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Remove a secret-dependent branch in Montgomery multiplication #3398

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merged 12 commits into from
Jun 9, 2020
Merged

Remove a secret-dependent branch in Montgomery multiplication #3398

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742f1a4
Add a const annotation to the non-changing argument of mpi_sub_mul
gilles-peskine-arm Jun 4, 2020
4e91d47
Revert "Shut up a clang-analyzer warning"
gilles-peskine-arm Jun 4, 2020
2a82f72
Document some internal bignum functions
gilles-peskine-arm Jun 4, 2020
f04d11e
Separate out low-level mpi_safe_cond_assign
gilles-peskine-arm Jun 4, 2020
132c097
Remove a secret-dependent branch in Montgomery multiplication
gilles-peskine-arm Jun 4, 2020
d55bfe9
Add changelog entry: fix #3394
gilles-peskine-arm Jun 4, 2020
026f555
Explicitly cast down from mbedtls_mpi_uint to unsigned char
gilles-peskine-arm Jun 5, 2020
37ecc61
More logical parameter order for mpi_sub_hlp
gilles-peskine-arm Jun 8, 2020
c097e9e
Move carry propagation out of mpi_sub_hlp
gilles-peskine-arm Jun 8, 2020
221626f
Simplify the final reduction in mpi_montmul
gilles-peskine-arm Jun 8, 2020
0e5faf6
mbedtls_mpi_sub_abs: check the range of the result when it happens
gilles-peskine-arm Jun 8, 2020
09ec10a
Clean up some comments
gilles-peskine-arm Jun 9, 2020
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24 changes: 15 additions & 9 deletions library/bignum.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1374,20 +1374,20 @@ static mbedtls_mpi_uint mpi_sub_hlp( size_t n,
}

/*
* Unsigned subtraction: X = |A| - |B| (HAC 14.9)
* Unsigned subtraction: X = |A| - |B| (HAC 14.9, 14.10)
*/
int mbedtls_mpi_sub_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi *B )
{
mbedtls_mpi TB;
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
size_t n;
mbedtls_mpi_uint c, z;
mbedtls_mpi_uint carry;
MPI_VALIDATE_RET( X != NULL );
MPI_VALIDATE_RET( A != NULL );
MPI_VALIDATE_RET( B != NULL );

if( mbedtls_mpi_cmp_abs( A, B ) < 0 )
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The initial check that A >= B is no longer needed, since the function now checks that the carry propagation terminates, which is equivalent.

Oops. The carry propagation check is equivalent, but that's assuming it is reached. If B->n > A->n, or more precisely if B without leading null limbs is larger than A (with its leading null limbs), then mpi_sub_hlp overflows X->p.

return( MBEDTLS_ERR_MPI_NEGATIVE_VALUE );
/* if( mbedtls_mpi_cmp_abs( A, B ) < 0 ) */
/* return( MBEDTLS_ERR_MPI_NEGATIVE_VALUE ); */
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Was it your intention to leave this code commented, or did you mean to remove it?


mbedtls_mpi_init( &TB );

Expand All @@ -1411,11 +1411,17 @@ int mbedtls_mpi_sub_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
if( B->p[n - 1] != 0 )
break;

c = mpi_sub_hlp( n, X->p, B->p );
while( c != 0 )
{
z = ( X->p[n] < c ); X->p[n] -= c;
c = z; n++;
carry = mpi_sub_hlp( n, X->p, B->p );
if( carry != 0 )
{
/* Propagate the carry to the first nonzero limb of X. */
for( ; n < X->n && X->p[n] == 0; n++ )
--X->p[n];
/* If we ran out of space for the carry, it means that the result
* is negative. */
if( n == X->n )
return( MBEDTLS_ERR_MPI_NEGATIVE_VALUE );
--X->p[n];
}

cleanup:
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