This commit splits off the RSA helper functions into separate headers and
compilation units to have a clearer separation of the public RSA interface,
intended to be used by end-users, and the helper functions which are publicly
provided only for the benefit of designers of alternative RSA implementations.
1) move the change into Features from Changes, in the changLog
2) Change the feature alternative configuration MBEDTLS_ECDH_ALT
definition to function alternative defintions
MBEDTLS_ECDH_COMPUTE_SHARED_ALT and MBEDTLS_ECDH_GEN_PUBLIC_ALT
1) update ChangLog to have new feature in Features instead of Changes
2) Change MBEDTLS_ECDSA_ALT to function specific alternative definitions:
MBEDTLS_ECDSA_SIGN_ALT, MBEDTLS_ECDSA_VERIFY_ALT and MBEDTLS_ECDSA_GENKEY_ALT
It is not necessary to pass a CSPRNG to `mbedtls_rsa_deduce_moduli`, as there
exist well-working static strategies, and even if a PRNG is preferred, a
non-secure one would be sufficient.
Further, the implementation is changed to use a static strategy for the choice
of candidates which according to some benchmarks even performs better than the
previous one using random candidate choices.
This commit restricts WANT_READ to indicate that no data is available on the
underlying transport. To signal the need for further processing - which was
previously also handled through this error code - a new internal error code
MBEDTLS_ERR_SSL_CONTINUE_PROCESSING is introduced.
- Enhances the documentation of mbedtls_ssl_get_bytes_avail (return
the number of bytes left in the current application data record, if
there is any).
- Introduces a new public function mbedtls_ssl_check_pending for
checking whether any data in the internal buffers still needs to be
processed. This is necessary for users implementing event-driven IO
to decide when they can safely idle until they receive further
events from the underlying transport.
Further, state explicitly that wrong key types need not be supported by alternative RSA implementations, and that those
may instead return the newly introduced error code MBEDTLS_ERR_RSA_UNSUPPORTED_OPERATION.
This commit returns to using constant macros instead of global variables for the DHM group constants. Further, macros
providing the binary encoding of the primes from RFC 3526 and RFC 7919 are added. The hex-string macros are deprecated.
Original intention was to be allowed to perform in-place operations like changing the byte-order before importing
parameters into an HSM. Now a copy is needed in this case, but there's no more danger of a user expecting the arguments
to be left untouched.
Document explicitly that `mbedtls_rsa_check_privkey` and `mbedtls_rsa_complete` succeeding does not guarantee the
consistency of the underlying RSA private key but only that enough information is present to perform a private key
operation.
This commit disables the new MBEDTLS_RSA_FORCE_BLINDING option by default to preserve backwards
compatibility. Further, it deprecates disabling to prepare for a future release in which blinding will be
unconditionally enforced.
* mbedtls-2.6: (27 commits)
Update version number to 2.6.0
Fix language in Changelog for clarity
Improve documentation of PKCS1 decryption functions
Fix style and missing item in ChangeLog
Add credit to Changelog to fix for #666
Fix naked call to time() with platform call
Fix ChangeLog for duplication after merge
Rename time and index parameter to avoid name conflict.
Correct comment
Adapt ChangeLog
Reliably zeroize sensitive data in AES sample application
Reliably zeroize sensitive data in Crypt-and-Hash sample application
Fix potential integer overflow parsing DER CRT
Fix potential integer overflow parsing DER CRL
Move the git scripts to correct path
Update after @sbutcher-arm comments
Fix slash direction for linux path
Add note for the git_hoos README file
Pre push hook script
Check return code of mbedtls_mpi_fill_random
...
This commit adds the function mbedtls_rsa_validate_crt for validating a set of CRT parameters. The function
mbedtls_rsa_check_crt is simplified accordingly.
Alternative RSA implementations can be provided by defining MBEDTLS_RSA_ALT in
config.h, defining an mbedtls_rsa_context struct in a new file rsa_alt.h and
re-implementing the RSA interface specified in rsa.h.
Through the previous reworkings, the adherence to the interface is the only
implementation obligation - in particular, implementors are free to use a
different layout for the RSA context structure.
This commit extends the RSA interface by import/export calls that can be used to
setup an RSA context from a subset of the core RSA parameters (N,P,Q,D,E).
