The numerical identifier of the CID extension hasn't been settled yet
and different implementations use values from different drafts. Allow
configuring the value at compile time.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Now that descriptions of error codes no longer have to be on the same line
for the sake of generate_errors.pl, move them to their own line before the
definition. This aligns them with what we do for other definitions, and
means that we no longer need to have very long lines containing both the C
definition and the comment.
```
perl -i -pe 's~^(#define +MBEDTLS_ERR_\w+ +-\w+) */\*[*!]<(.*)\*/~/**$2*/\n$1~' include/mbedtls/*.h
```
This commit does not change the output of generate_errors.pl.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Instances of `mbedtls_ssl_session` represent data enabling session resumption.
With the introduction of TLS 1.3, the format of this data changes. We therefore
need TLS-version field as part of `mbedtlsl_ssl_session` which allows distinguish
1.2 and 1.3 sessions.
This commit introduces such a TLS-version field to mbedtls_ssl_session.
The change has a few ramifications:
- Session serialization/deserialization routines need to be adjusted.
This is achieved by adding the TLS-version after the header of
Mbed TLS version+config, and by having the subsequent structure
of the serialized data depend on the value of this field.
The details are described in terms of the RFC 8446 presentation language.
The 1.2 session (de)serialization are moved into static helper functions,
while the top-level session (de)serialization only parses the Mbed TLS
version+config header and the TLS-version field, and dispatches according
to the found version.
This way, it will be easy to add support for TLS 1.3 sessions in the future.
- Tests for session serialization need to be adjusted
- Once we add support for TLS 1.3, with runtime negotiation of 1.2 vs. 1.3,
we will need to have some logic comparing the TLS version of the proposed session
to the negotiated TLS version. For now, however, we only support TLS 1.2,
and no such logic is needed. Instead, we just store the TLS version in the
session structure at the same point when we populate mbedtls_ssl_context.minor_ver.
The change introduces some overlap between `mbedtls_ssl_session.minor_ver` and
`mbedtls_ssl_context.minor_ver`, which should be studied and potentially resolved.
However, with both fields being private and explicitly marked so, this can happen
in a later change.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
Introduce new codes:
* MBEDTLS_ERR_SSL_UNSUPPORTED_EXTENSION
* MBEDTLS_ERR_SSL_NO_APPLICATION_PROTOCOL
These are returned when the corresponding alert is raised.
Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
New name MBEDTLS_ERR_SSL_BAD_CERTIFICATE
Also, replace some instances of MBEDTLS_ERR_SSL_BAD_HS_CERTIFICATE
by MBEDTLS_ERR_SSL_DECODE_ERROR and MBEDTLS_ERR_SSL_ILLEGAL_PARAMETER
as fit.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
Also remove preprocessor logic for MBEDTLS_CONFIG_FILE, since
build_info.h alreadyy handles it.
This commit was generated using the following script:
# ========================
#!/bin/sh
git ls-files | grep -v '^include/mbedtls/build_info\.h$' | xargs sed -b -E -i '
/^#if !?defined\(MBEDTLS_CONFIG_FILE\)/i#include "mbedtls/build_info.h"
//,/^#endif/d
'
# ========================
Signed-off-by: Bence Szépkúti <bence.szepkuti@arm.com>
This saves some code when compiling for Thumb, where access to
fields with offset index > 127 requires intermediate address
computations. Frequently used fields should therefore be located
at the top of the structure, while less frequently used ones --
such as the export callback -- can be moved to the back.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
TLS used to prefer larger curves, under the idea that a larger curve has a
higher security strength and is therefore harder to attack. However, brute
force attacks are not a practical concern, so this was not particularly
meaningful. If a curve is considered secure enough to be allowed, then we
might as well use it.
So order curves by resource usage. The exact definition of what this means
is purposefully left open. It may include criteria such as performance and
memory usage. Risk of side channels could be a factor as well, although it
didn't affect the current choice.
The current list happens to exactly correspond to the numbers reported by
one run of the benchmark program for "full handshake/s" on my machine.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Upgrade the default list of hashes and curves allowed for TLS. The list is
now aligned with X.509 certificate verification: hashes and curves with at
least 255 bits (Curve25519 included), and RSA 2048 and above.
Remove MBEDTLS_TLS_DEFAULT_ALLOW_SHA1_IN_KEY_EXCHANGE which would no
longer do anything.
