* origin/pr/2854:
Shorter version of mbedtls_ssl_send_fatal_handshake_failure
Resolve#2801 - remove repetitive assignment to ssl->in_msg (the first value was never used)
Resolve#2800 - move declaration to avoid unused variable warning in case MBEDTLS_SSL_PROTO_DTLS was undefined
Resolve#2717 - remove erroneous sizeof (the operator was applied to constant integer number)
Record checking fails if mbedtls_ssl_check_record() is called with
external buffer. Received record sequence number is available in the
incoming record but it is not available in the ssl contexts `in_ctr`-
variable that is used when decoding the sequence number.
To fix the problem, temporarily update ssl context `in_ctr` to
point to the received record header and restore value later.
In TLS, the master secret is always a key. But EAP-TLS uses the TLS
PRF to derive an IV with an empty string for the "secret" input. The
code always stored the secret into a key slot before calling the TLS
PRF, but this doesn't work when the secret is empty, since PSA Crypto
no longer supports empty keys. Add a special case for an empty secret.
The SSL context maintains a set of 'out pointers' indicating the
address at which to write the header fields of the next outgoing
record. Some of these addresses have a static offset from the
beginning of the record header, while other offsets can vary
depending on the active record encryption mechanism: For example,
if an explicit IV is in use, there's an offset between the end
of the record header and the beginning of the encrypted data to
allow the explicit IV to be placed in between; also, if the DTLS
Connection ID (CID) feature is in use, the CID is part of the
record header, shifting all subsequent information (length, IV, data)
to the back.
When setting up an SSL context, the out pointers are initialized
according to the identity transform + no CID, and it is important
to keep them up to date whenever the record encryption mechanism
changes, which is done by the helper function ssl_update_out_pointers().
During context deserialization, updating the out pointers according
to the deserialized record transform went missing, leaving the out
pointers the initial state. When attemping to encrypt a record in
this state, this lead to failure if either a CID or an explicit IV
was in use. This wasn't caught in the tests by the bad luck that
they didn't use CID, _and_ used the default ciphersuite based on
ChaChaPoly, which doesn't have an explicit IV. Changing either of
this would have made the existing tests fail.
This commit fixes the bug by adding a call to ssl_update_out_pointers()
to ssl_context_load() implementing context deserialization.
Extending test coverage is left for a separate commit.
Breaking into a series of statements makes things easier when stepping through
the code in a debugger.
Previous comments we stating the opposite or what the code tested for (what we
want vs what we're erroring out on) which was confusing.
Also expand a bit on the reasons for these restrictions.
Modelled after the config-checking header from session s11n.
The list of relevant config flags was established by manually checking the
fields serialized in the format, and which config.h flags they depend on.
This probably deserves double-checking by reviewers.
Since the type of cid_len is unsigned but shorter than int, it gets
"promoted" to int (which is also the type of the result), unless we make the
other operand an unsigned int which then forces the expression to unsigned int
as well.
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.
This mainly follows the design document (saving all fields marked "saved" in
the main structure and the transform sub-structure) with two exceptions:
- things related to renegotiation are excluded here (there weren't quite in
the design document as the possibility of allowing renegotiation was still
on the table, which is no longer is) - also, ssl.secure_renegotiation (which
is not guarded by MBEDTLS_SSL_RENEGOTIATION because it's used in initial
handshakes even with renegotiation disabled) is still excluded, as we don't
need it after the handshake.
- things related to Connection ID are added, as they weren't present at the
time the design document was written.
The exact format of the header (value of the bitflag indicating compile-time
options, whether and how to merge it with the serialized session header) will
be determined 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.
This commit makes use of the added space in the session header to
encode the state of those parts of the compile-time configuration
which influence the structure of the serialized session in the
present version of Mbed TLS. Specifically, these are
- the options which influence the presence/omission of fields
from mbedtls_ssl_session (which is currently shallow-copied
into the serialized session)
- the setting of MBEDTLS_X509_CRT_PARSE_C, which determines whether
the serialized session contains a CRT-length + CRT-value pair after
the shallow-copied mbedtls_ssl_session instance.
