This commit first changes the return convention of EccPoint_mult_safer() so
that it properly reports when faults are detected. Then all functions that
call it need to be changed to (1) follow the same return convention and (2)
properly propagate UECC_FAULT_DETECTED when it occurs.
Here's the reverse call graph from EccPoint_mult_safer() to the rest of the
library (where return values are translated to the MBEDTLS_ERR_ space) and test
functions (where expected return values are asserted explicitly).
EccPoint_mult_safer()
EccPoint_compute_public_key()
uECC_compute_public_key()
pkparse.c
tests/suites/test_suite_pkparse.function
uECC_make_key_with_d()
uECC_make_key()
ssl_cli.c
ssl_srv.c
tests/suites/test_suite_pk.function
tests/suites/test_suite_tinycrypt.function
uECC_shared_secret()
ssl_tls.c
tests/suites/test_suite_tinycrypt.function
uECC_sign_with_k()
uECC_sign()
pk.c
tests/suites/test_suite_tinycrypt.function
Note: in uECC_sign_with_k() a test for uECC_vli_isZero(p) is suppressed
because it is redundant with a more thorough test (point validity) done at the
end of EccPoint_mult_safer(). This redundancy was introduced in a previous
commit but not noticed earlier.
We don't really need a secure hash for that, something like CRC32 would
probably be enough - but we have SHA-256 handy, not CRC32, so use that for the
sake of simplicity.
By semi-internal I mean functions that are only public because they're used in
more than once compilation unit in the library (for example in ecc.c and
ecc_dsa.c) but should not really be part of the public-facing API.
Same motivation as for the other parameters. This is the last one, making the
curve structure empty, so it's left with a dummy parameter for legal reasons.
Validating the input is always a good idea. Validating the output protects
against some fault injections that would make the result invalid.
Note: valid_point() implies that the point is not zero.
Adding validation to mult_safer() makes it redundant in
compute_shared_secret().
This will make easier to add future counter-measures in a single place.
In practice this change means that:
- compute_public_key() now uses projective coordinate randomisation, which it
should as this is a protection against Template Attacks for example.
- mult_safer() now checks that the result is not the point at infinity, which
it can as the result is indeed never expected to be that
Inspection of the generated assembly showed that before this commit, armcc 5
was optimizing away the successive reads to the volatile local variable that's
used for double-checks. Inspection also reveals that inserting a call to an
external function is enough to prevent it from doing that.
The tested versions of ARM-GCC, Clang and Armcc 6 (aka armclang) all keep the
double read, with our without a call to an external function in the middle.
The inserted function can also be changed to insert a random delay if
desired in the future, as it is appropriately places between the reads.
This hardens against attacks that glitch the conditional branch by making it
necessary for the attacker to inject two consecutive faults instead of one. If
desired, we could insert a random delay in order to further protect against
double-glitch attacks.
Also, when a single glitch is detected we report it.
Previously it was returning 0 or 1, so flipping a single bit in the return
value reversed its meaning. Now it's returning the diff itself.
This is safe because in the two places it's used (signature verification and
point validation), invalid values will have a large number of bits differing
from the expected value, so diff will have a large Hamming weight.
An alternative would be to return for example -!(diff == 0), but the
comparison itself is prone to attacks (glitching the appropriate flag in the
CPU flags register, or the conditional branch if the comparison uses one). So
we'd need to protect the comparison, and it's simpler to just skip it and
return diff itself.
This is a first step in protecting against fault injection attacks: the
attacker can no longer change failure into success by flipping a single bit.
Additional steps are needed to prevent other attacks (instruction skip etc)
and will be the object of future commits.
The return value of uECC_vli_equal() should be protected as well, which will
be done in a future commit as well.
This avoids the need for each calling site to manually regularize the scalar
and randomize coordinates, which makes for simpler safe use and saves 50 bytes
of code size in the library.
