Context: There are two public key writing functions in Mbed TLS. First,
mbedtls_pk_write_pubkey(), which exports a public key in the form of a
SubjectPublicKey structure containing the raw keying material
(for example, EC point coordinates for an EC public key, without
reference to the underlying curve). Secondly, mbedtls_pk_write_pubkey_der(),
which exports a public key in the form of a SubjectPublicKeyInfo structure,
wrapping the SubjectPublicKey structure by additional information
identifying the type of public key (and for ECC, e.g., it'd also contain
the ECC group identifier). The implementation of mbedtls_pk_write_pubkey_der()
calls mbedtls_pk_write_pubkey() first and then adds the corresponding
algorithm identifier wrapper.
Both of these functions need to be provided for PSA-based opaque PK contexts,
based on PSA's public key export function.
Previously, PSA used the SubjectPublicKeyInfo structure as its export format,
so mbedtls_pk_write_pubkey_der() could be easily implemented, while
mbedtls_pk_write_pubkey() would need to trim the output of the PSA export.
The previous implementation of mbedtls_pk_write_pubkey() is not quite right
because it calls PSA export doesn't do any trimming, hence exporting the large
SubjectPublicKeyInfo structure instead of the small SubjectPublicKey.
mbedtls_pk_write_pubkey_der(), in turn, immediately returns after calling
mbedtls_pk_write_pubkey(), hence also returning the SubjectPublicKeyInfo
structure, which is correct.
By now, the PSA public key export format has changed to the smaller
SubjectPublicKey structure. This means that, now, mbedtls_pk_write_pubkey()
can be implemented by just calling the PSA export, and that
mbedtls_pk_write_pubkey_der() needs to add the algorithm information around
it, just as in the other types of PK contexts. While not correct for the
old format, the existing code for mbedtls_pk_write_pubkey() is therefore
correct for the new PSA public key format, and needs no change apart from
the missing pointer shift in the last commit.
The implementation of mbedtls_pk_write_pubkey_der() needs a special code
path for PSA-based opaque PK contexts, as the PK context only contains
the PSA key handle, and the PSA API needs to be used to extract the
underlying EC curve to be able to write the AlgorithmParameter structure
that's part of the SubjectPublicKeyInfo structure.
That's what this commit does, (hopefully) making both
mbedtls_pk_write_pubkey() and mbedtls_pk_write_pubkey_der() export
the correctly formatted public key based on the new PSA public key format.
Previously, PSA used SubjectPublicKeyInfo structures to serialize EC public keys.
This has recently been changed to using ECPoint structures instead, but the wrapper
making PSA ECDSA verification available through Mbed TLS' PK API hasn't yet been
adapted accordingly - which is what this commit does.
Luckily, Mbed TLS' PK API offers two functions mbedtls_pk_write_pubkey()
and mbedtls_pk_write_pubkey_der(), the latter exporting a SubjectPublicKeyInfo
structure and the former exporting an ECPoint structure in case of EC public
keys. For the adaptation of the ECDSA wrapper ecdsa_verify_wrap() it is therefore
sufficient to use mbedtls_pk_write_pubkey() instead of mbedtls_pk_write_pubkey_der().
Programs and tests need to be able to use PSA header files when
USE_CRYPTO_SUBMODULE and MBEDTLS_USE_PSA_CRYPTO are set. Add the crypto
submodule include folder, which contains psa headers, after the main
include folder so that psa headers can be found and crypto submodule
headers don't take precedence over mbedtls headers.
The check-files script contains the strings "TODO" and "todo" in order to
search for files that contain TODO items. So, any check-files script would
need to be excluded from the list of files that gets checked for "TODO".
Normally, the script excludes itself from checks, but with the addition of
the crypto submodule, there is another copy of the script present from the
project root. We must avoid checking check-files scripts for TODO items.
This also helps if you run check-files from another working tree in your
working tree.