- the setting of MBEDTLS_SSL_SESSION_TICKETS, which determines whether
the serialized session contains a session ticket.
This commit adds space for two bytes in the header of serizlied
SSL sessions which can be used to determine the structure of the
remaining serialized session in the respective version of Mbed TLS.
Specifically, if parts of the session depend on whether specific
compile-time options are set or not, the setting of these options
can be encoded in the added space.
This commit doesn't yet make use of the fields.
The format of serialized SSL sessions depends on the version and the
configuration of Mbed TLS; attempts to restore sessions established
in different versions and/or configurations lead to undefined behaviour.
This commit adds an 3-byte version header to the serialized session
generated and cleanly fails ticket parsing in case a session from a
non-matching version of Mbed TLS is presented.
This bug was present since cert digest had been introduced, which highlights
the need for testing.
While at it, fix a bug in the comment explaining the format - this was
introduced by me copy-pasting to hastily from current baremetal, that has a
different format (see next PR in the series for the same in development).
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 uncovered a bug that led to a double-free (in practice, in general could
be free() on any invalid value): initially the session structure is loaded
with `memcpy()` which copies the previous values of pointers peer_cert and
ticket to heap-allocated buffers (or any other value if the input is
attacker-controlled). Now if we exit before we got a chance to replace those
invalid values with valid ones (for example because the input buffer is too
small, or because the second malloc() failed), then the next call to
session_free() is going to call free() on invalid pointers.
This bug is fixed in this commit by always setting the pointers to NULL right
after they've been read from the serialised state, so that the invalid values
can never be used.
(An alternative would be to NULL-ify them when writing, which was rejected
mostly because we need to do it when reading anyway (as the consequences of
free(invalid) are too severe to take any risk), so doing it when writing as
well is redundant and a waste of code size.)
Also, while thinking about what happens in case of errors, it became apparent
to me that it was bad practice to leave the session structure in an
half-initialised state and rely on the caller to call session_free(), so this
commit also ensures we always clear the structure when loading failed.
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.
On client side, this is required for the main use case where of serialising a
session for later resumption, in case tickets are used.
On server side, this doesn't change much as ticket_len will always be 0.
This unblocks testing the functions by using them in ssl_client2, which will
be done in the next commit.
This finishes making these functions public. Next step is to get them tested,
but there's currently a blocker for that, see next commit (and the commit
after it for tests).
While 'session hash' is currently unique, so suitable to prove that the
intended code path has been taken, it's a generic enough phrase that in the
future we might add other debug messages containing it in completely unrelated
code paths. In order to future-proof the accuracy of the test, let's use a
more specific string.
The previous comment used "TLS" as a shortcut for "TLS 1.0/1.1" which was
confusing. This partially reflected the names of the calc_verify/finished that
go ssl, tls (for 1.0/1.1) tls_shaxxx (for 1.2), but still it's clearer to be
explicit in the comment - and perhaps in the long term the function names
could be clarified instead.
When using this function to deserialize, it's not a problem to have a session
structure as input as we'll have one around anyway (most probably freshly
deserialised).
However for tests it's convenient to be able to build a transform without
having a session structure around.
Also, removing this structure from parameters makes the function signature
more uniform, the only exception left being the ssl param at the end that's
hard to avoid for now.
Configs with no DEBUG_C are used for example in test-ref-configs.pl, which also
runs parts of compat.sh or ssl-opt.sh on them, so the added 'ssl = NULL'
statements will be exercised in those tests at least.
Make it more explicit what's used. Unfortunately, we still need ssl as a
parameter for debugging, and because calc_verify wants it as a parameter (for
all TLS versions except SSL3 it would actually only need handshake, but SSL3
also accesses session_negotiate).