The intended workflow is the following:
1. Call mbedtls_rsa_import one or multiple times to import the core parameters.
2. Call mbedtls_rsa_complete to deduce remaining core parameters as well as any
implementation-defined internal helper variables.
The RSA context is ready for use after this call.
The import function comes in two variants mbedtls_rsa_import and
mbedtls_rsa_import_raw, the former taking pointers to MPI's as input, the latter
pointers buffers holding to big-endian encoded MPI's.
The reason for this splitting is the following: When only providing an import
function accepting const MPI's, a user trying to import raw binary data into an
RSA context has to convert these to MPI's first which before passing them to the
import function, introducing an unnecessary copy of the data in memory. The
alternative would be to have another MPI-based import-function with
move-semantics, but this would be in contrast to the rest of the library's
interfaces.
Similarly, there are functions mbedtls_rsa_export and mbedtls_rsa_export_raw for
exporting the core RSA parameters, either as MPI's or in big-endian binary
format.
The main import/export functions deliberately do not include the additional
helper values DP, DQ and QP present in ASN.1-encoded RSA private keys. To
nonetheless be able to check whether given parameters DP, DQ and QP are in
accordance with a given RSA private key, the interface is extended by a function
mbedtls_rsa_check_opt (in line with mbedtls_rsa_check_privkey,
mbedtls_rsa_check_pubkey and mbedtls_rsa_check_pub_priv). Exporting the optional
parameters is taken care of by mbedtls_export_opt (currently MPI format only).
This commit adds convenience functions to the RSA module for computing a
complete RSA private key (with fields N, P, Q, D, E, DP, DQ, QP) from a subset
of core parameters, e.g. (N, D, E).
The fact that self-signed end-entity certs can be explicitly trusted by
putting them in the CA list even if they don't have the CA bit was not
documented though it's intentional, and tested by "Certificate verification #73
(selfsigned trusted without CA bit)" in test_suite_x509parse.data
It is unclear to me whether the restriction that explicitly trusted end-entity
certs must be self-signed is a good one. However, it seems intentional as it is
tested in tests #42 and #43, so I'm not touching it for now.
As noted in #557, several functions use 'index' resp. 'time'
as parameter names in their declaration and/or definition, causing name
conflicts with the functions in the C standard library of the same
name some compilers warn about.
This commit renames the arguments accordingly.
Allow forcing 64-bit integer type for bignum operations. Also introduce
the macro MBEDTLS_TYPE_UDBL to allow configuration of the double length
integer in unknown compilers.
Rename the macro MBEDTLS_PLATFORM_SETUP_ALT to
MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT to make the name more descriptive
as this macro enables/disables both functions.
Add the following two functions to allow platform setup and teardown
operations for the full library to be hooked in:
* mbedtls_platform_setup()
* mbedtls_platform_teardown()
An mbedtls_platform_context C structure is also added and two internal
functions that are called by the corresponding setup and teardown
functions above:
* mbedtls_internal_platform_setup()
* mbedtls_internal_plartform_teardown()
Finally, the macro MBEDTLS_PLATFORM_SETUP_ALT is also added to allow
mbedtls_platform_context and internal function to be overriden by the
user as needed for a platform.
The functions mbedtls_aes_decrypt and mbedtls_aes_encrypt have been
superseded by mbedtls_aes_internal_decrypt and
mbedtls_aes_internal_encrypt, respectively. Alternative
implementations should now only replace the latter, and leave the
maintenance wrapper definitions of the former untouched.
This commit clarifies this in the documentation of the respective
configuration options MBEDTLS_AES_DECRYPT_ALT and
MBEDTLS_AES_ENCRYPT_ALT.
Protecting the ECP hardware acceleratior with mutexes is inconsistent with the
philosophy of the library. Pre-existing hardware accelerator interfaces
leave concurrency support to the underlying platform.
Fixes#863
Document the preconditions on the input and output buffers for
the PKCS1 decryption functions
- mbedtls_rsa_pkcs1_decrypt,
- mbedtls_rsa_rsaes_pkcs1_v15_decrypt
- mbedtls_rsa_rsaes_oaep_decrypt
Allow forcing 64-bit integer type for bignum operations. Also introduce
the macro MBEDTLS_TYPE_UDBL to allow configuration of the double length
integer in unknown compilers.
