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DHM: Fix dhm_check_range() always returning 0

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Although the variable ret was initialised to an error, the
MBEDTLS_MPI_CHK macro was overwriting it. Therefore it ended up being
0 whenewer the bignum computation was successfull and stayed 0
independently of the actual check.
This commit is contained in:
Janos Follath 2017-09-20 15:33:24 +01:00
parent 5f9df9b2ad
commit 77359c93e4
2 changed files with 13 additions and 4 deletions
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6
ChangeLog
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@ -1,5 +1,11 @@
mbed TLS ChangeLog (Sorted per branch, date)
= mbed TLS 1.3.x branch released xxxx-xx-xx
Security
* Fix dhm_check_range() failing to detect trivial subgroups and potentially
leaking 1 bit of the private key. Reported by prashantkspatil.
= mbed TLS 1.3.21 branch released 2017-08-10
Security

11
library/dhm.c
View file

@ -91,6 +91,9 @@ static int dhm_read_bignum( mpi *X,
*
* Parameter should be: 2 <= public_param <= P - 2
*
* This means that we need to return an error if
* public_param < 2 or public_param > P-2
*
* For more information on the attack, see:
* http://www.cl.cam.ac.uk/~rja14/Papers/psandqs.pdf
* http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2005-2643
@ -98,17 +101,17 @@ static int dhm_read_bignum( mpi *X,
static int dhm_check_range( const mpi *param, const mpi *P )
{
mpi L, U;
int ret = POLARSSL_ERR_DHM_BAD_INPUT_DATA;
int ret = 0;
mpi_init( &L ); mpi_init( &U );
MPI_CHK( mpi_lset( &L, 2 ) );
MPI_CHK( mpi_sub_int( &U, P, 2 ) );
if( mpi_cmp_mpi( param, &L ) >= 0 &&
mpi_cmp_mpi( param, &U ) <= 0 )
if( mpi_cmp_mpi( param, &L ) < 0 ||
mpi_cmp_mpi( param, &U ) > 0 )
{
ret = 0;
ret = POLARSSL_ERR_DHM_BAD_INPUT_DATA;
}
cleanup: