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/*
* GCM Mode Encryption
* (C) 2013 Jack Lloyd
*
* Distributed under the terms of the Botan license
*/
#include <botan/gcm.h>
#include <botan/ctr.h>
#include <botan/internal/xor_buf.h>
#include <botan/loadstor.h>
namespace Botan {
namespace {
secure_vector<byte>
gcm_multiply(const secure_vector<byte>& x,
const secure_vector<byte>& y)
{
static const u64bit R = 0xE100000000000000;
u64bit V[2] = {
load_be<u64bit>(&y[0], 0),
load_be<u64bit>(&y[0], 1)
};
u64bit Z[2] = { 0, 0 };
for(size_t i = 0; i != 2; ++i)
{
u64bit X = load_be<u64bit>(&x[0], i);
for(size_t j = 0; j != 64; ++j)
{
if(X >> 63)
{
Z[0] ^= V[0];
Z[1] ^= V[1];
}
const u64bit r = (V[1] & 1) ? R : 0;
V[1] = (V[0] << 63) | (V[1] >> 1);
V[0] = (V[0] >> 1) ^ r;
X <<= 1;
}
}
secure_vector<byte> out(16);
store_be<u64bit>(&out[0], Z[0], Z[1]);
return out;
}
void ghash_update(const secure_vector<byte>& H,
secure_vector<byte>& ghash,
const byte input[], size_t length)
{
const size_t BS = 16;
/*
This assumes if less than block size input then we're just on the
final block and should pad with zeros
*/
while(length)
{
const size_t to_proc = std::min(length, BS);
xor_buf(&ghash[0], &input[0], to_proc);
ghash = gcm_multiply(ghash, H);
input += to_proc;
length -= to_proc;
}
}
void ghash_finalize(const secure_vector<byte>& H,
secure_vector<byte>& ghash,
size_t ad_len, size_t text_len)
{
secure_vector<byte> final_block(16);
store_be<u64bit>(&final_block[0], 8*ad_len, 8*text_len);
ghash_update(H, ghash, &final_block[0], final_block.size());
}
}
/*
* GCM_Mode Constructor
*/
GCM_Mode::GCM_Mode(BlockCipher* cipher, size_t tag_size) :
m_tag_size(tag_size),
m_cipher_name(cipher->name()),
m_H(16), m_H_ad(16), m_mac(16),
m_ad_len(0), m_text_len(0)
{
if(cipher->block_size() != BS)
throw std::invalid_argument("OCB requires a 128 bit cipher so cannot be used with " +
cipher->name());
m_ctr.reset(new CTR_BE(cipher)); // CTR_BE takes ownership of cipher
if(m_tag_size < 8 || m_tag_size > 16)
throw Invalid_Argument(name() + ": Bad tag size " + std::to_string(m_tag_size));
}
void GCM_Mode::clear()
{
zeroise(m_H);
zeroise(m_H_ad);
zeroise(m_mac);
zeroise(m_enc_y0);
m_ad_len = 0;
m_text_len = 0;
m_ctr.reset();
}
std::string GCM_Mode::name() const
{
return (m_cipher_name + "/GCM");
}
size_t GCM_Mode::update_granularity() const
{
return 4096; // CTR-BE's internal block size
}
Key_Length_Specification GCM_Mode::key_spec() const
{
return m_ctr->key_spec();
}
void GCM_Mode::key_schedule(const byte key[], size_t keylen)
{
m_ctr->set_key(key, keylen);
const std::vector<byte> zeros(BS);
m_ctr->set_iv(&zeros[0], zeros.size());
zeroise(m_H);
m_ctr->cipher(&m_H[0], &m_H[0], m_H.size());
}
void GCM_Mode::set_associated_data(const byte ad[], size_t ad_len)
{
zeroise(m_H_ad);
ghash_update(m_H, m_H_ad, ad, ad_len);
m_ad_len = ad_len;
}
secure_vector<byte> GCM_Mode::start(const byte nonce[], size_t nonce_len)
{
secure_vector<byte> y0(BS);
if(nonce_len == 12)
{
copy_mem(&y0[0], nonce, nonce_len);
y0[15] = 1;
}
else
{
ghash_update(m_H, y0, nonce, nonce_len);
ghash_finalize(m_H, y0, 0, nonce_len);
}
m_ctr->set_iv(&y0[0], y0.size());
m_enc_y0.resize(BS);
m_ctr->encipher(m_enc_y0);
m_text_len = 0;
m_mac = m_H_ad;
return secure_vector<byte>();
}
void GCM_Encryption::update(secure_vector<byte>& buffer)
{
m_ctr->cipher(&buffer[0], &buffer[0], buffer.size());
ghash_update(m_H, m_mac, &buffer[0], buffer.size());
m_text_len += buffer.size();
}
void GCM_Encryption::finish(secure_vector<byte>& buffer)
{
update(buffer);
ghash_finalize(m_H, m_mac, m_ad_len, m_text_len);
m_mac ^= m_enc_y0;
buffer += m_mac;
}
void GCM_Decryption::update(secure_vector<byte>& buffer)
{
ghash_update(m_H, m_mac, &buffer[0], buffer.size());
m_ctr->cipher(&buffer[0], &buffer[0], buffer.size());
m_text_len += buffer.size();
}
void GCM_Decryption::finish(secure_vector<byte>& buffer)
{
BOTAN_ASSERT(buffer.size() >= tag_size(),
"Have the tag as part of final input");
// handle any final input before the tag
if(size_t input_length = buffer.size() - tag_size())
{
ghash_update(m_H, m_mac, &buffer[0], input_length);
m_ctr->cipher(&buffer[0], &buffer[0], input_length);
m_text_len += input_length;
}
ghash_finalize(m_H, m_mac, m_ad_len, m_text_len);
m_mac ^= m_enc_y0;
const byte* included_tag = &buffer[buffer.size() - tag_size()];
if(!same_mem(&m_mac[0], included_tag, tag_size()))
throw Integrity_Failure("GCM tag check failed");
}
}
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