The SSL test programs can now use mbedtls_psa_get_random() rather than
entropy+DRBG as a random generator. This happens if
the configuration option MBEDTLS_USE_PSA_CRYPTO is enabled, or if
MBEDTLS_TEST_USE_PSA_CRYPTO_RNG is set at build time.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
With MBEDTLS_USE_PSA_CRYPTO, some of the randomness for the TLS
connection is generated inside the PSA crypto subsystem, which has no
reproducible mode. Whether there is a nonzero amount of randomness
coming from inside the PSA subsystem rather than from the random
generator set by mbedtls_ssl_conf_rng() depends on the choice of
cipher suite and other connection parameters as well as the level of
support for MBEDTLS_USE_PSA_CRYPTO. Rather than give unreliable
results, conservatively abort with a clear error message.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Move the declaration of the functions needed to use the test
implementation of mbedtls_psa_external_get_random() to a new header
file. Before, they were declared in
tests/include/test/psa_crypto_helpers.h, but this header file can't be
included in sample programs because it also includes headers from the
library directory which is not on the include path for sample
programs.
This fixes the build of the sample programs when
MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG and MBEDTLS_USE_PSA_CRYPTO are
enabled.
Move the implementation of the functions to a separate .c file as
well. This isn't strictly necessary, but makes the structure of the
source code easier to understand.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Currently, MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG is tested with a dummy
insecure implementation of mbedtls_psa_external_get_random. This
function needs to be explicitly enabled at runtime. This needs to
happen when the PSA external RNG is used, which currently is the case
in SSL test programs only when MBEDTLS_USE_PSA_CRYPTO is enabled.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Support HMAC_DRBG in ssl_client2 and ssl_server2, in addition to
CTR_DRBG. CTR_DRBG is still used if present, but it's now possible to
run the SSL test programs with CTR_DRBG disabled.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
In ssl_client2 and ssl_server2, to generate random data, go through a
level of indirection provided by ssl_test_lib. This way the programs
don't depend on a particular choice of RNG implementation, and only
ssl_test_lib.{h,c} explicitly reference CTR_DRBG.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is deliberately arranged to minimize code changes.
Subsequent commits will clean up the resulting code.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
In preparation for unifying the common RNG-related code of ssl_client2
and ssl_server2, make it possible to copy-paste that code out of these
programs' main() functions:
* Replaces reads of the non-unifiable structure opt by reads of a
separate variable.
* Replace references to the local variable rng by a pointer.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Group the random generation context (entropy and DRBG) into a struct.
This is in preparation for unifying the common RNG-related code of
ssl_client2 and ssl_server2, then generalizing that code to support
methods other than entropy+CTR_DRBG.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Declaring query_config() belongs with the query_config program, not in
ssl_test_lib.h, so move the declaration to a new header file
query_config.h.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Move from ssl_*2.c to ssl_test_lib.c:
* Functions that have exactly identical definitions in the two
programs, and that don't reference the global variable opt which
has a different type in the client and in the server. Also declare
these functions in ssl_test_lib.h.
Move from ssl_*2.c to ssl_test_common_source.c:
* Functions that have exactly identical definitions in the two
programs, but access fields of the global variable opt which
has a different structure type in the client and in the server.
* The array ssl_sig_hashes_for_test, because its type is incomplete.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Adjust whitespace and comments in code of ssl_client2.c and
ssl_server2.c that was almost identical to make these chunks exactly
identical.
Make the common functions non-static.
This is in preparation for moving the common code to ssl_test_lib.c.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Move from ssl_*2.c to ssl_test_lib.h:
* Include directives for headers included by both programs.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Move from ssl_*2.c to ssl_test_lib.h:
* The inclusion of the config header file (identical).
* The fallback definitions of platform functions (almost identical:
server2 also had snprintf and the order and whitespace was slightly
different).
* The compilation guards checking that required options are
enabled (tweaked to make them common, with MBEDTLS_SSL_{CLI,SRV}_C
separated out).
