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

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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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2
include/polarssl/bignum.h
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@ -58,7 +58,7 @@ typedef UINT64 uint64_t;
#define POLARSSL_ERR_MPI_NOT_ACCEPTABLE -0x000E /**< The input arguments are not acceptable. */ #define POLARSSL_ERR_MPI_NOT_ACCEPTABLE -0x000E /**< The input arguments are not acceptable. */
#define POLARSSL_ERR_MPI_MALLOC_FAILED -0x0010 /**< Memory allocation failed. */ #define POLARSSL_ERR_MPI_MALLOC_FAILED -0x0010 /**< Memory allocation failed. */
#define MPI_CHK(f) if( ( ret = f ) != 0 ) goto cleanup #define MPI_CHK(f) do { if( ( ret = f ) != 0 ) goto cleanup; } while( 0 )
/* /*
* Maximum size MPIs are allowed to grow to in number of limbs. * Maximum size MPIs are allowed to grow to in number of limbs.

25
library/bignum.c
View file

@ -1923,6 +1923,7 @@ int mpi_gen_prime( mpi *X, size_t nbits, int dh_flag,
{ {
int ret; int ret;
size_t k, n; size_t k, n;
t_uint r;
mpi Y; mpi Y;
if( nbits < 3 || nbits > POLARSSL_MPI_MAX_BITS ) 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 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 ) ); MPI_CHK( mpi_shift_r( &Y, 1 ) );
while( 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 ) if( ret != POLARSSL_ERR_MPI_NOT_ACCEPTABLE )
goto cleanup; goto cleanup;
/* Keep X = 3 mod 4 */ /*
MPI_CHK( mpi_add_int( X, X, 4 ) ); * Next candidates. We want to preserve
MPI_CHK( mpi_add_int( &Y, &Y, 2 ) ); * 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 ) );
} }
} }