This is the first in a series of commits adding client-side
support for PSA-based ECDHE.
Previously, the state of an ECDHE key agreement was maintained
in the field mbedtls_ssl_handshake_params::ecdh_ctx, of type
::mbedtls_ecdh_context and manipulated through the ECDH API.
The ECDH API will be superseeded by the PSA Crypto API for key
agreement, which needs the following data:
(a) A raw buffer holding the public part of the key agreement
received from our peer.
(b) A key slot holding the private part of the key agreement.
(c) The algorithm to use.
The commit adds fields to ::mbedtls_ssl_handshake_params
representing these three inputs to PSA-based key agreement.
Specifically, it adds a field for the key slot holding the
ECDH private key, a field for the EC curve identifier, and
a buffer holding the peer's public key.
Note: Storing the peer's public key buffer is slightly
inefficient, as one could perform the ECDH computation
as soon as the peer sends its public key, either working
with in-place or using a stack-buffer to reformat the
public key before passing it to PSA. This optimization
is left for a later commit.
PSA spec now defines more generic PSA storage types instead of the ITS
specific ones. This is necessary in order to integrate with
the newer implementation of PSA ITS landing in Mbed OS soon.
Changes include the following:
- psa_status_t replaces psa_its_status_t
- psa_storage_info_t replaces psa_its_info_t
- psa_storage_uid_t replaces psa_its_uid_t
Silence a compiler warning about implicit fallthrough by using a comment
format the compiler understand to mean that the fallthrough is
intentional.
In file included from library/cipher.c:63:0:
include/mbedtls/psa_util.h: In function ‘mbedtls_psa_translate_cipher_mode’:
include/mbedtls/psa_util.h:91:15: error: this statement may fall through [-Werror=implicit-fallthrough=]
if( taglen == 0 )
^
include/mbedtls/psa_util.h:94:9: note: here
default:
^~~~~~~
cc1: all warnings being treated as errors
$ gcc --version
gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Silence a compiler warning about implicit fallthrough by using a comment
format the compiler understand to mean that the fallthrough is
intentional.
In file included from library/cipher.c:63:0:
include/mbedtls/psa_util.h: In function ‘mbedtls_psa_translate_cipher_mode’:
include/mbedtls/psa_util.h:91:15: error: this statement may fall through [-Werror=implicit-fallthrough=]
if( taglen == 0 )
^
include/mbedtls/psa_util.h:94:9: note: here
default:
^~~~~~~
cc1: all warnings being treated as errors
$ gcc --version
gcc (Ubuntu 7.3.0-27ubuntu1~18.04) 7.3.0
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Resolve conflicts by performing the following.
- Take the upstream Mbed TLS ChangeLog verbatim.
- Reject changes to Makefiles and CMake that are related to using Mbed
Crypto as a submodule. It doesn't make sense to use Mbed Crypto as a
submodule of itself.
- Reject README changes, as Mbed Crypto has its own, different README.
- Reject PSA-related changes to config.h. We don't want to disable the
availability of the PSA Crypto API by default in the Mbed Crypto
config.h.
- Don't inadvertently revert dead code removal in
mbedtls_cipher_write_tag() which was added in f2a7529403 ("Fix
double return statement in cipher.c")
- Where Mbed Crypto already had some MBEDTLS_USE_PSA_CRYPTO code (from
past companion PRs) take the latest version from Mbed TLS which
includes integration with MBEDTLS_CHECK_PARAMS.
- Update the version of the shared library files to match what's
currently present in Mbed TLS.
- Reject removal of testing with PSA from config full tests.
- Resolve conflicts in test tests/suites/helpers.function, where both
Mbed Crypto and Mbed TLS both added documentation for TEST_ASSERT.
Combine text from both documentation efforts.
- Reject adding a submodule of ourselves.
- Reject addition of submodule tests in all.sh.
- Reject addition of submodule to library path in
tests/scripts/run-test-suites.pl.
- Avoid using USE_CRYPTO_SUBMODULE=1 in
component_test_use_psa_crypto_full_cmake_asan() in all.sh.
