This comment is no longer in the specific context of generating a
random point on an elliptic curve.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Slightly simplify is_significantly_above_a_power_of_2() to make it
easier to understand:
* Remove the explicit negative answer for x <= 4. The only functional
difference this makes is that is_significantly_above_a_power_of_2(3)
is now true.
* Shift the most significant bit of x to position 8 rather than 15.
This makes the final comparison easier to explain.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Add unit tests for mbedtls_mpi_fill_random() and mbedtls_mpi_random()
when the resulting MPI object previously had a nonzero value. I wrote
those to catch a bug that I introduced during the development of
mbedtls_mpi_random() (but does not appear in a committed version).
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
mbedtls_mpi_random() uses mbedtls_mpi_cmp_mpi_ct(), which requires its
two arguments to have the same storage size. This was not the case
when the upper bound passed to mbedtls_mpi_random() had leading zero
limbs.
Fix this by forcing the result MPI to the desired size. Since this is
not what mbedtls_mpi_fill_random() does, don't call it from
mbedtls_mpi_random(), but instead call a new auxiliary function.
Add tests to cover this and other conditions with varying sizes for
the two arguments.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Instead of generating blinding values and keys in a not-quite-uniform way
(https://github.com/ARMmbed/mbedtls/issues/4245) with copy-pasted code,
use mbedtls_mpi_random().
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
dhm_make_common includes a piece of code that is identical to
dhm_random_below except for returning a different error code in one
case. Call dhm_random_below instead of repeating the code.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
P-1 is as bad as 1 as a blinding value. Don't accept it.
The chance that P-1 would be randomly generated is infinitesimal, so
this is not a practical issue, but it makes the code cleaner. It was
inconsistent to accept P-1 as a blinding value but not as a private key.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Unify the common parts of mbedtls_dhm_make_params and mbedtls_dhm_make_public.
No intended behavior change, except that the exact error code may
change in some corner cases which are too exotic for the existing unit
tests.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
mbedtls_dhm_make_params() with x_size != size of P is not likely to be
useful, but it's supported, so test it.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Repeat a few tests that use random data. This way the code is
exercised with a few different random values.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Improve the validation of the output from mbedtls_dhm_make_params:
* Test that the output in the byte buffer matches the value in the
context structure.
* Test that the calculated values are in the desired range.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Instead of generating blinding values in a not-quite-uniform way
(https://github.com/ARMmbed/mbedtls/issues/4245) with copy-pasted
code, use mbedtls_mpi_random().
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Since mbedtls_mpi_random() is not specific to ECC code, move it from
the ECP module to the bignum module.
This increases the code size in builds without short Weierstrass
curves (including builds without ECC at all) that do not optimize out
unused functions.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Rename mbedtls_ecp_gen_privkey_sw to mbedtls_mpi_random since it has
no particular connection to elliptic curves beyond the fact that its
operation is defined by the deterministic ECDSA specification. This is
a generic function that generates a random MPI between 1 inclusive and
N exclusive.
Slightly generalize the function to accept a different lower bound,
which adds a negligible amount of complexity.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Add unit tests for private key generation on short Weierstrass curves.
These tests validate that the result is within the desired range.
Additionally, they validate that after performing many iterations, the
range is covered to an acceptable extent: for tiny ranges, all values
must be reached; for larger ranges, all value bits must reach both 0
and 1.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
mbedtls_ecp_gen_privkey_mx generates a random number with a certain
top bit set. Depending on the size, it would either generate a number
with that top bit being random, then forcibly set the top bit to
1 (when high_bit is not a multiple of 8); or generate a number with
that top bit being 0, then set the top bit to 1 (when high_bit is a
multiple of 8). Change it to always generate the top bit randomly
first.
This doesn't make any difference in practice: the probability
distribution is the same either way, and no supported or plausible
curve has a size of the form 8n+1 anyway. But it slightly simplifies
reasoning about the behavior of this function.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Don't calculate the bit-size of the initially generated random number.
This is not necessary to reach the desired distribution of private
keys, and creates a (tiny) side channel opportunity.
This changes the way the result is derived from the random number, but
does not affect the resulting distribution.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
The library rejected an RNG input of all-bits-zero, which led to the
key 2^{254} (for Curve25519) having a 31/32 chance of being generated
compared to other keys. This had no practical impact because the
probability of non-compliance was 2^{-256}, but needlessly
complicated the code.
The exception was added in 98e28a74e3 to
avoid the case where b - 1 wraps because b is 0. Instead, change the
comparison code to avoid calculating b - 1.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Test the exact output from known RNG input. This is overly
constraining, but ensures that the code has good properties.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
If a fallback is not explicitly configured in the
mbedtls_test_rnd_buf_info structure, fail after the buffer is
exhausted.
There is no intended behavior change in this commit: all existing uses
of mbedtls_test_rnd_buffer_rand() have been updated to set
mbedtls_test_rnd_std_rand as the fallback.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
For Montgomery keys, n_bits is actually the position of the highest
bit and not the number of bits, which would be 1 more (fence vs
posts). Rename the variable accordingly to lessen the confusion.
No semantic change.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Put the Montgomery and short Weierstrass implementations of
mbedtls_ecp_gen_privkey into their own function which can be tested
independently, but will not be part of the public ABI/API.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Prepare to isolate the Montgomery and short Weierstrass
implementations of mbedtls_ecp_gen_privkey into their own function.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Make input/output format documentation easier to find:
- Add direct reference to the steps from the input/output functions
- Move the format description directly to the step constants
Signed-off-by: Janos Follath <janos.follath@arm.com>
- Transformed setup description to a more explicit pseudocode based
approach.
- Explained implicit vs explicit key confirmation
Signed-off-by: Janos Follath <janos.follath@arm.com>
This step is not necessarily a memory-hard function. Memory-hard
functions are the best of the breed at the moment, but that's due to
current hardware designs, and CPU-hard-but-not-memory-hard functions
like PBKDF2 are acceptable as well. We're using “key stretching” as the
generic term for such functions.
Signed-off-by: Janos Follath <janos.follath@arm.com>
The key derivation operation passed to psa_pake_set_password_mhf() might
enter an error state before the function returns. If this happens, the
user needs to know about it so that they can properly abort it.
Signed-off-by: Janos Follath <janos.follath@arm.com>
The type of the key derivation operation was incorrect.
Also neither the PAKE nor key_derivation algorithm knows how many bytes
to transfer at this stage.
There is no optimal or recommended size, PAKEs don't mandate it either
(with the exception of OPAQUE, but that uses it internally and won't be
using this interface).
Adding an input length parameter to allow the application to control how
many bytes the PAKE takes from the key derivation.
Signed-off-by: Janos Follath <janos.follath@arm.com>
Using memory hard functions with PAKEs is the more secure option. It
should be as convenient and efficient to use as less secure options, but
so far it required creating an additional temporary key object.
With psa_pake_set_password_mhf() this eliminates the need for this.
Similarly we could add a convenience function to supply the password
directly from character strings, but that would make the less secure
option more convenient again and therfore we are not doing it now.
Signed-off-by: Janos Follath <janos.follath@arm.com>
PAKE constructions that have multiple key shares will always consume and
produce the key shares in numerical order. So using PSA_PAKE_DATA_XXX_X
would demand step-sequence validation, and provides no functional
utility over having only PSA_PAKE_DATA_XXX.
Signed-off-by: Janos Follath <janos.follath@arm.com>