Rename the macro MBEDTLS_PLATFORM_SETUP_ALT to
MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT to make the name more descriptive
as this macro enables/disables both functions.
Add the following two functions to allow platform setup and teardown
operations for the full library to be hooked in:
* mbedtls_platform_setup()
* mbedtls_platform_teardown()
An mbedtls_platform_context C structure is also added and two internal
functions that are called by the corresponding setup and teardown
functions above:
* mbedtls_internal_platform_setup()
* mbedtls_internal_plartform_teardown()
Finally, the macro MBEDTLS_PLATFORM_SETUP_ALT is also added to allow
mbedtls_platform_context and internal function to be overriden by the
user as needed for a platform.
The functions mbedtls_aes_decrypt and mbedtls_aes_encrypt have been
superseded by mbedtls_aes_internal_decrypt and
mbedtls_aes_internal_encrypt, respectively. Alternative
implementations should now only replace the latter, and leave the
maintenance wrapper definitions of the former untouched.
This commit clarifies this in the documentation of the respective
configuration options MBEDTLS_AES_DECRYPT_ALT and
MBEDTLS_AES_ENCRYPT_ALT.
Protecting the ECP hardware acceleratior with mutexes is inconsistent with the
philosophy of the library. Pre-existing hardware accelerator interfaces
leave concurrency support to the underlying platform.
Fixes#863
Document the preconditions on the input and output buffers for
the PKCS1 decryption functions
- mbedtls_rsa_pkcs1_decrypt,
- mbedtls_rsa_rsaes_pkcs1_v15_decrypt
- mbedtls_rsa_rsaes_oaep_decrypt
Allow forcing 64-bit integer type for bignum operations. Also introduce
the macro MBEDTLS_TYPE_UDBL to allow configuration of the double length
integer in unknown compilers.
Rename the macro MBEDTLS_PLATFORM_SETUP_ALT to
MBEDTLS_PLATFORM_SETUP_TEARDOWN_ALT to make the name more descriptive
as this macro enables/disables both functions.
Add the following two functions to allow platform setup and teardown
operations for the full library to be hooked in:
* mbedtls_platform_setup()
* mbedtls_platform_teardown()
An mbedtls_platform_context C structure is also added and two internal
functions that are called by the corresponding setup and teardown
functions above:
* mbedtls_internal_platform_setup()
* mbedtls_internal_plartform_teardown()
Finally, the macro MBEDTLS_PLATFORM_SETUP_ALT is also added to allow
mbedtls_platform_context and internal function to be overriden by the
user as needed for a platform.
The functions mbedtls_aes_decrypt and mbedtls_aes_encrypt have been
superseded by mbedtls_aes_internal_decrypt and
mbedtls_aes_internal_encrypt, respectively. Alternative
implementations should now only replace the latter, and leave the
maintenance wrapper definitions of the former untouched.
This commit clarifies this in the documentation of the respective
configuration options MBEDTLS_AES_DECRYPT_ALT and
MBEDTLS_AES_ENCRYPT_ALT.
Protecting the ECP hardware acceleratior with mutexes is inconsistent with the
philosophy of the library. Pre-existing hardware accelerator interfaces
leave concurrency support to the underlying platform.
Fixes#863
Document the preconditions on the input and output buffers for
the PKCS1 decryption functions
- mbedtls_rsa_pkcs1_decrypt,
- mbedtls_rsa_rsaes_pkcs1_v15_decrypt
- mbedtls_rsa_rsaes_oaep_decrypt
This change moves the calls to mbedtls_sha256_starts() and
mbedtls_sha512_starts() out of the mbedtls_entropy_init() function as
these now have return codes which need to be checked.
This patch modifies the internal md context structure in md_wrap.c to
add return values to the function pointers. This enables us to use the
new API in the corresponding MD modules so that failures can be
found at any point in an MD computation.
The following function calls are being deprecated to introduce int
return values.
* mbedtls_sha512()
* mbedtls_sha512_starts()
* mbedtls_sha512_update()
* mbedtls_sha512_finish()
* mbedtls_sha512_process()
The return codes can be used to return error values. This is important
when using hardware accelerators.
The following function calls are being deprecated to introduce int
return values.
* mbedtls_sha256()
* mbedtls_sha256_starts()
* mbedtls_sha256_update()
* mbedtls_sha256_finish()
* mbedtls_sha256_process()
The return codes can be used to return error values. This is important
when using hardware accelerators.
