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/*
* EAX Mode Encryption
* (C) 1999-2007 Jack Lloyd
* (C) 2016 Daniel Neus, Rohde & Schwarz Cybersecurity
*
* Botan is released under the Simplified BSD License (see license.txt)
*/
#include <botan/eax.h>
#include <botan/cmac.h>
#include <botan/ctr.h>
namespace Botan {
namespace {
/*
* EAX MAC-based PRF
*/
secure_vector<uint8_t> eax_prf(uint8_t tag, size_t block_size,
MessageAuthenticationCode& mac,
const uint8_t in[], size_t length)
{
for(size_t i = 0; i != block_size - 1; ++i)
{
mac.update(0);
}
mac.update(tag);
mac.update(in, length);
return mac.final();
}
}
/*
* EAX_Mode Constructor
*/
EAX_Mode::EAX_Mode(BlockCipher* cipher, size_t tag_size) :
m_tag_size(tag_size),
m_cipher(cipher),
m_ctr(new CTR_BE(m_cipher->clone())),
m_cmac(new CMAC(m_cipher->clone()))
{
if(m_tag_size < 8 || m_tag_size > m_cmac->output_length())
throw Invalid_Argument(name() + ": Bad tag size " + std::to_string(tag_size));
}
void EAX_Mode::clear()
{
m_cipher->clear();
m_ctr->clear();
m_cmac->clear();
reset();
}
void EAX_Mode::reset()
{
m_ad_mac.clear();
m_nonce_mac.clear();
// Clear out any data added to the CMAC calculation
try {
m_cmac->final();
}
catch(Key_Not_Set&) {}
}
std::string EAX_Mode::name() const
{
return (m_cipher->name() + "/EAX");
}
size_t EAX_Mode::update_granularity() const
{
/*
* For EAX this actually can be as low as 1 but that causes problems
* for applications which use update_granularity as the buffer size.
*/
return m_cipher->parallel_bytes();
}
Key_Length_Specification EAX_Mode::key_spec() const
{
return m_cipher->key_spec();
}
/*
* Set the EAX key
*/
void EAX_Mode::key_schedule(const uint8_t key[], size_t length)
{
/*
* These could share the key schedule, which is one nice part of EAX,
* but it's much easier to ignore that here...
*/
m_ctr->set_key(key, length);
m_cmac->set_key(key, length);
}
/*
* Set the EAX associated data
*/
void EAX_Mode::set_associated_data(const uint8_t ad[], size_t length)
{
if(m_nonce_mac.empty() == false)
throw Invalid_State("Cannot set AD for EAX while processing a message");
m_ad_mac = eax_prf(1, block_size(), *m_cmac, ad, length);
}
void EAX_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_nonce_mac = eax_prf(0, block_size(), *m_cmac, nonce, nonce_len);
m_ctr->set_iv(m_nonce_mac.data(), m_nonce_mac.size());
for(size_t i = 0; i != block_size() - 1; ++i)
m_cmac->update(0);
m_cmac->update(2);
}
size_t EAX_Encryption::process(uint8_t buf[], size_t sz)
{
BOTAN_STATE_CHECK(m_nonce_mac.size() > 0);
m_ctr->cipher(buf, buf, sz);
m_cmac->update(buf, sz);
return sz;
}
void EAX_Encryption::finish(secure_vector<uint8_t>& buffer, size_t offset)
{
BOTAN_ASSERT_NOMSG(m_nonce_mac.empty() == false);
update(buffer, offset);
secure_vector<uint8_t> data_mac = m_cmac->final();
xor_buf(data_mac, m_nonce_mac, data_mac.size());
if(m_ad_mac.empty())
{
m_ad_mac = eax_prf(1, block_size(), *m_cmac, nullptr, 0);
}
xor_buf(data_mac, m_ad_mac, data_mac.size());
buffer += std::make_pair(data_mac.data(), tag_size());
}
size_t EAX_Decryption::process(uint8_t buf[], size_t sz)
{
BOTAN_STATE_CHECK(m_nonce_mac.size() > 0);
m_cmac->update(buf, sz);
m_ctr->cipher(buf, buf, sz);
return sz;
}
void EAX_Decryption::finish(secure_vector<uint8_t>& buffer, size_t offset)
{
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(), "Have the tag as part of final input");
const size_t remaining = sz - tag_size();
if(remaining)
{
m_cmac->update(buf, remaining);
m_ctr->cipher(buf, buf, remaining);
}
const uint8_t* included_tag = &buf[remaining];
secure_vector<uint8_t> mac = m_cmac->final();
mac ^= m_nonce_mac;
if(m_ad_mac.empty())
{
m_ad_mac = eax_prf(1, block_size(), *m_cmac, nullptr, 0);
}
mac ^= m_ad_mac;
if(!constant_time_compare(mac.data(), included_tag, tag_size()))
throw Invalid_Authentication_Tag("EAX tag check failed");
buffer.resize(offset + remaining);
m_nonce_mac.clear();
}
}
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