Add mbedtls_ecp_read_key
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)
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 065a4cc..7dee3e3 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -1094,6 +1094,22 @@
void *p_rng );
/**
+ * \brief This function reads an ECP key.
+ *
+ * \param grp_id The ECP group identifier.
+ * \param key The destination key.
+ * \param buf The the buffer containing the binary representation of the
+ * key. (Big endian integer for Weierstrass curves, byte
+ * string for Montgomery curves.)
+ * \param buflen The length of the buffer in bytes.
+ *
+ * \return \c 0 on success.
+ * \return An \c MBEDTLS_ERR_ECP_XXX or \c MBEDTLS_MPI_XXX error code
+ * on failure.
+ */
+int mbedtls_ecp_read_key( mbedtls_ecp_group_id grp_id, mbedtls_ecp_keypair *key,
+ const unsigned char *buf, size_t buflen );
+/**
* \brief This function checks that the keypair objects
* \p pub and \p prv have the same group and the
* same public point, and that the private key in