It's also because of calc_verify that we can't make it const yet, but see next
commit.
* origin/pr/2790: (40 commits)
Fix possibly-lossy conversion warning from MSVC
Reintroduce length 0 check for records
Don't use memcpy() for 2-byte copy operation
Remove integer parsing macro
Fix alignment in record header parsing routine
Don't disallow 'record from another epoch' log msg in proxy ref test
Make sure 'record from another epoch' is displayed for next epoch
Implement record checking API
Mark ssl_parse_record_header() as `const` in SSL context
Make mbedtls_ssl_in_hdr_len() CID-unaware
Remove duplicate setting of ssl->in_msgtype and ssl->in_msglen
Move update of in_xxx fields in ssl_get_next_record()
Move update of in_xxx fields outside of ssl_prepare_record_content()
Reduce dependency of ssl_prepare_record_content() on in_xxx fields
Move ssl_update_in_pointers() to after record hdr parsing
Mark DTLS replay check as `const` on the SSL context
Move updating the internal rec ptrs to outside of rec hdr parsing
Mark ssl_decrypt_buf() as `const in the input SSL context
Adapt ssl_prepare_record_content() to use SSL record structure
Use record length from record structure when fetching content in TLS
...
* 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 commit implements the record checking API
mbedtls_ssl_check_record()
on top of the restructured incoming record stack.
Specifically, it makes use of the fact that the core processing routines
ssl_parse_record_header()
mbedtls_ssl_decrypt_buf()
now operate on instances of the SSL record structure mbedtls_record
instead of the previous mbedtls_ssl_context::in_xxx fields.
ssl_get_next_record() updates the legacy in_xxx fields in two places,
once before record decryption and once after. Now that record decryption
doesn't use or affect the in_xxx fields anymore, setting up the these
legacy fields can entirely be moved to the end of ssl_get_next_record(),
which is what this comit does.
This commit solely moves existing code, but doesn't yet simplify the
now partially redundant settings of the in_xxx fields. This will be
done in a separate commit.
Multiple record attributes such as content type and payload length
may change during record decryption, and the legacy in_xxx fields
in the SSL context therefore need to be updated after the record
decryption routine ssl_decrypt_buf() has been called.
After the previous commit has made ssl_prepare_record_content()
independent of the in_xxx fields, setting them can be moved
outside of ssl_prepare_record_content(), which is what this
commit does.
Previously, ssl_update_in_pointers() ensured that the in_xxx pointers
in the SSL context are set to their default state so that the record
header parsing function ssl_parse_record_header() could make use of them.
By now, the latter is independent of these pointers, so they don't need
to be setup before calling ssl_parse_record_header() anymore.
However, other parts of the messaging stack might still depend on it
(to be studied), and hence this commit does not yet reomve
ssl_update_in_pointers() entirely.
The stack maintains pointers mbedtls_ssl_context::in_xxx pointing to
various parts of the [D]TLS record header. Originally, these fields
were determined and set in ssl_parse_record_header(). By now,
ssl_parse_record_header() has been modularized to setup an instance
of the internal SSL record structure mbedtls_record, and to derive
the old in_xxx fields from that.
This commit takes a further step towards removing the in_xxx fields
by deriving them from the established record structure _outside_ of
ssl_parse_record_header() after the latter has succeeded.
One exception is the handling of possible client reconnects,
which happens in the case then ssl_parse_record_header() returns
MBEDTLS_ERR_SSL_UNEXPECTED_RECORD; since ssl_check_client_reconnect()
so far uses the in_xxx fields, they need to be derived from the
record structure beforehand.
This commit makes a first step towards modularizing the incoming record
processing by having it operate on instances of the structure mbedtls_record
representing SSL records.
So far, only record encryption/decryption operate in terms of record
instances, but the rest of the parsing doesn't. In particular,
ssl_parse_record_header() operates directly on the fixed input buffer,
setting the various ssl->in_xxx pointers and fields, and only directly
before/after calling ssl_decrypt_buf() these fields a converted to/from
mbedtls_record instances.