Context:
The existing API `mbedtls_x509_parse_crt_der()` for parsing DER
encoded X.509 CRTs unconditionally makes creates a copy of the
input buffer in RAM. While this comes at the benefit of easy use,
-- specifically: allowing the user to free or re-use the input
buffer right after the call -- it creates a significant memory
overhead, as the CRT is duplicated in memory (at least temporarily).
This might not be tolerable a resource constrained device.
As a remedy, this commit adds a new X.509 API call
`mbedtls_x509_parse_crt_der_nocopy()`
which has the same signature as `mbedtls_x509_parse_crt_der()`
and almost the same semantics, with one difference: The input
buffer must persist and be unmodified for the lifetime of the
established instance of `mbedtls_x509_crt`, that is, until
`mbedtls_x509_crt_free()` is called.
Resolve incompatibilties in the RSA module where changes made for
parameter validation prevent Mbed Crypto from working. Mbed Crypto
depends on being able to pass zero-length buffers that are NULL to RSA
encryption functions.
This reverts commit 2f660d047d.
The macro PSA_HASH_FINAL_SIZE no longer exists and all instances of it
should be replaced by PSA_HASH_SIZE. Replace all remaining instances of
PSA_HASH_FINAL_SIZE with PSA_HASH_SIZE.
Merge a development version of Mbed TLS 2.16.0 that doesn't have
parameter validation into development.
The following conflicts were resolved:
- Update ChangeLog to include release notes merged from development so
far, with a version of "2.14.0+01b34fb316a5" and release date of
"xxxx-xx-xx" to show this is not a released version, but instead a
snapshot of the development branch equivalent to version of the 2.14.0
with additional commits from the mbedtls/development branch up through
01b34fb316 included. Entries added for unreleased versions of Mbed
Crypto remain at the top of the file for Mbed TLS 2.xx.x.
- Replace the Mbed Crypto version of
mbedtls_rsa_rsaes_pkcs1_v15_decrypt() with the version from Mbed TLS
which fixes timing variations and memory access variations that could
lead to a Bleichenbacher-style padding oracle attack. This will
prevent using psa_asymmetric_decrypt() with zero-length output buffers
until a follow up commit is made to restore this capability.
- In ssl_srv.c, include changes for both the new ECDH interface and
opaque PSK as already added to development previously.
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.
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.
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
The file oid.c had conditional inclusion of functions based on a config.h
define that belongs to X.509, which is backwards. For now, just include those
functions unconditionally and rely on the linker to garbage-collect them if
not used.
In the longer term X.509-specific functions are likely to be removed from
libmbedcrypto, but at this step the goal is to preserve the API (and even ABI)
of libmbedcrypto for as long as possible while separating the source trees of
Mbed Crypto and Mbed TLS.
As agreed during the workshop, temporarily move definitions to oid.h even if
they might not semantically belong here, as a short-term measure allowing to
build libmbecrypto on its own (without X.509 files present in the source tree)
but still provide all the things Mbed TLS currently expects, and more
specifically preserve the API and ABI exposed by libmbedtls.
Remove front matter from our EC key format, to make it just the contents
of an ECPoint as defined by SEC1 section 2.3.3.
As a consequence of the simplification, remove the restriction on not
being able to use an ECDH key with ECDSA. There is no longer any OID
specified when importing a key, so we can't reject importing of an ECDH
key for the purpose of ECDSA based on the OID.
Remove pkcs-1 and rsaEncryption front matter from RSA public keys. Move
code that was shared between RSA and other key types (like EC keys) to
be used only with non-RSA keys.
New function psa_copy_key().
Conflicts:
* library/psa_crypto.c: trivial conflicts due to consecutive changes.
* tests/suites/test_suite_psa_crypto.data: the same code
was added on both sides, but with a conflict resolution on one side.
* tests/suites/test_suite_psa_crypto_metadata.function: the same code
was added on both sides, but with a conflict resolution on one side.
You can use PSA_ALG_ANY_HASH to build the algorithm value for a
hash-and-sign algorithm in a policy. Then the policy allows usage with
this hash-and-sign family with any hash.