The following function calls are being deprecated to introduce int
return values.
* mbedtls_ripemd160()
* mbedtls_ripemd160_starts()
* mbedtls_ripemd160_update()
* mbedtls_ripemd160_finish()
* mbedtls_ripemd160_process()
The return codes can be used to return error values. This is important
when using hardware accelerators.
The following function calls are being deprecated to introduce int
return values.
* mbedtls_md5()
* mbedtls_md5_starts()
* mbedtls_md5_update()
* mbedtls_md5_finish()
* mbedtls_md5_process()
The return codes can be used to return error values. This is important
when using hardware accelerators.
The following function calls are being deprecated to introduce int
return values.
* mbedtls_md4()
* mbedtls_md4_starts()
* mbedtls_md4_update()
* mbedtls_md4_finish()
* mbedtls_md4_process()
The return codes can be used to return error values. This is important
when using hardware accelerators.
The following function calls are being deprecated to introduce int
return values.
* mbedtls_md2()
* mbedtls_md2_starts()
* mbedtls_md2_update()
* mbedtls_md2_finish()
* mbedtls_md2_process()
The return codes can be used to return error values. This is important
when using hardware accelerators.
The following function calls are being deprecated to introduce int
return values.
* mbedtls_sha1()
* mbedtls_sha1_starts()
* mbedtls_sha1_update()
* mbedtls_sha1_finish()
* mbedtls_sha1_process()
The return codes can be used to return error values. This is important
when using hardware accelerators.
The previous version of `mbedtls_ssl_safer_memcmp` did not qualify the
pointers to the arrays to be compared as volatile, theoretically
opening the possibility for the compiler to notice that the loop
operation `diff |= A[i] ^ B[i]` is pointless if `diff = -1`. This
commit changes this. It also declares the stack variable `diff` as
volatile, to force read and write in every loop; omitting that, the
compiler would still be allowed to get away with reading `A[i]` and
`B[i]` but not doing the XOR and not updating `diff`.
This commit renames the new AES table packing option introduced in the
previous MBEDTLS_AES_PACK_TABLES and documents its use and memory vs.
speed tradeoff. It also enhances the documentation of the other
AES-related option MBEDTLS_AES_ROM_TABLES.
* restricted/iotssl-1398:
Add ChangeLog entry
Ensure application data records are not kept when fully processed
Add hard assertion to mbedtls_ssl_read_record_layer
Fix mbedtls_ssl_read
Simplify retaining of messages for future processing
There are situations in which it is not clear what message to expect
next. For example, the message following the ServerHello might be
either a Certificate, a ServerKeyExchange or a CertificateRequest. We
deal with this situation in the following way: Initially, the message
processing function for one of the allowed message types is called,
which fetches and decodes a new message. If that message is not the
expected one, the function returns successfully (instead of throwing
an error as usual for unexpected messages), and the handshake
continues to the processing function for the next possible message. To
not have this function fetch a new message, a flag in the SSL context
structure is used to indicate that the last message was retained for
further processing, and if that's set, the following processing
function will not fetch a new record.
This commit simplifies the usage of this message-retaining parameter
by doing the check within the record-fetching routine instead of the
specific message-processing routines. The code gets cleaner this way
and allows retaining messages to be used in other situations as well
without much effort. This will be used in the next commits.
By default, keep allowing SHA-1 in key exchange signatures. Disabling
it causes compatibility issues, especially with clients that use
TLS1.2 but don't send the signature_algorithms extension.
SHA-1 is forbidden in certificates by default, since it's vulnerable
to offline collision-based attacks.
There is now one test case to validate that SHA-1 is rejected in
certificates by default, and one test case to validate that SHA-1 is
supported if MBEDTLS_TLS_DEFAULT_ALLOW_SHA1 is #defined.
Default to forbidding the use of SHA-1 in TLS where it is unsafe: for
certificate signing, and as the signature hash algorithm for the TLS
1.2 handshake signature. SHA-1 remains allowed in HMAC-SHA-1 in the
XXX_SHA ciphersuites and in the PRF for TLS <= 1.1.
For easy backward compatibility for use in controlled environments,
turn on the MBEDTLS_TLS_DEFAULT_ALLOW_SHA1 compiled-time option.