This commit does not yet remove the ssl->in_xxx fields, but makes a step
towards extending the lifetime of mbedtls_record structure representing
incoming records, by modifying ssl_parse_record_header() to setup an
instance of mbedtls_record, and setting the ssl->in_xxx fields from that
instance. The instance so-constructed isn't used further so far, and in
particular it is not yet consolidated with the instance set up for use
in ssl_decrypt_record(). That's for a later commit.
Previously, ssl_parse_record_header() did not check whether the current
datagram is large enough to hold a record of the advertised size. This
could lead to records being silently skipped over or backed up on the
basis of an invalid record length. Concretely, the following would happen:
1) In the case of a record from an old epoch, the record would be
'skipped over' by setting next_record_offset according to the advertised
but non-validated length, and only in the subsequent mbedtls_ssl_fetch_input()
it would be noticed in an assertion failure if the record length is too
large for the current incoming datagram.
While not critical, this is fragile, and also contrary to the intend
that MBEDTLS_ERR_SSL_INTERNAL_ERROR should never be trigger-able by
external input.
2) In the case of a future record being buffered, it might be that we
backup a record before we have validated its length, hence copying
parts of the input buffer that don't belong to the current record.
This is a bug, and it's by luck that it doesn't seem to have critical
consequences.
This commit fixes this by modifying ssl_parse_record_header() to check that
the current incoming datagram is large enough to hold a record of the
advertised length, returning MBEDTLS_ERR_SSL_INVALID_RECORD otherwise.
We don't send alerts on other instances of ill-formed records,
so why should we do it here? If we want to keep it, the alerts
should rather be sent ssl_get_next_record().
As explained in the previous commit, if mbedtls_ssl_fetch_input()
is called multiple times, all but the first call are equivalent to
bounds checks in the incoming datagram.
In DTLS, if mbedtls_ssl_fetch_input() is called multiple times without
resetting the input buffer in between, the non-initial calls are functionally
equivalent to mere bounds checks ensuring that the incoming datagram is
large enough to hold the requested data. In the interest of code-size
and modularity (removing a call to a non-const function which is logically
const in this instance), this commit replaces such a call to
mbedtls_ssl_fetch_input() by an explicit bounds check in
ssl_parse_record_header().
Previously, `ssl_handle_possible_reconnect()` was part of
`ssl_parse_record_header()`, which was required to return a non-zero error
code to indicate a record which should not be further processed because it
was invalid, unexpected, duplicate, .... In this case, some error codes
would lead to some actions to be taken, e.g. `MBEDTLS_ERR_SSL_EARLY_MESSAGE`
to potential buffering of the record, but eventually, the record would be
dropped regardless of the precise value of the error code. The error code
`MBEDTLS_ERR_SSL_HELLO_VERIFY_REQUIRED` returned from
`ssl_handle_possible_reconnect()` did not receive any special treatment and
lead to silent dopping of the record - in particular, it was never returned
to the user.
In the new logic this commit introduces, `ssl_handle_possible_reconnect()` is
part of `ssl_check_client_reconnect()` which is triggered _after_
`ssl_parse_record_header()` found an unexpected record, which is already in
the code-path eventually dropping the record; we want to leave this code-path
only if a valid cookie has been found and we want to reset, but do nothing
otherwise. That's why `ssl_handle_possible_reconnect()` now returns `0` unless
a valid cookie has been found or a fatal error occurred.
Availability of sufficient incoming data should be checked when
it is needed, which is in mbedtls_ssl_fetch_input(), and this
function has the necessary bounds checks in place.
