siv.cpp

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/*
* SIV Mode Encryption
* (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/internal/siv.h>
 
#include <botan/block_cipher.h>
#include <botan/exceptn.h>
#include <botan/mem_ops.h>
#include <botan/internal/cmac.h>
#include <botan/internal/ct_utils.h>
#include <botan/internal/ctr.h>
#include <botan/internal/int_utils.h>
#include <botan/internal/poly_dbl.h>
 
namespace Botan {
 
SIV_Mode::SIV_Mode(std::unique_ptr<BlockCipher> cipher) :
      m_name(cipher->name() + "/SIV"),
      m_bs(cipher->block_size()),
      m_ctr(std::make_unique<CTR_BE>(cipher->new_object(), 8)),
      m_mac(std::make_unique<CMAC>(std::move(cipher))) {
   // Not really true but only 128 bit allowed at the moment
   if(m_bs != 16) {
      throw Invalid_Argument("SIV requires a 128 bit block cipher");
   }
}
 
SIV_Mode::~SIV_Mode() = default;
 
void SIV_Mode::clear() {
   m_ctr->clear();
   m_mac->clear();
   m_ad_macs.clear();
   reset();
}
 
void SIV_Mode::reset() {
   m_nonce.clear();
   m_msg_buf.clear();
   m_in_msg = false;
}
 
std::string SIV_Mode::name() const {
   return m_name;
}
 
bool SIV_Mode::valid_nonce_length(size_t /*length*/) const {
   return true;
}
 
size_t SIV_Mode::update_granularity() const {
   return 1;
}
 
size_t SIV_Mode::ideal_granularity() const {
   // Completely arbitrary value:
   return 128;
}
 
bool SIV_Mode::requires_entire_message() const {
   return true;
}
 
Key_Length_Specification SIV_Mode::key_spec() const {
   return m_mac->key_spec().multiple(2);
}
 
bool SIV_Mode::has_keying_material() const {
   return m_ctr->has_keying_material() && m_mac->has_keying_material();
}
 
void SIV_Mode::key_schedule(std::span<const uint8_t> key) {
   const size_t keylen = key.size() / 2;
   m_mac->set_key(key.first(keylen));
   m_ctr->set_key(key.last(keylen));
   m_ad_macs.clear();
   reset();
}
 
size_t SIV_Mode::maximum_associated_data_inputs() const {
   return block_size() * 8 - 2;
}
 
void SIV_Mode::set_associated_data_n(size_t n, std::span<const uint8_t> ad) {
   BOTAN_STATE_CHECK(!m_in_msg);
   const size_t max_ads = maximum_associated_data_inputs();
   if(n >= max_ads) {
      throw Invalid_Argument(name() + " allows no more than " + std::to_string(max_ads) + " ADs");
   }
 
   if(n > m_ad_macs.size()) {
      // If we are potentially skipping over AD elements in a way that will
      // create a gap, fill the gaps in with the mac of the empty string.
      const auto empty_mac = m_mac->process(std::span<const uint8_t>{});
      m_ad_macs.resize(n + 1, empty_mac);
   } else if(n == m_ad_macs.size()) {
      m_ad_macs.resize(n + 1);
   }
 
   m_ad_macs[n] = m_mac->process(ad);
}
 
void SIV_Mode::start_msg(const uint8_t nonce[], size_t nonce_len) {
   BOTAN_STATE_CHECK(!m_in_msg);
 
   if(!valid_nonce_length(nonce_len)) {
      throw Invalid_IV_Length(name(), nonce_len);
   }
 
   if(nonce_len > 0) {
      m_nonce = m_mac->process(nonce, nonce_len);
   } else {
      m_nonce.clear();
   }
 
   m_msg_buf.clear();
   m_in_msg = true;
}
 
size_t SIV_Mode::process_msg(uint8_t buf[], size_t sz) {
   // all output is saved for processing in finish
   m_msg_buf.insert(m_msg_buf.end(), buf, buf + sz);
   // SIV supports a "no start_msg" mode (deterministic, nonce-less): the
   // first process_msg locks AD just as start_msg would.
   m_in_msg = true;
   return 0;
}
 
