Part of the record encryption/decryption tests is to gradually
increase the space available at the front and/or at the back of
a record and observe when encryption starts to succeed. If exactly
one of the two parameters is varied at a time, the expectation is
that encryption will continue to succeed once it has started
succeeding (that's not true if both pre- and post-space are varied
at the same time).
Moreover, previously the test would take turns when choosing which
transform should be used for encryption, and which for decryption.
With the introduction of the CID feaature, this switching of transforms
doesn't align with the expectation of eventual success of the encryption,
since the overhead of encryption might be different for the parties,
because both parties may use different CIDs for their outgoing records.
This commit modifies the tests to not take turns between transforms,
but to always use the same transforms for encryption and decryption
during a single round of the test.
This commit modifies ssl_decrypt_buf() and ssl_encrypt_buf()
to include the CID into authentication data during record
protection.
It does not yet implement the new DTLSInnerPlaintext format
from https://tools.ietf.org/html/draft-ietf-tls-dtls-connection-id-04
This commit adds a static array `cid` to the internal structure
`mbedtls_record` representing encrypted and decrypted TLS records.
The expected evolution of state of this field is as follows:
- When handling an incoming record, the caller of `mbedtls_decrypt_buf()`
has to make sure the CID array field in `mbedtls_record` has been
properly set. Concretely, it will be copied from the CID from the record
header during record parsing.
- During decryption in `mbedtls_decrypt_buf()`, the transforms
incoming CID is compared to the CID in the `mbedtls_record`
structure representing the record to be decrypted.
- For an outgoing TLS record, the caller of `mbedtls_encrypt_buf()`
clears the CID in the `mbedtls_record` structure.
- During encryption in `mbedtls_encrypt_buf()`, the CID field in
`mbedtls_record` will be copied from the out-CID in the transform.
These will be copied from the CID fields in mbedtls_ssl_handshake_params
(outgoing CID) and mbedtls_ssl_context (incoming CID) when the transformation
is set up at the end of the handshake.
* 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.