The check is in terms of the internal input buffer length and is
hence likely to be originally intended to protect against overflow
of the input buffer when fetching data from the underlying
transport in mbedtls_ssl_fetch_input(). For locality of reasoning,
it's better to perform such a check close to where it's needed,
and in fact, mbedtls_ssl_fetch_input() _does_ contain an equivalent
bounds check, too, rendering the bounds check in question redundant.
mbedtls_ssl_decrypt_buf() asserts that the passed transform is not NULL,
but the function is only invoked in a single place, and this invocation
is clearly visible to be within a branch ensuring that the incoming
transform isn't NULL. Remove the assertion for the benefit of code-size.
The previous code performed architectural maximum record length checks
both before and after record decryption. Since MBEDTLS_SSL_IN_CONTENT_LEN
bounds the maximum length of the record plaintext, it suffices to check
only once after (potential) decryption.
This must not be confused with the internal check that the record
length is small enough to make the record fit into the internal input
buffer; this is done in mbedtls_ssl_fetch_input().
When MBEDTLS_SSL_ENCRYPT_THEN_MAC is enabled, but not
MBEDTLS_SSL_SOME_MODES_USE_MAC, mbedtls_ssl_derive_keys() and
build_transforms() will attempt to use a non-existent `encrypt_then_mac`
field in the ssl_transform.
Compile [ 93.7%]: ssl_tls.c
[Error] ssl_tls.c@865,14: 'mbedtls_ssl_transform {aka struct mbedtls_ssl_transform}' ha
s no member named 'encrypt_then_mac'
[ERROR] ./mbed-os/features/mbedtls/src/ssl_tls.c: In function 'mbedtls_ssl_derive_keys'
:
./mbed-os/features/mbedtls/src/ssl_tls.c:865:14: error: 'mbedtls_ssl_transform {aka str
uct mbedtls_ssl_transform}' has no member named 'encrypt_then_mac'
transform->encrypt_then_mac = session->encrypt_then_mac;
^~
Change mbedtls_ssl_derive_keys() and build_transforms() to only access
`encrypt_then_mac` if `encrypt_then_mac` is actually present.
Add a regression test to detect when we have regressions with
configurations that do not include any MAC ciphersuites.
Fixes d56ed2491b ("Reduce size of `ssl_transform` if no MAC ciphersuite is enabled")
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.
There are two options:
1. Don't set it, and don't use it during record protection,
guarding the respective paths by a check whether TLS or
DTLS is used.
2. Set it to the default value even for TLS, and avoid the
protocol-dependent branch during record protection.
This commit picks option 2.
This commit changes the stack's behaviour when facing a record
with a non-matching CID. Previously, the stack failed in this
case, while now we silently skip over the current record.
Previously, ssl_get_next_record() would fetch 13 Bytes for the
record header and hand over to ssl_parse_record_header() to parse
and validate these. With the introduction of CID-based records, the
record length is not known in advance, and parsing and validating
must happen at the same time. ssl_parse_record_header() is therefore
rewritten in the following way:
1. Fetch and validate record content type and version.
2. If the record content type indicates a record including a CID,
adjust the record header pointers accordingly; here, we use the
statically configured length of incoming CIDs, avoiding any
elaborate CID parsing mechanism or dependency on the record
epoch, as explained in the previous commit.
3. Fetch the rest of the record header (note: this doesn't actually
fetch anything, but makes sure that the datagram fetched in the
earlier call to ssl_fetch_input() contains enough data).
4. Parse and validate the rest of the record header as before.
This commit modifies the code surrounding the invocations of
ssl_decrypt_buf() and ssl_encrypt_buf() to deal with a change
of record content type during CID-based record encryption/decryption.
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.
Using the Connection ID extension increases the maximum record expansion
because
- the real record content type is added to the plaintext
- the plaintext may be padded with an arbitrary number of
zero bytes, in order to prevent leakage of information
through package length analysis. Currently, we always
pad the plaintext in a minimal way so that its length
is a multiple of 16 Bytes.
This commit adapts the various parts of the library to account
for that additional source of record expansion.
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.
