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Scale ops count for larger curves

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From a user's perspective, you want a "basic operation" to take approximately
the same amount of time regardless of the curve size, especially since max_ops
is a global setting: otherwise if you pick a limit suitable for P-384 then
when you do an operation on P-256 it will return way more often than needed.

Said otherwise, a user is actually interested in actual running time, and we
do the API in terms of "basic ops" for practical reasons (no timers) but then
we should make sure it's a good proxy for running time.
This commit is contained in:
Manuel Pégourié-Gonnard
2017-03-20 14:35:19 +01:00
parent d3a0ca8500
commit e685449004
2 changed files with 18 additions and 8 deletions
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8
library/ecp.c
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@ -166,10 +166,18 @@ static int ecp_check_budget( const mbedtls_ecp_group *grp, unsigned ops )
{
if( grp->rs != NULL )
{
/* scale depending on curve size: the chosen reference is 256-bit,
* and multiplication is quadratic. Round to the closest integer. */
if( grp->pbits >= 512 )
ops *= 4;
else if( grp->pbits >= 384 )
ops *= 2;
/* avoid infinite loops: always allow first step */
if( grp->rs->ops_done != 0 && grp->rs->ops_done + ops > ecp_max_ops )
return( MBEDTLS_ERR_ECP_IN_PROGRESS );
/* update running count */
grp->rs->ops_done += ops;
}