* hanno/sig_hash_compatibility:
Improve documentation
Split long lines
Remember suitable hash function for any signature algorithm.
Introduce macros and functions to characterize certain ciphersuites.
Note that the configuration structure must remain accessible. The
previous wording could have been taken as implying that it's ok to
change the structure but changes wouldn't be taken into account.
Also note that calling this function twice is not supported (it would
at least be a memory leak).
This patch modifies the following 2 functions in the AES module to
change the return type from void to int:
* mbedtls_aes_encrypt() -> mbedtls_internal_aes_encrypt()
* mbedtls_aes_decrypt() -> mbedtls_internal_aes_decrypt()
This change is necessary to allow users of MBEDTLS_AES_ALT,
MBEDTLS_AES_DECRYPT_ALT and MBEDTLS_AES_ENCRYPT_ALT to return an error
code when replacing the default with their own implementation, e.g.
a hardware crypto accelerator.
According to RFC5246 the server can indicate the known Certificate
Authorities or can constrain the aurhorisation space by sending a
certificate list. This part of the message is optional and if omitted,
the client may send any certificate in the response.
The previous behaviour of mbed TLS was to always send the name of all the
CAs that are configured as root CAs. In certain cases this might cause
usability and privacy issues for example:
- If the list of the CA names is longer than the peers input buffer then
the handshake will fail
- If the configured CAs belong to third parties, this message gives away
information on the relations to these third parties
Therefore we introduce an option to suppress the CA list in the
Certificate Request message.
Providing this feature as a runtime option comes with a little cost in
code size and advantages in maintenance and flexibility.
This commit changes `ssl_parse_signature_algorithms_ext` to remember
one suitable ( := supported by client and by our config ) hash
algorithm per signature algorithm.
It also modifies the ciphersuite checking function
`ssl_ciphersuite_match` to refuse a suite if there
is no suitable hash algorithm.
Finally, it adds the corresponding entry to the ChangeLog.
The routine `mbedtls_ssl_write_server_key_exchange` heavily depends on
what kind of cipher suite is active: some don't need a
ServerKeyExchange at all, some need (EC)DH parameters but no server
signature, some require both. Each time we want to restrict a certain
piece of code to some class of ciphersuites, it is guarded by a
lengthy concatentation of configuration checks determining whether at
least one of the relevant cipher suites is enabled in the config; on
the code level, it is guarded by the check whether one of these
cipher suites is the active one.
To ease readability of the code, this commit introduces several helper
macros and helper functions that can be used to determine whether a
certain class of ciphersuites (a) is active in the config, and
(b) contains the currently present ciphersuite.
With this commit the Elliptic Curve Point interface is rewised. Two
compile time options has been removed to simplify the interface and
the function names got a new prefix that indicates that these functions
are for internal use and not part of the public interface.
The primary use case behind providing an abstraction layer to enable
alternative Elliptic Curve Point arithmetic implementation, is making
use of cryptographic acceleration hardware if it is present.
To provide thread safety for the hardware accelerator we need a mutex
to guard it.
The compile time macros enabling the initialisation and deinitialisation
in the alternative Elliptic Curve Point arithmetic implementation had
names that did not end with '_ALT' as required by check-names.sh.
When provided with an empty line, mpi_read_file causes a numeric
underflow resulting in a stack underflow. This commit fixes this and
adds some documentation to mpi_read_file.
The modular inversion function hangs when provided with the modulus 1. This commit refuses this modulus with a BAD_INPUT error code. It also adds a test for this case.
In the ecdsa.c sample application we don't use hashing, we use ecdsa
directly on a buffer containing plain text. Although the text explains
that it should be the message hash it still can be confusing.
Any misunderstandings here are potentially very dangerous, because ECDSA
truncates the message hash if necessary and this can lead to trivial
signature forgeries if the API is misused and the message is passed
directly to the function without hashing.
This commit adds a hash computation step to the ecdsa.c sample
application and clarification to the doxygen documentation of the
ECDSA functions involved.
Fix an incorrect condition in ssl_check_ctr_renegotiate() that compared
64 bits of record counter instead of 48 bits as described in RFC 6347
Section 4.3.1. This would cause the function's return value to be
occasionally incorrect and the renegotiation routines to be triggered
at unexpected times.
