN_STATE_CHECK(m_state.empty() == false);
 
   const size_t shift = feedback();
 
   size_t left = sz;
 
   if(m_keystream_pos != 0) {
      const size_t take = std::min<size_t>(left, shift - m_keystream_pos);
 
      xor_buf(m_keystream.data() + m_keystream_pos, buf, take);
      copy_mem(buf, m_keystream.data() + m_keystream_pos, take);
 
      m_keystream_pos += take;
      left -= take;
      buf += take;
 
      if(m_keystream_pos == shift) {
         shift_register();
      }
   }
 
   while(left >= shift) {
      xor_buf(m_keystream.data(), buf, shift);
      copy_mem(buf, m_keystream.data(), shift);
 
      left -= shift;
      buf += shift;
      shift_register();
   }
 
   if(left > 0) {
      xor_buf(m_keystream.data(), buf, left);
      copy_mem(buf, m_keystream.data(), left);
      m_keystream_pos += left;
   }
 
   return sz;
}
 
void CFB_Encryption::finish_msg(secure_vector<uint8_t>& buffer, size_t offset) {
   update(buffer, offset);
}
 
namespace {
 
inline void xor_copy(uint8_t buf[], uint8_t key_buf[], size_t len) {
   for(size_t i = 0; i != len; ++i) {
      const uint8_t k = key_buf[i];
      key_buf[i] = buf[i];
      buf[i] ^= k;
   }
}
 
}  // namespace
 
size_t CFB_Decryption::process_msg(uint8_t buf[], size_t sz) {
   assert_key_material_set();
   BOTAN_STATE_CHECK(m_state.empty() == false);
 
   const size_t shift = feedback();
 
   size_t left = sz;
 
   if(m_keystream_pos != 0) {
      const size_t take = std::min<size_t>(left, shift - m_keystream_pos);
 
      xor_copy(buf, m_keystream.data() + m_keystream_pos, take);
 
      m_keystream_pos += take;
      left -= take;
      buf += take;
 
      if(m_keystream_pos == shift) {
         shift_register();
      }
   }
 
   while(left >= shift) {
      xor_copy(buf, m_keystream.data(), shift);
      left -= shift;
      buf += shift;
      shift_register();
   }
 
   if(left > 0) {
      xor_copy(buf, m_keystream.data(), left);
      m_keystream_pos += left;
   }
 
   return sz;
}
 
void CFB_Decryption::finish_msg(secure_vector<uint8_t>& buffer, size_t offset) {
   update(buffer, offset);
}
 
}  // namespace Botan