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gen_prime: ensure X = 2 mod 3 -> 2.5x speedup

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This commit is contained in:
Manuel Pégourié-Gonnard
2013-11-22 17:54:59 +01:00
parent 711507a726
commit 0160eacc82
2 changed files with 22 additions and 5 deletions
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25
library/bignum.c
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@ -1923,6 +1923,7 @@ int mpi_gen_prime( mpi *X, size_t nbits, int dh_flag,
{
int ret;
size_t k, n;
t_uint r;
mpi Y;
if( nbits < 3 || nbits > POLARSSL_MPI_MAX_BITS )
@ -1952,7 +1953,19 @@ int mpi_gen_prime( mpi *X, size_t nbits, int dh_flag,
}
else
{
MPI_CHK( mpi_sub_int( &Y, X, 1 ) );
/*
* An necessary condition for Y and X = 2Y + 1 to be prime
* is X = 2 mod 3 (which is equivalent to Y = 2 mod 3).
* Make sure it is satisfied, while keeping X = 3 mod 4
*/
MPI_CHK( mpi_mod_int( &r, X, 3 ) );
if( r == 0 )
MPI_CHK( mpi_add_int( X, X, 8 ) );
else if( r == 1 )
MPI_CHK( mpi_add_int( X, X, 4 ) );
/* Set Y = (X-1) / 2, which is X / 2 because X is odd */
MPI_CHK( mpi_copy( &Y, X ) );
MPI_CHK( mpi_shift_r( &Y, 1 ) );
while( 1 )
@ -1969,9 +1982,13 @@ int mpi_gen_prime( mpi *X, size_t nbits, int dh_flag,
if( ret != POLARSSL_ERR_MPI_NOT_ACCEPTABLE )
goto cleanup;
/* Keep X = 3 mod 4 */
MPI_CHK( mpi_add_int( X, X, 4 ) );
MPI_CHK( mpi_add_int( &Y, &Y, 2 ) );
/*
* Next candidates. We want to preserve
* Y = 1 mod 2 and Y = 2 mod 3 (eq X = 3 mod 4 and X = 2 mod 3)
* so up Y by 6 and X by 12.
*/
MPI_CHK( mpi_add_int( X, X, 12 ) );
MPI_CHK( mpi_add_int( &Y, &Y, 6 ) );
}
}