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Create a new source file for code that is shared between ssl_client2.c
and ssl_server2.c, but cannot be compiled separately and instead needs
to be #include'd in each program that uses it.
This is for code that references macros or types that are defined
differently in the two programs.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Create a new source file for code that is shared between ssl_client2.c
and ssl_server2.c. This commit only creates the file; subsequent
commits will populate it.
Add it to the official build systems (make, cmake, VS).
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
mbedtls_ssl_conf_dtls_srtp_protection_profiles stores the pointer to the
profiles in the configuration.
Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
In cmake version < 3.0, the SOURCES property on targets cannot be
modified after the target is defined. There are several instances in
the code that were using `target_properties()`, which is not available
in the older versions of cmake. Unfortunately, the workaround in #3801
(381c1078fc) assumes that this SOURCES property can be modified.
Work around this by building up any necessary sources before declaring
the target. This is more awkward, but needed to continue to be able to
support the old versions of cmake.
Fixes#3788.
Signed-off-by: David Brown <david.brown@linaro.org>
PSA and SSL programs are PSA clients thus should use
psa_key_id_t as the type for key identifiers, not
mbedtls_svc_key_id_t.
As a consequence, PSA, ssl_server2 and ssl_client2
programs cannot compile and must not be compiled if
MBEDTLS_PSA_CRYPTO_KEY_ID_ENCODES_OWNER is defined.
Thus, add MBEDTLS_PSA_CRYPTO_KEY_ID_ENCODES_OWNER
compilation guard to those programs.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>
Define psa_key_handle_t to be equal to
mbedtls_svc_key_id_t. Make the handle of a persistent
key be equal to its key identifier. For volatile keys,
make the key handle equal to the volatile key
identifier of the created volatile key.
The unit tests are modified just to make them compile
not to make them run successfully. They are fixed in
the subsequent commits.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>
CMake versions less than 3.0 do not support the `target_sources`
command. In order to be able to support v2.8.12.2 of cmake, directly
set the SOURCES property instead of using the target_sources command.
A future patch could reverse this, if the project decides to forgo
support for cmake versions less than 3.0.
Fixes#3801
Signed-off-by: David Brown <david.brown@linaro.org>
Use the export keys functionality, to call the public API
`mbedtls_ssl_tls_prf()`, and remove the function
`mbedtls_ssl_get_dtls_srtp_key_material()`.
Signed-off-by: Johan Pascal <johan.pascal@belledonne-communications.com>
Fix compilation errors when `MBEDTLS_DTLS_SRTP` not set
1. Add file missed in previous commmit.
2. In sample applications, set `DFL_FORCE_SRTP_PROFILE` to 0.
Signed-off-by: Johan Pascal <johan.pascal@belledonne-communications.com>
1. Set correct mki from the `use_srtp` extension.
2. Use mki value received from the client as the mki used by server.
3. Use `mbedtls_ssl_dtls_srtp_set_mki_value()` as a client API only.
Signed-off-by: Johan Pascal <johan.pascal@belledonne-communications.com>
Add dtls-srtp to `ssl_client2` and `ssl_server2` examples,
for reference and for allowing in tests.
Signed-off-by: Johan Pascal <johan.pascal@belledonne-communications.com>
In `read_next_b64_code()`, the result of fgetc() is stored into a char,
but later compared against EOF, which is generally -1. On platforms
where char is unsigned, this generates a compiler warning/error that the
comparison will never be true (causing a build failure). The value will
never match, with the function ultimately bailing with a "Too many bad
symbols are detected" error.
On platforms with signed char, EOF is detected, but a file containing a
0xFF character will causes a premature end of file exit of the loop.
Fix this by changing the result to an int.
Fixes#3794.
Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Signed-off-by: David Brown <david.brown@linaro.org>
Allows required targets to have prefixes added to them, so that external
projects can avoid target names clashing.
Signed-off-by: Raef Coles <raef.coles@arm.com>
If MBEDTLS_KEY_EXCHANGE_SOME_PSK_ENABLED is defined, then the return value will be overridden by the extra code running after the removed return instruction.