Test that PSA_ALG_ANY_HASH-based policies allow a specific hash, but
not a different hash-and-sign family. Test that PSA_ALG_ANY_HASH is
not valid for operations, only in policies.
Test for a subclass of public-key algorithm: those that perform
full-domain hashing, i.e. algorithms that can be broken down as
sign(key, hash(message)).
Remove the type and bits arguments to psa_allocate_key() and
psa_create_key(). They can be useful if the implementation wants to
know exactly how much space to allocate for the slot, but many
implementations (including ours) don't care, and it's possible to work
around their lack by deferring size-dependent actions to the time when
the key material is created. They are a burden to applications and
make the API more complex, and the benefits aren't worth it.
Change the API and adapt the implementation, the units test and the
sample code accordingly.
Change the key derivation API to take inputs in multiple steps,
instead of a single one-site-fits-poorly function.
Conflicts:
* include/psa/crypto.h: merge independent changes in the documentation
of psa_key_agreement (public_key from the work on public key formats
vs general description and other parameters in the work on key derivation).
* tests/suites/test_suite_psa_crypto.data: update the key agreement
tests from the work on key derivation to the format from the work on
public key formats.
* tests/suites/test_suite_psa_crypto_metadata.function: reconcile the
addition of unrelated ALG_IS_xxx macros
Get rid of "key selection" algorithms (of which there was only one:
raw key selection).
Encode key agreement by combining a raw key agreement with a KDF,
rather than passing the KDF as an argument of a key agreement macro.
The normal way is to pass the shared secret to a key derivation.
Having an ad hoc function will allow us to simplify the possible
behaviors of key agreement and get rid of "key selection" algorithms
which are a hard-to-understand invention of this API.
It isn't a good fit. It's overly complex for what the API can do now,
which is Diffie-Hellman. Consider it again later for more complex use
cases such as authenticated key exchanges.
Use separate step types for a KDF secret and for the private key in a
key agreement.
Determine which key type is allowed from the step type, independently
of the KDF.
Forbid raw inputs for certain steps. They definitely should be
forbidden for asymmetric keys, which are structured. Also forbid them
for KDF secrets: the secrets are supposed to be keys, even if they're
unstructured.
Add the ability to specify Diffie-Hellman key exchange keys. Specify the
import/export format as well, even though importing and exporting isn't
implemented yet.
Remove front matter and DSS parameters from our DSA key formats, both
keypair and public key, to make it just a representation of the integer
private key, `x`, or the public key, `y`, respectively.
DSA and static DH need extra domain parameters. Instead of passing these
in with the keys themselves, add get and set functions to set and
retrieve this information about keys.
Add a new function mbedtls_asn1_write_named_bitstring() that removes
trailing 0s at the end of DER encoded bitstrings. The function is
implemented according to Hanno Becker's suggestions.
This commit also changes the functions x509write_crt_set_ns_cert_type
and crt_set_key_usage to call the new function as the use named
bitstrings instead of the regular bitstrings.
Remove front matter from our EC key format, to make it just the contents
of an ECPoint as defined by SEC1 section 2.3.3.
As a consequence of the simplification, remove the restriction on not
being able to use an ECDH key with ECDSA. There is no longer any OID
specified when importing a key, so we can't reject importing of an ECDH
key for the purpose of ECDSA based on the OID.
You can use PSA_ALG_ANY_HASH to build the algorithm value for a
hash-and-sign algorithm in a policy. Then the policy allows usage with
this hash-and-sign family with any hash.
Test that PSA_ALG_ANY_HASH-based policies allow a specific hash, but
not a different hash-and-sign family. Test that PSA_ALG_ANY_HASH is
not valid for operations, only in policies.
Finish changing "start" to "set up".
Correct the way to set an IV for decryption: it's set_iv(), not
update().
When decrypting, the IV is given, not random.
Test for a subclass of public-key algorithm: those that perform
full-domain hashing, i.e. algorithms that can be broken down as
sign(key, hash(message)).
