gcm.cpp

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/*
* GCM Mode Encryption
* (C) 2013,2015 Jack Lloyd
* (C) 2016 Daniel Neus, Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
 
#include <botan/internal/gcm.h>
 
#include <botan/block_cipher.h>
#include <botan/exceptn.h>
#include <botan/mem_ops.h>
#include <botan/internal/ct_utils.h>
#include <botan/internal/ctr.h>
#include <botan/internal/fmt.h>
#include <botan/internal/ghash.h>
#include <botan/internal/int_utils.h>
#include <array>
 
namespace Botan {
 
/*
* GCM_Mode Constructor
*/
GCM_Mode::GCM_Mode(std::unique_ptr<BlockCipher> cipher, size_t tag_size) :
      m_tag_size(tag_size), m_cipher_name(cipher->name()) {
   if(cipher->block_size() != GCM_BS) {
      throw Invalid_Argument("Invalid block cipher for GCM");
   }
 
   /* We allow any of the values 128, 120, 112, 104, or 96 bits as a tag size */
   /* 64 bit tag is still supported but deprecated and will be removed in the future */
   if(m_tag_size != 8 && (m_tag_size < 12 || m_tag_size > 16)) {
      throw Invalid_Argument(fmt("{} cannot use a tag of {} bytes", name(), m_tag_size));
   }
 
   m_ctr = std::make_unique<CTR_BE>(std::move(cipher), 4);
   m_ghash = std::make_unique<GHASH>();
}
 
GCM_Mode::~GCM_Mode() = default;
 
void GCM_Mode::clear() {
   m_ctr->clear();
   m_ghash->clear();
   reset();
}
 
void GCM_Mode::reset() {
   m_ghash->reset_state();
   m_in_msg = false;
}
 
std::string GCM_Mode::name() const {
   return fmt("{}/GCM({})", m_cipher_name, tag_size());
}
 
std::string GCM_Mode::provider() const {
   return m_ghash->provider();
}
 
size_t GCM_Mode::update_granularity() const {
   return 1;
}
 
size_t GCM_Mode::ideal_granularity() const {
   return GCM_BS * std::max<size_t>(2, BlockCipher::ParallelismMult);
}
 
bool GCM_Mode::valid_nonce_length(size_t len) const {
   // GCM does not support empty nonces
   return (len > 0);
}
 
Key_Length_Specification GCM_Mode::key_spec() const {
   return m_ctr->key_spec();
}
 
bool GCM_Mode::has_keying_material() const {
   return m_ctr->has_keying_material();
}
 
void GCM_Mode::key_schedule(std::span<const uint8_t> key) {
   reset();
   m_ctr->set_key(key);
 
   std::array<uint8_t, GCM_BS> zeros{};
   m_ctr->set_iv(zeros);
 
   uint8_t H[GCM_BS] = {0};
   m_ctr->encipher(H);
   m_ghash->set_key(H);
}
 
void GCM_Mode::set_associated_data_n(size_t idx, std::span<const uint8_t> ad) {
   BOTAN_ARG_CHECK(idx == 0, "GCM: cannot handle non-zero index in set_associated_data_n");
   m_ghash->set_associated_data(ad);
}
 
void GCM_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);
   }
 
   std::array<uint8_t, GCM_BS> y0 = {};
 
   if(nonce_len == 12) {
      copy_mem(y0.data(), nonce, nonce_len);
      y0[15] = 1;
   } else {
      m_ghash->nonce_hash(std::span<uint8_t, GCM_BS>(y0), {nonce, nonce_len});
   }
 
   m_ctr->set_iv(y0.data(), y0.size());
 
   clear_mem(y0.data(), y0.size());
   m_ctr->encipher(y0);
 
   m_ghash->start(y0);
   secure_scrub_memory(y0);
   m_in_msg = true;
}
 
size_t GCM_Encryption::output_length(size_t input_length) const {
   return add_or_throw(input_length, tag_size(), "GCM input too large");
}
 
size_t GCM_Encryption::process_msg(uint8_t buf[], size_t sz) {
   BOTAN_STATE_CHECK(m_in_msg);
   BOTAN_ARG_CHECK(sz % update_granularity() == 0, "Invalid buffer size");
   m_ctr->cipher(buf, buf, sz);
   m_ghash->update({buf, sz});
   return sz;
}
 
void GCM_Encryption::finish_msg(secure_vector<uint8_t>& buffer, size_t offset) {
   BOTAN_STATE_CHECK(m_in_msg);
   BOTAN_ARG_CHECK(offset <= buffer.size(), "Invalid offset");
   const size_t sz = buffer.size() - offset;
   uint8_t* buf = buffer.data() + offset;
 
   m_ctr->cipher(buf, buf, sz);
   m_ghash->update({buf, sz});
 
   std::array<uint8_t, 16> mac = {0};
   m_ghash->final(std::span(mac).first(tag_size()));
   buffer += std::make_pair(mac.data(), tag_size());
   m_in_msg = false;
}
 
size_t GCM_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();
}
 
size_t GCM_Decryption::process_msg(uint8_t buf[], size_t sz) {
   BOTAN_STATE_CHECK(m_in_msg);
   BOTAN_ARG_CHECK(sz % update_granularity() == 0, "Invalid buffer size");
   m_ghash->update({buf, sz});
   m_ctr->cipher(buf, buf, sz);
   return sz;
}
 
void GCM_Decryption::finish_msg(secure_vector<uint8_t>& buffer, size_t offset) {
   BOTAN_STATE_CHECK(m_in_msg);
   BOTAN_ARG_CHECK(offset <= buffer.size(), "Invalid offset");
   const size_t sz = buffer.size() - offset;
   uint8_t* buf = buffer.data() + offset;
 
   BOTAN_ARG_CHECK(sz >= tag_size(), "input did not include the tag");
 
   const size_t remaining = sz - tag_size();
 
   // handle any final input before the tag
   if(remaining > 0) {
      m_ghash->update({buf, remaining});
      m_ctr->cipher(buf, buf, remaining);
   }
 
   std::array<uint8_t, 16> mac = {0};
   m_ghash->final(std::span(mac).first(tag_size()));
 
   const uint8_t* included_tag = &buffer[remaining + offset];
 
   m_in_msg = false;
 
   if(!CT::is_equal(mac.data(), included_tag, tag_size()).as_bool()) {
      clear_mem(std::span{buffer}.subspan(offset, remaining));
      throw Invalid_Authentication_Tag("GCM tag check failed");
   }
 
   buffer.resize(offset + remaining);
}
 
}  // namespace Botan