Signed-off-by: gabor-mezei-arm <gabor.mezei@arm.com>
The extra code running after the removed return instruction should not generate any output. Only the read config value must be printed.
Signed-off-by: gabor-mezei-arm <gabor.mezei@arm.com>
As a result, the copyright of contributors other than Arm is now
acknowledged, and the years of publishing are no longer tracked in the
source files.
Also remove the now-redundant lines declaring that the files are part of
MbedTLS.
This commit was generated using the following script:
# ========================
#!/bin/sh
# Find files
find '(' -path './.git' -o -path './3rdparty' ')' -prune -o -type f -print | xargs sed -bi '
# Replace copyright attribution line
s/Copyright.*Arm.*/Copyright The Mbed TLS Contributors/I
# Remove redundant declaration and the preceding line
$!N
/This file is part of Mbed TLS/Id
P
D
'
# ========================
Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
The example application programs/ssl/ssl_client2 allows the
configuration of a client CRT through the parameters
- crt_file, key_file
However, password protected key files are not supported.
This commit adds a new command line option
- key_pwd
which allow to specify a password for the key file specified
in the key_file parameter.
The example application programs/ssl/ssl_server2 allows the
configuration of up to two CRTs through the command line
parameters
- crt_file, key_file
- crt_file2, key_file2.
However, password protected key files are not supported.
This commit adds command line options
- key_pwd
- key_pwd2
which allow to specify passwords for the key files specified
in key_file and key_file2, respectively.
Use list of executables to:
- factorize the code to define executables
- highlight the similarities and differences of the executable definitions
- avoid list duplication
Use alphabetic order for executables in lists.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>
The path to source files were relative which triggered
warnings when generating the build system.
Move to absolute paths based on CMAKE_CURRENT_SOURCE_DIR.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>
1. When `ssl_server2` export key functionality fails,
don't exit the server, but reset it, to have the
server recover for next connection.
2. Add text filters for `export keys functionality` test in ssl-opt.sh
to check for additional output, to verify if the export suceeded.
This was discovered in the `ssl-opt.sh` script, where the server exited,
before the test tried to kill the server priocess, resulting in a
`kill: No such process` message.
Fixes#2662
Signed-off-by: Ron Eldor <Ron.Eldor@arm.com>
Don't use string literals that are longer than 4095 bytes, which is
the minimum that C99 compilers are required to support. Compilers are
extremely likely to support longer literals, but `gcc -std=c99 -pedantic`
complains.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Since the server might want to have a different maximum fragment length
for the outgoing messages than the negotiated one - introduce a new way of
computing it. This commit also adds additional ssl-opt.sh tests ensuring
that the maximum fragment lengths are set as expected.
mbedtls_ssl_get_max_frag_len() is now a deprecated function,
being an alias to mbedtls_ssl_get_output_max_frag_len(). The behaviour
of this function is the same as before.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
Merge the latest state of the target branch (mbedtls/development) into the
pull request to merge mbed-crypto into mbedtls.
Conflicts:
* ChangeLog: add/add conflict. Resolve by using the usual section order.
Rename identifiers containing double-underscore (`__`) to avoid `__`.
The reason to avoid double-underscore is that all identifiers
containing double-underscore are reserved in C++. Rename all such
identifiers that appear in any public header, including ssl_internal.h
which is in principle private but in practice is installed with the
public headers.
This commit makes check-names.sh pass.
```
perl -i -pe 's/\bMBEDTLS_SSL__ECP_RESTARTABLE\b/MBEDTLS_SSL_ECP_RESTARTABLE_ENABLED/g; s/\bMBEDTLS_KEY_EXCHANGE_(_\w+)_(_\w+)\b/MBEDTLS_KEY_EXCHANGE${1}${2}/g' include/mbedtls/*.h library/*.c programs/*/*.c scripts/data_files/rename-1.3-2.0.txt tests/suites/*.function
```
The ssl-opt.sh test cases using session resumption tend to fail occasionally
on the CI due to a race condition in how ssl_server2 and ssl_client2 handle
the reconnection cycle.
