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/**
* (C) Copyright Projet SECRET, INRIA, Rocquencourt
* (C) Bhaskar Biswas and Nicolas Sendrier
*
* (C) 2014 cryptosource GmbH
* (C) 2014 Falko Strenzke fstrenzke@cryptosource.de
*
* Distributed under the terms of the Botan license
*
*/
#include <botan/goppa_code.h>
#include <botan/gf2m_rootfind_dcmp.h>
#include <botan/code_based_util.h>
namespace Botan {
namespace {
void matrix_arr_mul(std::vector<u32bit> matrix,
u32bit numo_rows,
u32bit words_per_row,
const byte* input_vec,
u32bit* output_vec, u32bit output_vec_len)
{
for(size_t j = 0; j < numo_rows; j++)
{
if((input_vec[j / 8] >> (j % 8)) & 1)
{
for(size_t i = 0; i < output_vec_len; i ++)
{
output_vec[i] ^= matrix[ j * (words_per_row) + i];
}
}
}
}
/**
* returns the error vector to the syndrome
*/
secure_vector<gf2m> goppa_decode(const polyn_gf2m & syndrom_polyn,
const polyn_gf2m & g,
const std::vector<polyn_gf2m> & sqrtmod,
const std::vector<gf2m> & Linv)
{
gf2m a;
u32bit code_length = Linv.size();
u32bit t = g.get_degree();
std::shared_ptr<gf2m_small_m::Gf2m_Field> sp_field = g.get_sp_field();
std::pair<polyn_gf2m, polyn_gf2m> h__aux = polyn_gf2m::eea_with_coefficients( syndrom_polyn, g, 1);
polyn_gf2m & h = h__aux.first;
polyn_gf2m & aux = h__aux.second;
a = sp_field->gf_inv(aux.get_coef(0));
gf2m log_a = sp_field->gf_log(a);
for(int i = 0; i <= h.get_degree(); ++i)
{
h.set_coef(i,sp_field->gf_mul_zrz(log_a,h.get_coef(i)));
}
// compute h(z) += z
h.add_to_coef( 1, 1);
// compute S square root of h (using sqrtmod)
polyn_gf2m S(t - 1, g.get_sp_field());
for(u32bit i=0;i<t;i++)
{
a = sp_field->gf_sqrt(h.get_coef(i));
if(i & 1)
{
for(u32bit j=0;j<t;j++)
{
S.add_to_coef( j, sp_field->gf_mul(a, sqrtmod[i/2].get_coef(j)));
}
}
else
{
S.add_to_coef( i/2, a);
}
} /* end for loop (i) */
S.get_degree();
std::pair<polyn_gf2m, polyn_gf2m> v__u = polyn_gf2m::eea_with_coefficients(S, g, t/2+1);
polyn_gf2m & u = v__u.second;
polyn_gf2m & v = v__u.first;
// sigma = u^2+z*v^2
polyn_gf2m sigma ( t , g.get_sp_field());
const size_t u_deg = u.get_degree();
for(size_t i = 0; i <= u_deg; ++i)
{
sigma.set_coef(2*i, sp_field->gf_square(u.get_coef(i)));
}
const size_t v_deg = v.get_degree();
for(size_t i = 0; i <= v_deg; ++i)
{
sigma.set_coef(2*i+1, sp_field->gf_square(v.get_coef(i)));
}
secure_vector<gf2m> res = find_roots_gf2m_decomp(sigma, code_length);
size_t d = res.size();
secure_vector<gf2m> result(d);
for(u32bit i = 0; i < d; ++i)
{
gf2m current = res[i];
gf2m tmp;
tmp = gray_to_lex(current);
if(tmp >= code_length) /* invalid root */
{
result[i] = i;
}
result[i] = Linv[tmp];
}
return result;
}
}
/**
* @param p_err_pos_len must point to the available length of err_pos on input, the
* function will set it to the actual number of errors returned in the err_pos
* array */
secure_vector<byte> mceliece_decrypt(
secure_vector<gf2m> & error_pos,
const byte *ciphertext, u32bit ciphertext_len,
const McEliece_PrivateKey & key)
{
u32bit dimension = key.get_dimension();
u32bit codimension = key.get_codimension();
u32bit t = key.get_goppa_polyn().get_degree();
polyn_gf2m syndrome_polyn(key.get_goppa_polyn().get_sp_field()); // init as zero polyn
const unsigned unused_pt_bits = dimension % 8;
const unsigned char unused_pt_bits_mask = (1 << unused_pt_bits) - 1;
if(ciphertext_len != (key.get_code_length()+7)/8)
{
throw Invalid_Argument("wrong size of McEliece ciphertext");
}
u32bit cleartext_len = (key.get_message_word_bit_length()+7)/8;
if(cleartext_len != bit_size_to_byte_size(dimension))
{
throw Invalid_Argument("mce-decryption: wrong length of cleartext buffer");
}
secure_vector<u32bit> syndrome_vec(bit_size_to_32bit_size(codimension));
matrix_arr_mul(
key.get_H_coeffs(),
key.get_code_length(),
bit_size_to_32bit_size(codimension),
ciphertext,
&syndrome_vec[0], syndrome_vec.size() );
secure_vector<byte> syndrome_byte_vec(bit_size_to_byte_size(codimension));
u32bit syndrome_byte_vec_size = syndrome_byte_vec.size();
for(u32bit i = 0; i < syndrome_byte_vec_size; i++)
{
syndrome_byte_vec[i] = syndrome_vec[i/4] >> (8* (i % 4));
}
syndrome_polyn = polyn_gf2m( t-1, &syndrome_byte_vec[0],bit_size_to_byte_size(codimension), key.get_goppa_polyn().get_sp_field());
syndrome_polyn.get_degree();
error_pos = goppa_decode(syndrome_polyn, key.get_goppa_polyn(), key.get_sqrtmod(), key.get_Linv());
u32bit nb_err = error_pos.size();
secure_vector<byte> cleartext(cleartext_len);
std::memcpy(&cleartext[0], ciphertext, cleartext_len);
for(u32bit i = 0; i < nb_err; i++)
{
gf2m current = error_pos[i];
if(current >= cleartext_len * 8)
{
// an invalid position, this shouldn't happen
continue;
}
cleartext[current / 8] ^= (1 << (current % 8));
}
if(unused_pt_bits)
{
cleartext[cleartext_len - 1] &= unused_pt_bits_mask;
}
return cleartext;
}
}
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