Extend scope of TC in ECDH-param extraction from CRT
Previously, TinyCrypt was only used for ECDHE-ECDSA/RSA ciphersuites.
This commit is a step towards using it for _all_ ciphersuites involving
ECDHE (specifically: ECDHE, ECDHE-PSK, static ECDH), extending the scope
of the use of TinyCrypt in the writing of the ClientKeyExchange message.
Extend scope of TC in ECDH-param extraction from CRT
Previously, TinyCrypt was only used for ECDHE-ECDSA/RSA ciphersuites.
This commit is a step towards using it for _all_ ciphersuites involving
ECDHE (specifically: ECDHE, ECDHE-PSK, static ECDH), extending the scope
of the use of TinyCrypt in the parsing of the ServerKeyExchange message.
mbedtls/ecp.h defines constants
MBEDTLS_ECP_PF_UNCOMPRESSED
MBEDTLS_ECP_PF_COMPRESSED
MBEDTLS_ECP_TLS_NAMED_CURVE
which regard the encoding of elliptic curves and curve point formats in TLS.
As such, they should be defined in the SSL namespace. Asides, this will help
replacing the legacy ECC crypto by alternative ECC implementations.
This function is called on client-only once the ciphersuite has
been chosen and it it is known which digest the client will need
for the handshake transcript throughout the handshake, and causes
all other unneeded handshake transcripts to be discontinued.
(On the server, we cannot call this function because we don't know
which hash the client will those in its CertificateVerify message).
However, the benefit of this call is marginal, since transcript hash
computation is negligible compared to asymmetric crypto, and moreover
the handshake transcript contexts for the unused digests are still
stored in the SSL handshake parameter structure and not freed until
the end of the handshake.
Finally, if we're running on a _really_ constrained client, there
will be only one hash function enabled anyway, and in this case
the checksum optimization has no effect.
This commit therefore removes checksum optimization altogether,
saving some code on constrained systems.
Now function mbedtls_ssl_set_hostname is compile-time configurable
in config.h with define MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION.
This affects to many x509 API's. See config.h for details.
Previously, MBEDTLS_KEY_EXCHANGE_ECDH[E]_XXX_ENABLED would imply
that MBEDTLS_ECDH_C is set, but with the introduction of tinyCrypt
as an alternative ECDH implementation, this is no longer the case.
Eventually, all HS parsing/writing functions should take an arbitrary buffer +
length pair as their argument, and return MBEDTLS_ERR_SSL_BUFFER_TOO_SMALL if
the provided buffer is too short. So far, we've only made a first step by
allowing to pass an arbitrary buffer, but don't yet add bounds checks
throughout. While deliberate for now, this must be clearly documented.
This makes grepping the functions more difficult, and also leads to compilation failures
when trying to build the library from a single source file (which might be useful for
code-size reasons).
IAR doesn't like `((void) var);` as a means to indicate an unused
variable if that variable hasn't been initialized before. Make it
happy by initializing the variable before.
ssl_server_key_exchange_parse() is compiled even if there's no ciphersuite
enabled which uses it (for example, that's the case in RSA-only builds).
The rationale for that is to avoid cluttering the code with numerous
compile-time guards. A consequence, however, is the top of
ssl_server_key_exchange_parse() contains declarations for variables
which are never put to use, and rightfully leading to compiler warnings.
This commit silences these warnings by putting `((void) VAR);` statements
in the branch which detects if we ever happen to call the function in an
unexpected ciphersuite.
In the PSK and RSA-PSK ciphersuites, the ServerKeyExchange message
MAY be skipped. This commit moves the code-path peeking at the
incoming message to decide whether it's probably a ServerKeyExchange
to the new coordination function ssl_server_key_exchange_coordinate().
This commit moves the code checking whether a SrvKeyExchange message
is expected or not to the new function ssl_srv_key_exchange_coordinate().
Note that the potential static DH extraction is done prior to the
coordination step.
This code moves the code-path that extracts static DH parameters
from the server's CRT (if applicable) to the new function
ssl_server_key_exchange_prepare().
This commit adds declarations and dummy implementations for
the restructured incoming server key exchange handling that
will replace the previous ssl_parse_server_key_exchange().