The server does the following in order:
- S1 send application data
- S2 send a close_notify alert
- S3 close the client socket
- S4 wait for a "new connection" (actually a new datagram)
- S5 start a handshake
The client does the following in order:
- C1 wait for and read application data from the server
- C2 send a close_notify alert
- C3 close the server socket
- C4 reset session data and re-open a server socket
- C5 start a handshake
If the client has been able to send the close_notify (C2) and if has been
delivered to the server before if closes the client socket (S3), when the
server reaches S4, the datagram that we start the new connection will be the
ClientHello and everything will be fine.
However if S3 wins the race and happens before the close_notify is delivered,
in S4 the close_notify is what will be seen as the first datagram in a new
connection, and then in S5 this will rightfully be rejected as not being a
valid ClientHello and the server will close the connection (and go wait for
another one). The client will then fail to read from the socket and exit
non-zero and the ssl-opt.sh harness will correctly report this as a failure.
In order to avoid this race condition in test using ssl_client2 and
ssl_server2, this commits introduces a new command-line option
skip_close_notify to ssl_client2 and uses it in all ssl-opt.sh tests that use
session resumption with DTLS and ssl_server2.
This works because ssl_server2 knows how many messages it expects in each
direction and in what order, and closes the connection after that rather than
relying on close_notify (which is also why there was a race in the first
place).
Tests that use another server (in practice there are two of them, using
OpenSSL as a server) wouldn't work with skip_close_notify, as the server won't
close the connection until the client sends a close_notify, but for the same
reason they don't need it (there is no race between receiving close_notify and
closing as the former is the cause of the later).
An alternative approach would be to make ssl_server2 keep the connection open
until it receives a close_notify. Unfortunately it creates problems for tests
where we simulate a lossy network, as the close_notify could be lost (and the
client can't retransmit it). We could modify udp_proxy with an option to never
drop alert messages, but when TLS 1.3 comes that would no longer work as the
type of messages will be encrypted.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
As the SSL programs, like ssl_client2 and ssl_server2, are dependent on
SSL and therefore about to be removed, the only consumer of query_config
is the query_compile_time_config test. As such, it makes sense to move
query_config to be next to what uses it.
If `context_buf = mbedtls_calloc( 1, buf_len )` failed,
`context_buf_len` was not initialized. Noticed by
`gcc -Os -Werror=maybe-uninitialized`.
This was only a problem in ssl_server2 (a test program), only with
MBEDTLS_SSL_CONTEXT_SERIALIZATION enabled.
Make client_random and server_random const in
mbedtls_ssl_export_keys_ext_t, so that the key exporter is discouraged
from modifying the client/server hello.
Update examples and tests use const for hello.random as well, to ensure
that the export callbacks are of the proper type.
Fixes#2759
This commit adds command line options
- nss_keylog=0/1
- nss_keylog_file=FILENAME
to the example programs ssl/ssl_client2 and ssl/ssl_server2 which
allow to print and export the session keys in the NSS keylog
for debugging purposes.
* origin/pr/2803:
Add a ChangeLog entry for mbedtls_net_close()
Added mbedtls_net_close and use it in ssl_fork_server to correctly disassociate the client socket from the parent process and the server socket from the child process.
The code wants timer callbacks to be set (checked in fetch_input()), and can't
easily check whether we're using nbio, so it seems easier to require the
callbacks to be always set rather than only with nbio as was previously done.
context_buf was never free()d. Moreover, since we want to free it on error
paths as well, and even properly zeroize it in order to demonstrate good
memory hygiene, we need to make it and its length main()-scoped.
Previously it was missing reset in case 1, and in case 2 the code was never
executed as the option value was reset to 0.
Tighten checking of return values of save(NULL, 0) now that it works.
Also, improve the printed output as well as the comments.
I checked manually that everything now works and fail in the expected way:
save, reset-or-reinit and load all succeed, but the subsequent read or write
fails.
This is enabled by default as we generally enable things by default unless
there's a reason not to (experimental, deprecated, security risk).
