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.
* origin/pr/2410:
Update change log
Document the default value for the maximum fragment length
Improve clarity of mbedtls_ssl_conf_max_frag_len documentation
Reword ssl_conf_max_frag_len documentation
Fix typos and miswording in the mbedtls_ssl_conf_max_frag_len documentation comment
Reword ssl_conf_max_frag_len documentation to clarify its necessity
Add an additional function `mbedtls_ssl_export_keys_ext_t()`
for exporting key, that adds additional information such as
the used `tls_prf` and the random bytes.
Resolve conflicts by performing the following:
- Ensure calls to mbedtls_x509_crt_verify_* are made with callbacks
* origin/pr/2539:
Make CRT callback tests more robust
Rename constant in client2.c
Fix typo
Add test for configuration specific CRT callback
Fix doxygen documentation of mbedtls_ssl_set_verify()
Add test exercising context-specific CRT callback to ssl-opt.sh
Add cmd to use context-specific CRT callback in ssl_client2
Implement context-specific verification callbacks
Add context-specific CRT verification callbacks
Improve documentation of mbedtls_ssl_conf_verify()
Additional changes to temporarily enable running tests:
ssl_srv.c and test_suite_ecdh use mbedtls_ecp_group_load instead of
mbedtls_ecdh_setup
test_suite_ctr_drbg uses mbedtls_ctr_drbg_update instead of
mbedtls_ctr_drbg_update_ret
This commit adds two public API functions
mbedtls_ssl_conf_psk_opaque()
mbedtls_ssl_set_hs_psk_opaque()
which allow to configure the use of opaque, PSA-maintained PSKs
at configuration time or run time.
The previous comment in ecp.h that only functions that take a "restart
context" argument can restart was wrong due to ECDH and SSL functions.
Changing that criterion to "document says if can return IN PROGRESS".
This requires updating the documentation of the SSL functions to mention this
explicitly, but it's something we really ought to do anyway, a bit
embarrassing that this wasn't done already - callers need to know what
`MBEDTLS_ERR_SSL_xxx` error codes to special-case. Note that the documentation
of the relevant functions was in a suboptimal state, so it was improved in the
process - it could use some more improvement, but only the changes that helped
cleanly insert the info about the IN_PROGRESS part were done here.
Also, while updating the ecp.h comment, I noticed several functions in the
ECDH module were wrongfully documented as restartable, which is probably a
left-over from the days before `mbedtls_ecdh_enable_restart()` was introduced.
Fixing that as well, to make the criterion used in ecp.h correct.
