- a comment regarding the implementation of hmac_drbg_reseed_core()
was misplaced.
- add more references to the standard, and add details on how the
comments in the code refer to various parts of the standard.
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.
According to SP800-90A, the DRBG seeding process should use a nonce
of length `security_strength / 2` bits as part of the DRBG seed. It
further notes that this nonce may be drawn from the same source of
entropy that is used for the first `security_strength` bits of the
DRBG seed. The present HMAC DRBG implementation does that, requesting
`security_strength * 3 / 2` bits of entropy from the configured entropy
source in total to form the initial part of the DRBG seed.
However, some entropy sources may have thresholds in terms of how much
entropy they can provide in a single call to their entropy gathering
function which may be exceeded by the present HMAC DRBG implementation
even if the threshold is not smaller than `security_strength` bits.
Specifically, this is the case for our own entropy module implementation
which only allows requesting at most 32 Bytes of entropy at a time
in configurations disabling SHA-512, and this leads to runtime failure
of HMAC DRBG when used with Mbed TLS' own entropy callbacks in such
configurations.
This commit fixes this by splitting the seed entropy acquisition into
two calls, one requesting `security_strength` bits first, and another
one requesting `security_strength / 2` bits for the nonce.
The use of tinyCrypt is restricted Secp256r1-only, and a check in
ssl_ciphersuite_is_match() ensures that an EC ciphersuite is chosen
only if the client advertised support for Secp256r1, too.
In a way inconsistent with the rest of the library restricting the
use of tinyCrypt to pure-ECDHE, the previous ServerKeyExchange writing
routine would use tinyCrypt also for ECDHE-PSK-based ciphersuites.
This commit fixes this.
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_parse_client_key_exchange()
has been entirely moved to one of the newly introduced subroutines
and is no longer needed. This commit removes it.
After parsing and performing key generation operations,
the server-side incoming 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 comes done after the ClientKeyExchange
has been parsed and the respective keying material has been generated,
and this commit moves it to the new postprocessing function
ssl_client_key_exchange_postprocess().
This commit moves the generation of the master secret and session keys
from the premaster secret (done in mbedtlsssl_derive_keys()) from the
previous ClientKeyExchange parsing function ssl_parse_client_key_exchange()
to the new postprocessing function ssl_client_key_exchange_postprocess().
This commit adds declarations and dummy implementations for
the restructured incoming client key exchange handling that
will replace the previous ssl_parse_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:
- Fetching: Read next message from the messaging layer
and check that it has the correct type.
The ClientKeyExchange message is never
omitted, so there is no ambiguity in what
to expect, and hence no dedicated preparation
step as for other handshake states.
- Parsing: Parse the ClientKeyExchange message and
use the information in it to derive keying
material such as the shared (EC)DHE secret.
- Postprocessing:
Compute the session keys from the available
keying material. This splits in two steps:
(1) Build the PreMasterSecret (PMS) from the
available keying material, e.g. concatenate
the (EC)DHE secret with a PSK, if used.
(2) Extract the MasterSecret and Session Keys
from the PreMasterSecret.
The subsequent commits will scatter the code from the previous
monolithic function ssl_parse_client_key_exchange() among those
dedicated functions, commenting out each part of
ssl_parse_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_parse_client_key_exchange() will be removed.
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.
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.
After the rewrite of incoming record processing to use the internal
SSL record structure mbedtls_record (which contains the data_offset
field to indicate where the IV resides), this field is no longer
necessary.
Note: This is an API break.
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.