We need a compile-time option because, even though the functions themselves
can be easily garbage-collected by the linker, implementing them will require
saving 64 bytes of Client/ServerHello.random values after the handshake, that
would otherwise not be needed, and people who don't need this feature
shouldn't have to pay the price of increased RAM usage.
We have explicit recommendations to use US spelling for technical writing, so
let's apply this to code as well for uniformity. (My fingers tend to prefer UK
spelling, so this needs to be fixed in many places.)
sed -i 's/\([Ss]eriali\)s/\1z/g' **/*.[ch] **/*.function **/*.data ChangeLog
This allows callers to discover what an appropriate size is. Otherwise they'd
have to either try repeatedly, or allocate an overly large buffer (or some
combination of those).
Adapt documentation an example usage in ssl_client2.
Avoid useless copy with mbedtls_ssl_get_session() before serialising.
Used in ssl_client2 for testing and demonstrating usage, but unfortunately
that means mbedtls_ssl_get_session() is no longer tested, which will be fixed
in the next commit.
This provides basic testing for the session (de)serialisation functions, as
well as an example of how to use them.
Tested locally with tests/ssl-opt.sh -f '^Session resume'.
* origin/pr/2781:
Documentation fixes according to review
Remove unused label in ssl_client2/ssl_server2
Add missing word in documentation of mbedtls_ssl_check_record()
cli/srv ex: Add dbg msg if record checking gives inconsistent result
Fix minor issues in documentation of mbedtls_ssl_check_record()
State that record checking is DTLS only and doesn't check content type
Update version_features.c
Pass dgrams to mbedtls_ssl_check_record in ssl_client2/server2
Add IO wrappers to ssl_server2 as interm's between NET and SSL layer
Add IO wrappers to ssl_client2 as interm's between NET and SSL layer
Introduce configuration option and API for SSL record checking
This is done to account for platforms, for which we want custom behavior
upon the program termination, hence we call `mbedtls_exit()` instead of
returning from `main()`.
* origin/pr/2697:
Update crypto submodule
Add all.sh component that exercises invalid_param checks
Remove mbedtls_param_failed from programs
Make it easier to define MBEDTLS_PARAM_FAILED as assert
Make test suites compatible with #include <assert.h>
Pass -m32 to the linker as well
Don't systematically rebuild programs
* origin/pr/2681:
unconditional mbedtls_ssl_conf_rng
Adding usage and removing dummy random
comment alignment
Commit to C90 style
Adds missing ret definition
More clarity for ifdef orders
Only warns if MBEDTLS_HAVE_TIME
Option used added in string
Code review
Style fixes
SSL reproducible test mode
* origin/pr/2260:
Update crypto submodule
Remove heading spaces in tests/data_files/Makefile
Re-generate library/certs.c from script
Add new line at the end of test-ca2.key.enc
Use strict syntax to annotate origin of test data in certs.c
Add run to all.sh exercising !MBEDTLS_PEM_PARSE_C + !MBEDTLS_FS_IO
Allow DHM self test to run without MBEDTLS_PEM_PARSE_C
ssl-opt.sh: Auto-skip tests that use files if MBEDTLS_FS_IO unset
Document origin of hardcoded certificates in library/certs.c
Adapt ChangeLog
Rename server1.der to server1.crt.der
Add DER encoded files to git tree
Add build instructions to generate DER versions of CRTs and keys
Document "none" value for ca_path/ca_file in ssl_client2/ssl_server2
ssl_server2: Skip CA setup if `ca_path` or `ca_file` argument "none"
ssl_client2: Skip CA setup if `ca_path` or `ca_file` argument "none"
Correct white spaces in ssl_server2 and ssl_client2
Adapt ssl_client2 to parse DER encoded test CRTs if PEM is disabled
Adapt ssl_server2 to parse DER encoded test CRTs if PEM is disabled
All sample and test programs had a definition of mbedtls_param_failed.
