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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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37 changes: 25 additions & 12 deletions library/bignum.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1343,12 +1343,23 @@ int mbedtls_mpi_add_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
}

/*
* Helper for mbedtls_mpi subtraction:
* d -= s where d and s have the same size and d >= s.
* Helper for mbedtls_mpi subtraction.
*
* Calculate d - s where d and s have the same size.
* This function operates modulo (2^ciL)^n and returns the carry
* (1 if there was a wraparound, i.e. if `d < s`, and 0 otherwise).
*
* \param n Number of limbs of \p d and \p s.
* \param[in,out] d On input, the left operand.
* On output, the result of the subtraction:
* \param[s] The right operand.
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Syntax error: I think you mean param[in] s.

(Btw, less important, but you're using doxygen syntax in something that's not a doxygen comment. Maybe it would be cleaner to start the comment with /**, so that various tools such as clang's -Wdocumentation can check it.)

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Regarding the second part, I was a bit surprised check-doxy-blocks didn't complain. I checked the CI results, and it did complain.

*
* \return 1 if `d < s`.
* 0 if `d >= s`.
*/
static void mpi_sub_hlp( size_t n,
mbedtls_mpi_uint *d,
const mbedtls_mpi_uint *s )
static mbedtls_mpi_uint mpi_sub_hlp( size_t n,
mbedtls_mpi_uint *d,
const mbedtls_mpi_uint *s )
{
size_t i;
mbedtls_mpi_uint c, z;
Expand All @@ -1359,11 +1370,7 @@ static void mpi_sub_hlp( size_t n,
c = ( *d < *s ) + z; *d -= *s;
}

while( c != 0 )
{
z = ( *d < c ); *d -= c;
c = z; d++;
}
return( c );
}

/*
Expand All @@ -1374,6 +1381,7 @@ int mbedtls_mpi_sub_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
mbedtls_mpi TB;
int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
size_t n;
mbedtls_mpi_uint c, z;
MPI_VALIDATE_RET( X != NULL );
MPI_VALIDATE_RET( A != NULL );
MPI_VALIDATE_RET( B != NULL );
Expand Down Expand Up @@ -1403,7 +1411,12 @@ int mbedtls_mpi_sub_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
if( B->p[n - 1] != 0 )
break;

mpi_sub_hlp( n, X->p, B->p );
c = mpi_sub_hlp( n, X->p, B->p );
while( c != 0 )
{
z = ( X->p[n] < c ); X->p[n] -= c;
c = z; n++;
}

cleanup:

Expand Down Expand Up @@ -2047,7 +2060,7 @@ static void mpi_montmul( mbedtls_mpi *A, const mbedtls_mpi *B, const mbedtls_mpi
* timing attacks. */
/* Set d to A + (2^biL)^n - N. */
d[n] += 1;
mpi_sub_hlp( n, d, N->p );
d[n] -= mpi_sub_hlp( n, d, N->p );
/* Now d - (2^biL)^n = A - N so d >= (2^biL)^n iff A >= N.
* So we want to copy the result of the subtraction iff d->p[n] != 0.
* Note that d->p[n] is either 0 or 1 since A - N <= N <= (2^biL)^n. */
Expand Down