The entry point for the SrvKeyExchange handling that is called
from the handshake state machine is
`ssl_process_server_key_exchange()`,
splitting the processing into the following steps:
- Preparation: For a static DH key exchange, extract
DH parameters from the server's CRT.
- Coordination: Check if a SrvKeyExchange message is expected
(e.g., it isn't for a RSA-based key exchange)
- Reading: Fetch and check content and handshake type
of incoming message.
- Parsing: Parse and store the ServerKeyExchange message.
- Postprocessing: Update handstate state machine.
The subsequent commits will scatter the code from the previous
monolithic function ssl_parse_server_key_exchange() among those
dedicated functions, commenting out each part of
ssl_parse_server_key_exchange() that has already been dealt with.
This gradual progression is meant to ease reviewing. Once all
code has been moved and all changes explained,
ssl_parse_server_key_exchange() will be removed.
The postprocessing code for the server-side incoming client key
exchange and the client-side outgoing client key exchange both
contain the same code-paths for building the premaster secret
depending on the chosen ciphersuite (e.g., for ECDHE-PSK,
concatenating the ECDHE secret with the chosen PSK).
This commit moves this common code to ssl_tls.c, allowing
client- and server-side to share it.
The code from the previous function ssl_write_client_key_exchange()
has been entirely moved to one of the newly introduced subroutines
and is no longer needed. This commit removes it.
This commit moves the code responsible for
(a) generating the client's private and public (EC)DHE keys
(b) writing it to the message buffer
to the new writing function ssl_client_key_exchange_write().
As mentioned in the previous commit message, (a) and (b) are
currently inseparable at the (EC)DHE API level, which is why
(a) can't be moved to the preparation step.
For RSA or RSA-PSK exchanges, the PMS contains 46 random bytes
picked by the client. These bytes are generated prior to the
writing of the ClientKeyExchange message.
This commit splits the previous function ssl_write_encrypted_pms() into
PPMS-GEN: ssl_rsa_generate_partial_pms()
PPMS-ENC: ssl_rsa_encrypt_partial_pms().
The prefix 'partial' is meant to emphasize that the generation of the PMS
is not always entirely done by these functions: For RSA-PSK e.g., the
PSK still needs to be added.
The two calls of ssl_write_encrypted_pms() in
ssl_write_client_key_exchange() will split in calls of the functions
PPMS-GEN and PPMS-ENC each, with PPMS-GEN being moved to the new
preparation function ssl_client_key_exchange_prepare() in this commit,
and PPMS-ENC being moved to ssl_client_key_exchange_write() in the
next commit.
After and performing key generation operations,
the client-side outgoing ClientKeyExchange handling includes
code-paths to assembly the PreMasterSecret (PMS) from the
available keying material, the exact assembly procedure
depending on which ciphersuite is in use. E.g., in an
(EC)DHE-PSK ciphersuite, the (EC)DHE secret would be concatenated
with the PSK to form the PMS.
This assembly of the PMS logically can be done after the ClientKeyExchange
has been written and the respective keying material has been generated,
and this commit moves it to the new postprocessing function
ssl_client_key_exchange_postprocess().
Ideally, the PMS assembly could be done prior to writing the
ClientKeyExchange message, but the (EC)DHE API does currently
not allow splitting secret-generation and secret-export; as
long as that's the case, we to generation and exporting in the
message writing function, forcing PMS assembly to be done in
the postprocessing.
This commit adds declarations and dummy implementations for
the restructured outgoing client key exchange handling that
will replace the previous ssl_write_client_key_exchange().
The entry point for the CliKeyExchange handling that is called
from the handshake state machine is
`ssl_process_client_key_exchange()`,
splitting the processing into the following steps:
- Preparation
Compute the keying material to be sent.
* For (EC)DH: Pick parameters and compute PMS.
* For ECJPAKE: Run round 2
* For RSA: Encrypt PMS
- Writing: Prepare the writing of a new messae.
- Postprocessing: Update handstate state machine.
The subsequent commits will scatter the code from the previous
monolithic function ssl_write_client_key_exchange() among those
dedicated functions, commenting out each part of
ssl_write_client_key_exchange() that has already been dealt with.
This gradual progression is meant to ease reviewing. Once all
code has been moved and all changes explained,
ssl_write_client_key_exchange() will be removed.