Document more precisely what is allowed by default.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
No change here, these were already mandatory, it just wasn't explicit in
the documentation.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
Conflicts:
include/mbedtls/ssl.h
include/psa/crypto_struct.h
Conflicts fixed by using the code from development branch
and manually re-applying the MBEDTLS_PRIVATE wrapping.
ssl_server2 had a check that we never try to use a minor version lower
than 2 with DTLS, but that check is no longer needed, as there's no way
that would happen now that MBEDTLS_SSL_MINOR_VERSION_1 is no longer
public.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
This commit removes the API
```
mbedtls_ssl_conf_ciphersuites_for_version()
```
which allows to configure lists of acceptable ciphersuites
for each supported version of SSL/TLS: SSL3, TLS 1.{0,1,2}.
With Mbed TLS 3.0, support for SSL3, TLS 1.0 and TLS 1.1
is dropped. Moreover, upcoming TLS 1.3 support has a different
notion of cipher suite and will require a different API.
This means that it's only for TLS 1.2 that we require
a ciphersuite configuration API, and
```
mbedtls_ssl_conf_ciphersuites()
```
can be used for that. The version-specific ciphersuite
configuration API `mbedtls_ssl_conf_ciphersuites_for_version()`,
in turn, is no longer needed.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
The previous documentation could be read as exhaustively listing
the factors that go into computing the maximum outgoing record
plaintext size -- we should give examples, but allow ourselves
to add more factors in the future.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit makes the API
- mbedtls_ssl_get_output_max_frag_len()
- mbedtls_ssl_get_input_max_frag_len()
- mbedtls_ssl_get__max_frag_len()
for querying the state of the Maximum Fragment Length
extension internal.
Rationale: The value those APIs provide to the user is in
upper bounds for the size of incoming and outgoing records,
which can be used to size application data buffers apporpriately
before passing them to mbedtls_ssl_{read,write}(). However,
there are other factors which influence such upper bounds,
such as the MTU or other extensions (specifically, the
record_size_limit extension which is still to be implemented)
which should be taken into account.
There should be more general APIs for querying the maximum
size of incoming and outgoing records.
For the maximum size of outgoing records, we already have such,
namely mbedtls_ssl_get_max_out_record_payload().
For the maximum size of incoming records, a new API will be
added in a subsequent commit.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
Conflicts:
library/version_features.c
programs/test/query_config.c
Files were removed in development branch and modified by current branch.
Conflicts fixes by removing them.
Conflicts:
* configs/config-psa-crypto.h: modified here, removed in development
* tests/suites/test_suite_x509parse.data: all conflicts are in depends_on
lines where development made a change unrelated to MBEDTLS_SHAxxx and our
branch either changed `MBEDTLS_SHA256_C` to `MBEDTLS_SHA224_C` or
`MBEDTLS_SHA512_C:!MBEDTLS_SHA512_NO_SHA384` to ``MBEDTLS_SHA384_C`, with
no change to what the test does. Pick the other branch's dependency
changes then apply our SHA dpeendency change.
mbedtls_ssl_{get,set}_session() exhibited idempotent behaviour
in Mbed TLS 2.x. Multiple calls to those functions are not useful
in TLS 1.2, and the idempotent nature is unsuitable for support of
TLS 1.3 which introduces the availabilty to offer multiple tickets
for resumption, as well as receive multiple tickets.
In preparation for TLS 1.3 support, this commit relaxes the semantics
of `mbedtls_ssl_{get,set}_session()` by allowing implementations to
fail gracefully, and leveraging this freedom by modifying the
existing TLS 1.2 implementation to only accept one call to
`mbedtls_ssl_{get,set}_session()` per context, and non-fatally
failing all subsequent invocations.
For TLS 1.3, it will be leveraged by making multiple calls to
`mbedtls_ssl_get_session()` issue one ticket a time until no more
tickets are available, and by using multiple calls to
`mbedtls_ssl_set_session()` to allow the client to offer multiple
tickets to the server.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
Rework SHA224 and SHA283 documentation.
Define MBEDTLS_SHAxxx_C options in alphabetic order.
Fix SHA224 and SHA384 dependencies in test suites.
Signed-off-by: Mateusz Starzyk <mateusz.starzyk@mobica.com>
This commit removes the unused error code
```
MBEDTLS_ERR_SSL_INVALID_VERIFY_HASH
```
from the public API for Mbed TLS 3.0.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This error is used when the output buffer isn't large enough
to hold our own certificate.
In the interest of cleaning up the error space for 3.0, this commit
removes MBEDTLS_ERR_SSL_CERTIFICATE_TOO_LARGE and replaces its single
use by MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
The SSL error code MBEDTLS_ERR_SSL_PEER_VERIFY_FAILED is unused.
