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mbedtls_mpi_exp_mod: remove spurious copy of the output variable

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Clear some confusion between `X` as the output variable and "X" as a name
given to the accumulator. Previous iterations of the code used the variable
`X` as the accumulator, but now that the accumulator is `W[x_index]`, some
of the comments didn't make sense.

Remove the copy of the initial value of `X` into `W[x_index]`, which was
meaningless: the initial value of an output variable should not, and did
not, matter. `W[x_index]` is later overridden unconditionally to take the
value `RR`.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2023-08-10 15:59:28 +02:00
parent 8630906d9b
commit e6cb45e68e
1 changed files with 8 additions and 10 deletions
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18
library/bignum.c
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@ -1680,8 +1680,9 @@ int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
* and squarings. Firstly, when multiplying by an element of the window
* W[i], we do a constant-trace table lookup to obfuscate i. This leaves
* squarings as having a different memory access patterns from other
* multiplications. So secondly, we put the accumulator X in the table as
* well, and also do a constant-trace table lookup to multiply by X.
* multiplications. So secondly, we put the accumulator in the table as
* well, and also do a constant-trace table lookup to multiply by the
* accumulator which is W[x_index].
*
* This way, all multiplications take the form of a lookup-and-multiply.
* The number of lookup-and-multiply operations inside each iteration of
@ -1694,19 +1695,16 @@ int mbedtls_mpi_exp_mod(mbedtls_mpi *X, const mbedtls_mpi *A,
* observe both memory accesses and branches. However, branch prediction
* exploitation typically requires many traces of execution over the same
* data, which is defeated by randomized blinding.
*
* To achieve this, we make a copy of X and we use the table entry in each
* calculation from this point on.
*/
const size_t x_index = 0;
mbedtls_mpi_init(&W[x_index]);
mbedtls_mpi_copy(&W[x_index], X);
j = N->n + 1;
/* All W[i] and X must have at least N->n limbs for the mpi_montmul()
* and mpi_montred() calls later. Here we ensure that W[1] and X are
* large enough, and later we'll grow other W[i] to the same length.
* They must not be shrunk midway through this function!
/* All W[i] including the accumulator must have at least N->n limbs for
* the mpi_montmul() and mpi_montred() calls later. Here we ensure that
* W[1] and the accumulator W[x_index] are large enough. later we'll grow
* other W[i] to the same length. They must not be shrunk midway through
* this function!
*/
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(&W[x_index], j));
MBEDTLS_MPI_CHK(mbedtls_mpi_grow(&W[1], j));