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Move some more code to new function

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Manuel Pégourié-Gonnard 2017-03-09 13:23:50 +01:00
parent 22be635d13
commit 4b2336d7f6
1 changed files with 37 additions and 25 deletions
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62
library/ecp.c
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@ -1407,6 +1407,41 @@ cleanup:
return( ret );
}
/*
* Pick window size based on curve size and whether we optimize for base point
*/
static unsigned char ecp_pick_window_size( const mbedtls_ecp_group *grp,
unsigned char p_eq_g )
{
unsigned char w;
/*
* Minimize the number of multiplications, that is minimize
* 10 * d * w + 18 * 2^(w-1) + 11 * d + 7 * w, with d = ceil( nbits / w )
* (see costs of the various parts, with 1S = 1M)
*/
w = grp->nbits >= 384 ? 5 : 4;
/*
* If P == G, pre-compute a bit more, since this may be re-used later.
* Just adding one avoids upping the cost of the first mul too much,
* and the memory cost too.
*/
if( p_eq_g )
w++;
/*
* Make sure w is within bounds.
* (The last test is useful only for very small curves in the test suite.)
*/
if( w > MBEDTLS_ECP_WINDOW_SIZE )
w = MBEDTLS_ECP_WINDOW_SIZE;
if( w >= grp->nbits )
w = 2;
return( w );
}
/*
* Multiplication using the comb method,
* for curves in short Weierstrass form
@ -1445,31 +1480,8 @@ static int ecp_mul_comb( mbedtls_ecp_group *grp, mbedtls_ecp_point *R,
p_eq_g = 0;
#endif
/*
* Minimize the number of multiplications, that is minimize
* 10 * d * w + 18 * 2^(w-1) + 11 * d + 7 * w, with d = ceil( nbits / w )
* (see costs of the various parts, with 1S = 1M)
*/
w = grp->nbits >= 384 ? 5 : 4;
/*
* If P == G, pre-compute a bit more, since this may be re-used later.
* Just adding one avoids upping the cost of the first mul too much,
* and the memory cost too.
*/
if( p_eq_g )
w++;
/*
* Make sure w is within bounds.
* (The last test is useful only for very small curves in the test suite.)
*/
if( w > MBEDTLS_ECP_WINDOW_SIZE )
w = MBEDTLS_ECP_WINDOW_SIZE;
if( w >= grp->nbits )
w = 2;
/* Other sizes that depend on w */
/* Window size and others that depend on it */
w = ecp_pick_window_size( grp, p_eq_g );
pre_len = 1U << ( w - 1 );
d = ( grp->nbits + w - 1 ) / w;