Fix an incorrect condition in ssl_check_ctr_renegotiate() that compared
64 bits of record counter instead of 48 bits as described in RFC 6347
Section 4.3.1. This would cause the function's return value to be
occasionally incorrect and the renegotiation routines to be triggered
at unexpected times.
Fixes many typos, and errors in comments.
* Clarifies many comments
* Grammar correction in config.pl help text
* Removed comment about MBEDTLS_X509_EXT_NS_CERT_TYPE.
* Comment typo fix (Dont => Don't)
* Comment typo fix (assure => ensure)
* Comment typo fix (byes => bytes)
* Added citation for quoted standard
* Comment typo fix (one complement => 1's complement)
The is some debate about whether to prefer "one's complement", "ones'
complement", or "1's complement". The more recent RFCs related to TLS
(RFC 6347, RFC 4347, etc) use " 1's complement", so I followed that
convention.
* Added missing ")" in comment
* Comment alignment
* Incorrect comment after #endif
Fix implementation and documentation missmatch for the function
arguments to mbedtls_gcm_finish(). Also, removed redundant if condition
that always evaluates to true.
Minor fixes following review including:
* formatting changes including indentation and code style
* corrections
* removal of debug code
* clarification of code through variable renaming
* memory leak
* compiler warnings
Change the CMAC interface to match the mbedtls_md_hmac_xxxx() interface. This
changes the overall design of the CMAC interface to make it more consistent with
the existing HMAC interface, and will allow incremental updates of input data
rather than requiring all data to be presented at once, which is what the
current interface requires.
The library/net.c and its corresponding include/mbedtls/net.h file are
renamed to library/net_sockets.c and include/mbedtls/net_sockets.h
respectively. This is to avoid naming collisions in projects which also
have files with the common name 'net'.
Allow the size of the entry_name character array in x509_crt.c to be
configurable through a macro in config.h. entry_name holds a
path/filename string. The macro introduced in
MBEDTLS_X509_MAX_FILE_PATH_LEN.
Ensure that the entropy self test always fails whenever
MBEDTLS_TEST_NULL_ENTROPY is defined. This is because the option is
meant to be for testing and development purposes rather than production
quality software. Also, this patch enhances the documentation for
mbedtls_entropy_source_self_test() and mbedtls_entropy_self_test().
Instead of polling the hardware entropy source a single time and
comparing the output with itself, the source is polled at least twice
and make sure that the separate outputs are different.
The self test is a quick way to check at startup whether the entropy
sources are functioning correctly. The self test only polls 8 bytes
from the default entropy source and performs the following checks:
- The bytes are not all 0x00 or 0xFF.
- The hardware does not return an error when polled.
- The entropy does not provide data in a patter. Only check pattern
at byte, word and long word sizes.
* Check time platform abstraction macro definitions
This patch adds some checks to check_config.h to ensure that macro
definitions for the time platform abstraction are acceptable. In this
case the requirements are:
- MBEDTLS_PLATFORM_C and MBEDTLS_HAVE_TIME must be defined whenever
MBEDTLS_PLATFORM_TIME_ALT, MBEDTLS_PLATFORM_TIME_TYPE_MACRO or
MBEDTLS_PLATFORM_TIME_MACRO is defined.
- MBEDTLS_PLATFORM_STD_TIME and MBEDTLS_PLATFORM_TIME_ALT cannot be
defined simultaneously with MBEDTLS_PLATFORM_TIME_TYPE_MACRO or
MBEDTLS_PLATFORM_TIME_MACRO.
- MBEDTLS_HAVE_TIME and MBEDTLS_PLATFORM_TIME_ALT must be defined
whenever MBEDTLS_PLATFORM_STD_TIME is defined.
* Document requirements for time abstraction macros
Document that time platform abstraction macros
MBEDTLS_PLATFORM_TIME_ALT, MBEDTLS_PLATFORM_TIME_MACRO,
MBEDTLS_PLATFORM_TIME_TYPE_MACRO and MBEDTLS_PLATFORM_STD_TIME require
MBEDTLS_HAVE_TIME to be defined in config.h.
* Fix requires comment in config.h
* Split preprocessor condition for simplicity
Separates platform time abstraction into it's own header from the
general platform abstraction as both depend on different build options.
(MBEDTLS_PLATFORM_C vs MBEDTLS_HAVE_TIME)