Remove pkcs-1 and rsaEncryption front matter from RSA public keys. Move
code that was shared between RSA and other key types (like EC keys) to
be used only with non-RSA keys.
Copy the nice and clear documentation from psa_export_key() as to what
implementations are allowed to do regarding key export formats, as the
same applies to public keys.
hash_setup and mac_setup used to be called hash_start and mac_start,
but we've now converged on _setup as names. Finish making the
terminology in the documentation consistent.
We've added documentation for how context objects for multi-part
operations must be initialized consistently for key policy, hash,
cipher, and MAC. Update the generator documentation to be consistent
with how we've documented the other operations.
Add new initializers for cipher operation objects and use them in our
tests and library code. Prefer using the macro initializers due to their
straightforwardness.
The struct psa_cipher_operation_s is built with a
mbedtls_cipher_context_t. The shape of mbedtls_cipher_context_t and an
initializer that works with Clang 5.0 and its
-Wmissing-field-initializers varies based on the configuration of the
library. Instead of making multiple initializers based on a maze of
ifdefs for all combinations of MBEDTLS_CIPHER_MODE_WITH_PADDING,
MBEDTLS_CMAC_C, and MBEDTLS_USE_PSA_CRYPTO, add a dummy variable to
psa_cipher_operation_s's union that encloses mbedtls_cipher_context_t.
This allows us to initialize the dummy with a Clang-approved initializer
and have it properly initialize the entire object.
Add new initializers for MAC operation objects and use them in our tests
and library code. Prefer using the macro initializers due to their
straightforwardness.
Add new initializers for hash operation objects and use them in our
tests and library code. Prefer using the macro initializers due to their
straightforwardness.
Add new initializers for key policies and use them in our docs, example
programs, tests, and library code. Prefer using the macro initializers
due to their straightforwardness.
As there are some definitions that are defined regardless of
whether MBEDTLS_ECP_RESTARTABLE is defined or not, these definitions
need to be moved outside the MBEDTLS_ECP_ALT guards. This is a simple
move as MBEDTLS_ECP_ALT and MBEDTLS_ECP_RESTARTABLE are mutually
exclusive options.
Split crypto_driver.h into 4:
* crypto_driver_common.h for common definitions, not meant to be
included directly by driver code.
* crypto_accel_driver.h for drivers that work with transparent
key material.
* crypto_se_driver.h for drivers that work with opaque key
material.
* crypto_entropy_driver.h for drivers of entropy sources.
There is no code change in this commit, I only moved some code around.
Now that the type definitions that are useful for driver are in a
separate header file from the application interface function
declarations, include that header file in crypto_driver.h.
Some parts of the library, and crypto drivers, need to see key types,
algorithms, policies, etc. but not API functions. Move portable
integral types and macros to build and analyze values of these types
to a separate headers crypto_types.h and crypto_values.h.
No functional changes, code was only moved from crypto.h to the new headers.
Define psa_status_t to int32_t unconditionally. There's no reason to
refer to psa_error_t here: psa_error_t is int32_t if it's present. We
would only need a conditional definition if psa_defs.h and
psa_crypto.h used the same type name.
Keep the conditional definition of PSA_SUCCESS. Although the C
preprocessor allows a duplicate definition for a macro, it has to be
the exact same token sequence, not merely an equivalent way to build
the same value.
Document when a context must be initialized or not, when it must be
set up or not, and whether it needs a private key or a public key will
do.
The implementation is sometimes more liberal than the documentation,
accepting a non-set-up context as a context that can't perform the
requested information. This preserves backward compatibility.
For mbedtls_pk_parse_key and mbedtls_pk_parse_keyfile, the password is
optional. Clarify what this means: NULL is ok and means no password.
Validate parameters and test accordingly.
A 0-length buffer for the key is a legitimate edge case. Ensure that
it works, even with buf=NULL. Document the key and keylen parameters.
There are already test cases for parsing an empty buffer. A subsequent
commit will add tests for writing to an empty buffer.