This was necessary because we wanted to be able to build them in a
configuration with MBEDTLS_CHECK_PARAMS set but without a definition
of MBEDTLS_PARAM_FAILED. Now that we activate the sample definition of
MBEDTLS_PARAM_FAILED in config.h when testing with
MBEDTLS_CHECK_PARAMS set, this boilerplate code is no longer needed.
Introduce a new configuration option MBEDTLS_CHECK_PARAMS_ASSERT,
which is disabled by default. When this option is enabled,
MBEDTLS_PARAM_FAILED defaults to assert rather than to a call to
mbedtls_param_failed, and <assert.h> is included.
This fixes#2671 (no easy way to make MBEDTLS_PARAM_FAILED assert)
without breaking backward compatibility. With this change,
`config.pl full` runs tests with MBEDTLS_PARAM_FAILED set to assert,
so the tests will fail if a validation check fails, and programs don't
need to provide their own definition of mbedtls_param_failed().
This allows to test PSK-based ciphersuites via ssl_server2 in builds
which have MBEDTLS_X509_CRT_PARSE_C enabled but both MBEDTLS_FS_IO and
MBEDTLS_CERTS_C disabled.
This allows to test PSK-based ciphersuites via ssl_client2 in builds
which have MBEDTLS_X509_CRT_PARSE_C enabled but both MBEDTLS_FS_IO and
MBEDTLS_CERTS_C disabled.
A similar change is applied to the `crt_file` and `key_file` arguments.
This commit modifies the CID configuration API mbedtls_ssl_conf_cid_len()
to allow the configuration of the stack's behaviour when receiving an
encrypted DTLS record with unexpected CID.
Context:
The CID draft does not require that the length of CIDs used for incoming
records must not change in the course of a connection. Since the record
header does not contain a length field for the CID, this means that if
CIDs of varying lengths are used, the CID length must be inferred from
other aspects of the record header (such as the epoch) and/or by means
outside of the protocol, e.g. by coding its length in the CID itself.
Inferring the CID length from the record's epoch is theoretically possible
in DTLS 1.2, but it requires the information about the epoch to be present
even if the epoch is no longer used: That's because one should silently drop
records from old epochs, but not the entire datagrams to which they belong
(there might be entire flights in a single datagram, including a change of
epoch); however, in order to do so, one needs to parse the record's content
length, the position of which is only known once the CID length for the epoch
is known. In conclusion, it puts a significant burden on the implementation
to infer the CID length from the record epoch, which moreover mangles record
processing with the high-level logic of the protocol (determining which epochs
are in use in which flights, when they are changed, etc. -- this would normally
determine when we drop epochs).
Moreover, with DTLS 1.3, CIDs are no longer uniquely associated to epochs,
but every epoch may use a set of CIDs of varying lengths -- in that case,
it's even theoretically impossible to do record header parsing based on
the epoch configuration only.
We must therefore seek a way for standalone record header parsing, which
means that we must either (a) fix the CID lengths for incoming records,
or (b) allow the application-code to configure a callback to implement
an application-specific CID parsing which would somehow infer the length
of the CID from the CID itself.
Supporting multiple lengths for incoming CIDs significantly increases
complexity while, on the other hand, the restriction to a fixed CID length
for incoming CIDs (which the application controls - in contrast to the
lengths of the CIDs used when writing messages to the peer) doesn't
appear to severely limit the usefulness of the CID extension.
Therefore, the initial implementation of the CID feature will require
a fixed length for incoming CIDs, which is what this commit enforces,
in the following way:
In order to avoid a change of API in case support for variable lengths
CIDs shall be added at some point, we keep mbedtls_ssl_set_cid(), which
includes a CID length parameter, but add a new API mbedtls_ssl_conf_cid_len()
which applies to an SSL configuration, and which fixes the CID length that
any call to mbetls_ssl_set_cid() which applies to an SSL context that is bound
to the given SSL configuration must use.
While this creates a slight redundancy of parameters, it allows to
potentially add an API like mbedtls_ssl_conf_cid_len_cb() later which
could allow users to register a callback which dynamically infers the
length of a CID at record header parsing time, without changing the
rest of the API.