secure_vector<uint8_t> SIV_Mode::S2V(const uint8_t* text, size_t text_len) {
   // S2V processes at most block_size()*8 - 1 (127 for a 128-bit block)
   // components; the associated data, the nonce (if present), and the plaintext
   // are all components, so reject inputs that would exceed the limit.
   const size_t s2v_components = m_ad_macs.size() + (m_nonce.empty() ? 0 : 1) + 1;
   if(s2v_components > block_size() * 8 - 1) {
      throw Invalid_Argument(name() + ": too many S2V components");
   }
 
   const std::vector<uint8_t> zeros(block_size());
 
   secure_vector<uint8_t> V = m_mac->process(zeros.data(), zeros.size());
 
   for(const auto& ad_mac : m_ad_macs) {
      poly_double_n(V.data(), V.size());
      V ^= ad_mac;
   }
 
   if(!m_nonce.empty()) {
      poly_double_n(V.data(), V.size());
      V ^= m_nonce;
   }
 
   if(text_len < block_size()) {
      poly_double_n(V.data(), V.size());
      xor_buf(V.data(), text, text_len);
      V[text_len] ^= 0x80;
      return m_mac->process(V);
   }
 
   m_mac->update(text, text_len - block_size());
   xor_buf(V.data(), &text[text_len - block_size()], block_size());
   m_mac->update(V);
 
   return m_mac->final();
}
 
void SIV_Mode::set_ctr_iv(secure_vector<uint8_t> V) {
   V[m_bs - 8] &= 0x7F;
   V[m_bs - 4] &= 0x7F;
 
   ctr().set_iv(V.data(), V.size());
}
 
size_t SIV_Encryption::output_length(size_t input_length) const {
   return add_or_throw(input_length, tag_size(), "SIV input too large");
}
 
void SIV_Encryption::finish_msg(secure_vector<uint8_t>& buffer, size_t offset) {
   BOTAN_ARG_CHECK(buffer.size() >= offset, "Offset is out of range");
 
   buffer.insert(buffer.begin() + offset, msg_buf().begin(), msg_buf().end());
 
   const secure_vector<uint8_t> V = S2V(buffer.data() + offset, buffer.size() - offset);
 
   buffer.insert(buffer.begin() + offset, V.begin(), V.end());
 
   if(buffer.size() != offset + V.size()) {
      set_ctr_iv(V);
      ctr().cipher1(&buffer[offset + V.size()], buffer.size() - offset - V.size());
   }
 
   // Drop m_nonce as well as the in-message flag. S2V() consumes m_nonce,
   // so leaving it live would let a subsequent finish_msg() without an
   // intervening start_msg() silently re-use the prior nonce instead of
   // running nonce-less SIV.
   reset();
}
 
size_t SIV_Decryption::output_length(size_t input_length) const {
   BOTAN_ARG_CHECK(input_length >= tag_size(), "Message too short to be valid");
   return input_length - tag_size();
}
 
void SIV_Decryption::finish_msg(secure_vector<uint8_t>& buffer, size_t offset) {
   BOTAN_ARG_CHECK(buffer.size() >= offset, "Offset is out of range");
 
   if(!msg_buf().empty()) {
      buffer.insert(buffer.begin() + offset, msg_buf().begin(), msg_buf().end());
      msg_buf().clear();
   }
 
   const size_t sz = buffer.size() - offset;
 
   BOTAN_ARG_CHECK(sz >= tag_size(), "input did not include the tag");
 
   secure_vector<uint8_t> V(buffer.data() + offset, buffer.data() + offset + block_size());
 
   if(buffer.size() != offset + V.size()) {
      set_ctr_iv(V);
 
      ctr().cipher(buffer.data() + offset + V.size(), buffer.data() + offset, buffer.size() - offset - V.size());
   }
 
   const secure_vector<uint8_t> T = S2V(buffer.data() + offset, buffer.size() - offset - V.size());
 
   // See SIV_Encryption::finish_msg for why this is reset() and not just
   // clearing the in-message flag.
   reset();
 
   if(!CT::is_equal<uint8_t>(T, V).as_bool()) {
      clear_mem(std::span{buffer}.subspan(offset, buffer.size() - offset - V.size()));
      throw Invalid_Authentication_Tag("SIV tag check failed");
   }
 
   buffer.resize(buffer.size() - tag_size());
}
 
}  // namespace Botan