The library is able to perform computations and cryptographic schemes on
curves with x coordinate ladder representation. Here we add the
capability to export such points.
The function `mbedtls_mpi_write_binary()` writes big endian byte order,
but we need to be able to write little endian in some caseses. (For
example when handling keys corresponding to Montgomery curves.)
Used `echo xx | tac -rs ..` to transform the test data to little endian.
The private keys used in ECDH differ in the case of Weierstrass and
Montgomery curves. They have different constraints, the former is based
on big endian, the latter little endian byte order. The fundamental
approach is different too:
- Weierstrass keys have to be in the right interval, otherwise they are
rejected.
- Any byte array of the right size is a valid Montgomery key and it
needs to be masked before interpreting it as a number.
Historically it was sufficient to use mbedtls_mpi_read_binary() to read
private keys, but as a preparation to improve support for Montgomery
curves we add mbedtls_ecp_read_key() to enable uniform treatment of EC
keys.
For the masking the `mbedtls_mpi_set_bit()` function is used. This is
suboptimal but seems to provide the best trade-off at this time.
Alternatives considered:
- Making a copy of the input buffer (less efficient)
- removing the `const` constraint from the input buffer (breaks the api
and makes it less user friendly)
- applying the mask directly to the limbs (violates the api between the
modules and creates and unwanted dependency)
The library is able to perform computations and cryptographic schemes on
curves with x coordinate ladder representation. Here we add the
capability to import such points.
The function `mbedtls_mpi_read_binary()` expects big endian byte order,
but we need to be able to read from little endian in some caseses. (For
example when handling keys corresponding to Montgomery curves.)
Used `echo xx | tac -rs .. | tr [a-z] [A-Z]` to transform the test data
to little endian and `echo "ibase=16;xx" | bc` to convert to decimal.
Add a test case for doing an ECDH calculation by calling
mbedtls_ecdh_get_params on both keys, with keys belonging to
different groups. This should fail, but currently passes.
Don't unconditionally enable PSA Crypto for all tests. Only enable it in
tests that require it. This allows crypto tests to check that
psa_crypto_init() fails when it is supposed to fail, since we want to
perform some action in a test, and then call psa_crypto_init() and check
the result without it having been called previously.
Additional changes to temporarily enable running tests:
ssl_srv.c and test_suite_ecdh use mbedtls_ecp_group_load instead of
mbedtls_ecdh_setup
test_suite_ctr_drbg uses mbedtls_ctr_drbg_update instead of
mbedtls_ctr_drbg_update_ret