Remove it for Mbed TLS 3.0.
The code being unused comes as a surprise, at is seems to be
reasonable to report it to the user upon peer CRT verification
failure. However, this study (can potentially re-introduction
of the code) can be left for 3.x, while the error code removal
can only happen in 3.0.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
The SSL error code MBEDTLS_ERR_SSL_UNKNOWN_CIPHER is unused.
Remove it for Mbed TLS 3.0 and leave a comment indicating the
gap in the error code space it creates.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
Moved new check_crypto_config.h file from include/psa to library
directory and the file is now included from *.c instead of the
crypto_config.h file. Fixed guards in PSA crypto library based
on review comments for new PSA crypto config features.
Signed-off-by: John Durkop <john.durkop@fermatsoftware.com>
Move all the PSA crypto APIs using key handles
to use key identifiers but psa_key_open() and
psa_key_close(). This is done without modifying
any test as key handles and key identifiers are
now the same.
Update the library modules using PSA crypto APIs
to get rid of key handles.
Programs and unit tests are updated to not use
key handles in subsequent commits, not in this
one.
Signed-off-by: Ronald Cron <ronald.cron@arm.com>
In the original attempt to add RSA support to PSA crypto config was too
generic. This set of changes adds support for the following RSA features:
PSA_WANT_ALG_RSA_PKCS1V15_CRYPT, PSA_WANT_ALG_RSA_PKCS1V15_SIGN,
PSA_WANT_ALG_RSA_OAEP, PSA_WANT_ALG_RSA_PSS, PSA_WANT_KEY_TYPE_RSA_KEY_PAIR,
and PSA_WANT_KEY_TYPE_RSA_PUBLIC_KEY.
There were also some updates to ensure the proper inclusion of PSA crypto
library code when certain features are enabled. These updates were made to
address warnings and errors in builds from the new tests for these
features being added for PSA crypto configuration.
Signed-off-by: John Durkop <john.durkop@fermatsoftware.com>
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>
Improve readability of the code:
1. move common code to `ssl_internal.h` as `static inline`.
2. Add comments.
3. Use local variables for extension size.
4. Change function signature, by adding buffer size and output length.
5. Take server srtp profile out of the loop.
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>
Make the key material length in mbedtls_ssl_get_dtls_srtp_key_material
to be in\out, like it is done all over the library
Signed-off-by: Johan Pascal <johan.pascal@belledonne-communications.com>
1. Add check for prerequisites in check_config.h
2. Add mki value to use_srtp extension
3. address some review comments
Signed-off-by: Johan Pascal <johan.pascal@belledonne-communications.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>
Description referred to mbedtls_ssl_sent_t callback,
but the callback is named mbedtls_ssl_send_t.
Signed-off-by: Christopher Moynihan <christophm@gmail.com>
This is to avoid confusion with the class of macros
MBEDTLS_SSL_PROTO_TLS1_X
which have an underscore between major and minor version number.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
TLS 1.3 record protection allows the addition of an arbitrary amount
of padding.
This commit introduces a configuration option
```
MBEDTLS_SSL_TLS13_PADDING_GRANULARITY
```
The semantics of this option is that padding is chosen in a minimal
way so that the padded plaintext has a length which is a multiple of
MBEDTLS_SSL_TLS13_PADDING_GRANULARITY.
For example, setting MBEDTLS_SSL_TLS13_PADDING_GRANULARITY to 1024
means that padded plaintexts will have length 1024, 2048, ..., while
setting it to 1 means that no padding will be used.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
This commit adds an error condition for bad user configurations
and updates the number of SSL module errors in error.h.
Signed-off-by: Ronald Cron <ronald.cron@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
```
Add a description of MBEDTLS_ERR_SSL_PEER_CLOSE_NOTIFY to the
documentation, as suggested by hanno-arm.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>
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
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.
The number of meaning of the flags will be determined later, when handling the
relevant struct members. For now three bytes are reserved as an example, but
this number may change later.
Enforce restrictions indicated in the documentation.
This allows to make some simplifying assumptions (no need to worry about
saving IVs for CBC in TLS < 1.1, nor about saving handshake data) and
guarantees that all values marked as "forced" in the design document have the
intended values and can be skipped when serialising.
Some of the "forced" values are not checked because their value is a
consequence of other checks (for example, session_negotiated == NULL outside
handshakes). We do however check that session and transform are not NULL (even
if that's also a consequence of the initial handshake being over) as we're
going to dereference them and static analyzers may appreciate the info.
