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Improve comments on parity trick

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Manuel Pégourié-Gonnard 2017-08-24 14:21:05 +02:00
parent 95aedfea33
commit fd87e354f6
2 changed files with 5 additions and 2 deletions
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2
include/mbedtls/ecp.h
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@ -119,7 +119,7 @@ mbedtls_ecp_point;
* 1. Short Weierstrass y^2 = x^3 + A x + B mod P (SEC1 + RFC 4492)
* 2. Montgomery, y^2 = x^3 + A x^2 + x mod P (Curve25519 + draft)
* In both cases, a generator G for a prime-order subgroup is fixed. In the
* short weierstrass, this subgroup is actually the whole curve, and its
* Short Weierstrass case, this subgroup is actually the whole curve, and its
* cardinal is denoted by N.
*
* In the case of Short Weierstrass curves, our code requires that N is an odd

5
library/ecp.c
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@ -1751,6 +1751,9 @@ cleanup:
* this wrapper ensures that by replacing m by N - m if necessary, and
* informs the caller that the result of multiplication will be negated.
*
* This works because we only support large prime order for Short Weierstrass
* curves, so N is always odd hence either m or N - m is.
*
* See ecp_comb_recode_core() for background.
*/
static int ecp_comb_recode_scalar( const mbedtls_ecp_group *grp,
@ -1766,7 +1769,7 @@ static int ecp_comb_recode_scalar( const mbedtls_ecp_group *grp,
mbedtls_mpi_init( &M );
mbedtls_mpi_init( &mm );
/* N is odd with all real-world curves, just make extra sure */
/* N is always odd (see above), just make extra sure */
if( mbedtls_mpi_get_bit( &grp->N, 0 ) != 1 )
return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );