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Remove bias in EC private key generation

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Manuel Pégourié-Gonnard 2014-01-03 12:35:05 +01:00
parent 217a29c844
commit 79f73b96d9
1 changed files with 14 additions and 5 deletions
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19
library/ecp.c
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@ -1771,17 +1771,26 @@ int ecp_gen_keypair( ecp_group *grp, mpi *d, ecp_point *Q,
{
/* SEC1 3.2.1: Generate d such that 1 <= n < N */
int count = 0;
unsigned char rnd[POLARSSL_ECP_MAX_BYTES];
/*
* Match the procedure given in RFC 6979 (deterministic ECDSA):
* - use the same byte ordering;
* - keep the leftmost nbits bits of the generated octet string;
* - try until result is in the desired range.
* This also avoids any biais, which is especially important for ECDSA.
*/
do
{
mpi_fill_random( d, n_size, f_rng, p_rng );
while( mpi_cmp_mpi( d, &grp->N ) >= 0 )
mpi_shift_r( d, 1 );
f_rng( p_rng, rnd, n_size );
mpi_read_binary( d, rnd, n_size );
mpi_shift_r( d, 8 * n_size - grp->nbits );
if( count++ > 10 )
return( POLARSSL_ERR_ECP_RANDOM_FAILED );
}
while( mpi_cmp_int( d, 1 ) < 0 );
while( mpi_cmp_int( d, 1 ) < 0 ||
mpi_cmp_mpi( d, &grp->N ) >= 0 );
}
else
#endif