This commit introduces a new SSL error code
`MBEDTLS_ERR_SSL_VERSION_MISMATCH`
which can be used to indicate operation failure due to a
mismatch of version or configuration.
It is put to use in the implementation of `mbedtls_ssl_session_load()`
to signal the attempt to de-serialize a session which has been serialized
in a build of Mbed TLS using a different version or configuration.
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.
The next commit with make the implementation publicly available as well.
For now the API is kept unchanged. The save function API has a serious drawback in that the user
must guess what an appropriate buffer size is.
Internally so far this didn't matter because we were only using that API for
ticket creation, and tickets are written to the SSL output buffer whose size
is fixed anyway, but for external users this might not be suitable. Improving
that is left for later.
Also, so far the functions are defined unconditionally. Whether we want to
re-use existing flags or introduce a new one is left for later.
Finally, currently suggested usage of calling get_session() then
session_save() is memory-inefficient in that get_session() already makes a
copy. I don't want to recommend accessing `ssl->session` directly as we want
to prohibit direct access to struct member in the future. Providing a clean
and efficient way is also left to a later commit.
* 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
To help the build system find the correct include files, paths starting
with "mbedtls/" or "psa/" must be used. Otherwise, you can run into
build failures like the following when building Mbed Crypto as a
submodule.
In file included from chachapoly.c:31:0:
../../include/mbedtls/chachapoly.h:43:10: fatal error: poly1305.h: No such file or directory
#include "poly1305.h"
^~~~~~~~~~~~
compilation terminated.
This commit modifies mbedtls_ssl_get_peer_cid() to also allow passing
NULL pointers in the arguments for the peer's CID value and length, in
case this information is needed.
For example, some users might only be interested in whether the use of
the CID was negotiated, in which case both CID value and length pointers
can be set to NULL. Other users might only be interested in confirming
that the use of CID was negotiated and the peer chose the empty CID,
in which case the CID value pointer only would be set to NULL.
It doesn't make sense to pass a NULL pointer for the CID length but a
non-NULL pointer for the CID value, as the caller has no way of telling
the length of the returned CID - and this case is therefore forbidden.
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.
Currently, the stack silently ignores DTLS frames with an unexpected CID.
However, in a system which performs CID-based demultiplexing before passing
datagrams to the Mbed TLS stack, unexpected CIDs are a sign of something not
working properly, and users might want to know about it.
This commit introduces an SSL error code MBEDTLS_ERR_SSL_UNEXPECTED_CID
which the stack can return in response to an unexpected CID. It will
conditionally be put to use in subsequent commits.
mbedtls_ssl_context contains pointers in_buf, in_hdr, in_len, ...
which point to various parts of the header of an incoming TLS or
DTLS record; similarly, there are pointers out_buf, ... for
outgoing records.
This commit adds fields in_cid and out_cid which point to where
the CID of incoming/outgoing records should reside, if present,
namely prior to where the record length resides.
Quoting https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-04:
The DTLSInnerPlaintext value is then encrypted and the CID added to
produce the final DTLSCiphertext.
struct {
ContentType special_type = tls12_cid; /* 25 */
ProtocolVersion version;
uint16 epoch;
uint48 sequence_number;
opaque cid[cid_length]; // New field
uint16 length;
opaque enc_content[DTLSCiphertext.length];
} DTLSCiphertext;
For outgoing records, out_cid is set in ssl_update_out_pointers()
based on the settings in the current outgoing transform.
For incoming records, ssl_update_in_pointers() sets in_cid as if no
CID was present, and it is the responsibility of ssl_parse_record_header()
to update the field (as well as in_len, in_msg and in_iv) when parsing
records that do contain a CID. This will be done in a subsequent commit.
Finally, the code around the invocations of ssl_decrypt_buf()
and ssl_encrypt_buf() is adapted to transfer the CID from the
input/output buffer to the CID field in the internal record
structure (which is what ssl_{encrypt/decrypt}_buf() uses).
Note that mbedtls_ssl_in_hdr_len() doesn't need change because
it infers the header length as in_iv - in_hdr, which will account
for the CID for records using such.
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.
* mbedtls_ssl_context gets fields indicating whether the CID extension
should be negotiated in the next handshake, and, if yes, which CID
the user wishes the peer to use.
This information does not belong to mbedtls_ssl_handshake_params
because (a) it is configured prior to the handshake, and (b) it
applies to all subsequent handshakes.
* mbedtls_ssl_handshake_params gets fields indicating the state of CID
negotiation during the handshake. Specifically, it indicates if the
use of the CID extension has been negotiated, and if so, which CID
the peer wishes us to use for outgoing messages.
