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Fix EC J-PAKE failing when the payload is all-bits-zero

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Fix function mbedtls_ecp_mul_shortcuts() to skip multiplication when m
is 0 and simply assignt 0 to R. Additionally fix ecjpake_zkp_read() to
return MBEDTLS_ERR_ECP_INVALID_KEY when the above condintion is met.

Fix #1792

Signed-off-by: TRodziewicz <rodziewicz@gmail.com>
This commit is contained in:
TRodziewicz 2021-03-04 17:59:39 +01:00
parent 2ac5f8c04b
commit 9edff740e1
3 changed files with 17 additions and 2 deletions
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4
ChangeLog.d/issue1792.txt Normal file
View file

@ -0,0 +1,4 @@
Bugfix
* Fix a bug in EC J-PAKE that would cause it fail when the payload is all-
bits-zero.
Found by Gilles Peskine, reported in #1792.

7
library/ecjpake.c
View file

@ -286,6 +286,13 @@ static int ecjpake_zkp_read( const mbedtls_md_info_t *md_info,
* Verification * Verification
*/ */
MBEDTLS_MPI_CHK( ecjpake_hash( md_info, grp, pf, G, &V, X, id, &h ) ); MBEDTLS_MPI_CHK( ecjpake_hash( md_info, grp, pf, G, &V, X, id, &h ) );
if( mbedtls_mpi_cmp_int( &r,0 ) == 0 )
{
ret = MBEDTLS_ERR_ECP_INVALID_KEY;
goto cleanup;
}
MBEDTLS_MPI_CHK( mbedtls_ecp_muladd( (mbedtls_ecp_group *) grp, MBEDTLS_MPI_CHK( mbedtls_ecp_muladd( (mbedtls_ecp_group *) grp,
&VV, &h, X, &r, G ) ); &VV, &h, X, &r, G ) );

8
library/ecp.c
View file

@ -2795,7 +2795,7 @@ cleanup:
#if defined(MBEDTLS_ECP_SHORT_WEIERSTRASS_ENABLED) #if defined(MBEDTLS_ECP_SHORT_WEIERSTRASS_ENABLED)
/* /*
* R = m * P with shortcuts for m == 1 and m == -1 * R = m * P with shortcuts for m == 0, m == 1 and m == -1
* NOT constant-time - ONLY for short Weierstrass! * NOT constant-time - ONLY for short Weierstrass!
*/ */
static int mbedtls_ecp_mul_shortcuts( mbedtls_ecp_group *grp, static int mbedtls_ecp_mul_shortcuts( mbedtls_ecp_group *grp,
@ -2806,7 +2806,11 @@ static int mbedtls_ecp_mul_shortcuts( mbedtls_ecp_group *grp,
{ {
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED; int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
if( mbedtls_mpi_cmp_int( m, 1 ) == 0 ) if ( mbedtls_mpi_cmp_int( m, 0 ) == 0 )
{
MBEDTLS_MPI_CHK( mbedtls_ecp_set_zero( R ) );
}
else if( mbedtls_mpi_cmp_int( m, 1 ) == 0 )
{ {
MBEDTLS_MPI_CHK( mbedtls_ecp_copy( R, P ) ); MBEDTLS_MPI_CHK( mbedtls_ecp_copy( R, P ) );
} }