The previous introduction of constant deprecation macros
in platform_util.h lead to failure of tests/scrips/check-names.sh
because the regular expressions in the latter choked on the brackets
in the part `__attribute__((deprecated))` of the definition of the
helper type `mbedtls_deprecated_{numeric|string}_constant_t`.
Postponing any further study and potential robustness improvements
in check-names.sh to another time, this commit circumvents this
problem by temporarily abbreviating `__attribute__((deprecated))`
as `MBEDTLS_DEPRECATED`, which doesn't lead to problems with
check-names.sh.
This commit introduces macros
* MBEDTLS_DEPRECATED_STRING_CONSTANT
* MBEDTLS_DEPRECATED_NUMERIC_CONSTANT
to platform_util.h which can be used to deprecate public macro constants.
Their definition is essentially taken from dhm.h where the
MBEDTLS_DEPRECATED_STRING_CONSTANT was used to deprecate
insecure hardcoded DHM primes.
Add inclusion to configration file in header files,
instead of relying on other header files to include
the configuration file. This issue resolves#1371
- Be specific about the constraints: be a readable/writable buffer of length
X, be an initialized context, be a context initialized and bound to a key...
- Always use full sentences with all the required pronouns.
Some of the documentation is obsolete in its reference to key slots
when it should discuss key handles. This may require a further pass,
possibly with some reorganization of error codes.
Update the documentation of functions that modify key slots (key
material creation and psa_set_key_policy()) to discuss how they affect
storage.
This commit marks the beginning of the removal of support for direct
access to key slots. From this commit on, programs that use
psa_key_slot_t will no longer compile.
Subsequent commits will remove the now-unused legacy support in
psa_crypto.c.
Replace `psa_key_slot_t key` by `psa_key_handle_t` in function
declarations.
This is a transition period during which handles are key slot numbers
and the whole library can still be used by accessing a key slot number
without allocating a handle.
It's better if the macro receives the condition as an expression rather than a
string - that way it can choose to use it as is or stringify it. Also, the
documentation states that the parameter is an expression, not a string.
The test framework for validation of parameters depends on the macro
MBEDTLS_PARAM_FAILED() being set to its default value when building the
library. So far the test framework attempted to define this macro but this was
the wrong place - this definition wouldn't be picked by the library.
Instead, a different approach is taken: skip those tests when the macro is
defined in config.h, as in that case we have no way to know if it will indeed
end up calling mbedtls_param_failed() as we need it to.
This commit was tested by manually ensuring that aes_invalid_params:
- passes (and is not skipped) in the default configuration
- is skipped when MBEDTLS_PARAM_FAILED() is defined in config.h
The previous prototype gave warnings are the strings produced by #cond and
__FILE__ are const, so we shouldn't implicitly cast them to non-const.
While at it modifying most example programs:
- include the header that has the function declaration, so that the definition
can be checked to match by the compiler
- fix whitespace
- make it work even if PLATFORM_C is not defined:
- CHECK_PARAMS is not documented as depending on PLATFORM_C and there is
no reason why it should
- so, remove the corresponding #if defined in each program...
- and add missing #defines for mbedtls_exit when needed
The result has been tested (make all test with -Werror) with the following
configurations:
- full with CHECK_PARAMS with PLATFORM_C
- full with CHECK_PARAMS without PLATFORM_C
- full without CHECK_PARAMS without PLATFORM_C
- full without CHECK_PARAMS with PLATFORM_C
Additionally, it has been manually tested that adding
mbedtls_aes_init( NULL );
near the normal call to mbedtls_aes_init() in programs/aes/aescrypt2.c has the
expected effect when running the program.
It was inconsistent between files: sometimes 3 arguments, sometimes one.
Align to 1 argument for the macro and 3 for the function, because:
- we don't need 3 arguments for the macro, it can add __FILE__ and __LINE__
in its expansion, while the function needs them as parameters to be correct;
- people who re-defined the macro should have flexibility, and 3 arguments
can give the impression they they don't have as much as they actually do;
- the design document has the macro with 1 argument, so let's stick to that.
The function called through the macro MBEDTLS_PARAM_FAILED() must be supplied by
users and makes no sense as a library function, apart from debug and test.
