The function mbedtls_mpi_sub_abs first checked that A >= B and then
performed the subtraction, relying on the fact that A >= B to
guarantee that the carry propagation would stop, and not taking
advantage of the fact that the carry when subtracting two numbers can
only be 0 or 1. This made the carry propagation code a little hard to
follow.
Write an ad hoc loop for the carry propagation, checking the size of
the result. This makes termination obvious.
The initial check that A >= B is no longer needed, since the function
now checks that the carry propagation terminates, which is equivalent.
This is a slight performance gain.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
There was some confusion during review about when A->p[n] could be
nonzero. In fact, there is no need to set A->p[n]: only the
intermediate result d might need to extend to n+1 limbs, not the final
result A. So never access A->p[n]. Rework the explanation of the
calculation in a way that should be easier to follow.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
The function mpi_sub_hlp had confusing semantics: although it took a
size parameter, it accessed the limb array d beyond this size, to
propagate the carry. This made the function difficult to understand
and analyze, with a potential buffer overflow if misused (not enough
room to propagate the carry).
Change the function so that it only performs the subtraction within
the specified number of limbs, and returns the carry.
Move the carry propagation out of mpi_sub_hlp and into its caller
mbedtls_mpi_sub_abs. This makes the code of subtraction very slightly
less neat, but not significantly different.
In the one other place where mpi_sub_hlp is used, namely mpi_montmul,
this is a net win because the carry is potentially sensitive data and
the function carefully arranges to not have to propagate it.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
mpi_sub_hlp performs a subtraction A - B, but took parameters in the
order (B, A). Swap the parameters so that they match the usual
mathematical syntax.
This has the additional benefit of putting the output parameter (A)
first, which is the normal convention in this module.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Let code analyzers know that this is deliberate. For example MSVC
warns about the conversion if it's implicit.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
In mpi_montmul, an auxiliary function for modular
exponentiation (mbedtls_mpi_mod_exp) that performs Montgomery
multiplication, the last step is a conditional subtraction to force
the result into the correct range. The current implementation uses a
branch and therefore may leak information about secret data to an
adversary who can observe what branch is taken through a side channel.
Avoid this potential leak by always doing the same subtraction and
doing a contant-trace conditional assignment to set the result.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Separate out a version of mpi_safe_cond_assign that works on
equal-sized limb arrays, without worrying about allocation sizes or
signs.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This reverts commit 2cc69fffcf.
A check was added in mpi_montmul because clang-analyzer warned about a
possibly null pointer. However this was a false positive. Recent
versions of clang-analyzer no longer emit a warning (3.6 does, 6
doesn't).
Incidentally, the size check was wrong: mpi_montmul needs
T->n >= 2 * (N->n + 1), not just T->n >= N->n + 1.
Given that this is an internal function which is only used from one
public function and in a tightly controlled way, remove both the null
check (which is of low value to begin with) and the size check (which
would be slightly more valuable, but was wrong anyway). This allows
the function not to need to return an error, which makes the source
code a little easier to read and makes the object code a little
smaller.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Similarly to the recently-added tests for dependencies on CTR_DRBG:
constrained environments will probably want only one DRBG module, and we
should make sure that tests pass in such a configuration.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
Enable branch coverage output in basic_build_test.sh. This
includes enabling branch coverage output to the lcov make target,
which is disabled by default.
Signed-off-by: Dan Handley <dan.handley@arm.com>
People who prefer to rely on HMAC_DRBG (for example because they use it for
deterministic ECDSA and don't want a second DRBG for code size reasons) should
be able to build and run the tests suites without CTR_DRBG.
Ideally we should make sure the level of testing (SSL) is the same regardless
of which DRBG modules is enabled, but that's a more significant piece of work.
For now, just ensure everything builds and `make test` passes.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
While at it, declare deps on ENTROPY as well.
A non-regression test will be added in a follow-up commit.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
Running the entropy unit test creates a suitable seedfile, but this
only works due to the happy accident that no prior unit test needs one
(specifically, test_suite_entropy runs before test_suite_rsa). So
create one explicitly, both for robustness and to keep the script
closer to the version in development where the explicit seedfile
creation is required.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Convert all text files to Unix line endings unless they're Windows
stuff.
Make sure that all text files have a trailing newline.
Remove whitespace at the end of lines.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
We're only interested in files that are committed and pushed to be
included in Mbed TLS, not in any other files that may be lying around.
So ask git for the list of file names.
This script is primarily intended to run on the CI, and there it runs
on a fresh Git checkout plus potentially some other checkouts or
leftovers from a previous part of the CI job. It should also run
reasonably well on developer machines, where there may be various
additional files. In both cases, git is available.
Ad hoc directory exclusions are no longer needed.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Have an explicit list of exemptions for specific checks rather than
whitelisting files to check. Some checks, such as permissions, should
apply to all files.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
When generate_errors.pl was first written, there was no asn1.h. But
now there is one and it does not need any special treatment.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Make the contributing document link to how to create a changelog rather
than just linking to the Changelog itself. Backport to 2.16
Signed-off-by: Paul Elliott <paul.elliott@arm.com>
Add output of python3 version to output_env.sh.
Added in addition to the version of `python` as some
project's scripts try both executable names.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>
