171a7efd02
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) |
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