`mbedtls_ssl_handshake_params::extended_ms` holds the state of the
ExtendedMasterSecret extension in the current handshake. Initially
set to 'disabled' for both client and server,
- the client sets it to 'enabled' as soon as it finds the ExtendedMS
extension in the `ServerHello` and it has advertised that extension
in its ClientHello,
- the server sets it to 'enabled' as soon as it finds the ExtendedMS
extension in the `ClientHello` and is willing to advertise is in its
`ServerHello`.
This commit slightly restructures this logic in prepraration for the
removal of `mbedtls_ssl_handshake_params::extended_ms` in case both
the use and the enforcement of the ExtendedMasterSecret extension have
been fixed at compile-time. Namely, in this case there is no need for
the `extended_ms` field in the handshake structure, as the ExtendedMS
must be in use if the handshake progresses beyond the Hello stage.
Paving the way for the removal of mbedtls_ssl_handshake_params::extended_ms
this commit introduces a temporary variable tracking the presence of the
ExtendedMS extension in the ClientHello/ServerHello messages, leaving
the derivation of `extended_ms` (and potential failure) to the end of
the parsing routine.
This commit is the first in a series demonstrating how code-size
can be reduced by hardcoding parts of the SSL configuration at
compile-time, focusing on the example of the configuration of
the ExtendedMasterSecret extension.
The flexibility of an SSL configuration defined a runtime vs.
compile-time is necessary for the use of Mbed TLS as a
dynamically linked library, but is undesirable in constrained
environments because it introduces the following overhead:
- Definition of SSL configuration API (code-size overhead)
(and on the application-side: The API needs to be called)
- Additional fields in the SSL configuration (RAM overhead,
and potentially code-size overhead if structures grow
beyond immediate-offset bounds).
- Dereferencing is needed to obtain configuration settings.
- Code contains branches and potentially additional structure
fields to distinguish between different configurations.
Considering the example of the ExtendedMasterSecret extension,
this instantiates as follows:
- mbedtls_ssl_conf_extended_master_secret() and
mbedtls_ssl_conf_extended_master_secret_enforced()
are introduced to configure the ExtendedMasterSecret extension.
- mbedtls_ssl_config contains bitflags `extended_ms` and
`enforce_extended_master_secret` reflecting the runtime
configuration of the ExtendedMasterSecret extension.
- Whenever we need to access these fields, we need a chain
of dereferences `ssl->conf->extended_ms`.
- Determining whether Client/Server should write the
ExtendedMasterSecret extension needs a branch
depending on `extended_ms`, and the state of the
ExtendedMasterSecret negotiation needs to be stored in a new
handshake-local variable mbedtls_ssl_handshake_params::extended_ms.
Finally (that's the point of ExtendedMasterSecret) key derivation
depends on this handshake-local state of ExtendedMasterSecret.
All this is unnecessary if it is known at compile-time that the
ExtendedMasterSecret extension is used and enforced:
- No API calls are necessary because the configuration is fixed
at compile-time.
- No SSL config fields are necessary because there are corresponding
compile-time constants instead.
- Accordingly, no dereferences for field accesses are necessary,
and these accesses can instead be replaced by the corresponding
compile-time constants.
- Branches can be eliminated at compile-time because the compiler
knows the configuration. Also, specifically for the ExtendedMasterSecret
extension, the field `extended_ms` in the handshake structure
is unnecessary, because we can fail immediately during the Hello-
stage of the handshake if the ExtendedMasterSecret extension
is not negotiated; accordingly, the non-ExtendedMS code-path
can be eliminated from the key derivation logic.
A way needs to be found to allow fixing parts of the SSL configuration
at compile-time which removes this overhead in case it is used,
while at the same time maintaining readability and backwards
compatibility.
This commit proposes the following approach:
From the user perspective, for aspect of the SSL configuration
mbedtls_ssl_config that should be configurable at compile-time,
introduce a compile-time option MBEDTLS_SSL_CONF_FIELD_NAME.
If this option is not defined, the field is kept and configurable
at runtime as usual. If the option is defined, the field is logically
forced to the value of the option at compile time.
Internally, read-access to fields in the SSL configuration which are
configurable at compile-time gets replaced by new `static inline` getter
functions which evaluate to the corresponding field access or to the
constant MBEDTLS_SSL_CONF_FIELD_NAME, depending on whether the latter
is defined or not.
Write-access to fields which are configurable at compile-time needs
to be removed: Specifically, the corresponding API itself either
needs to be removed or replaced by a stub function without effect.
This commit takes the latter approach, which has the benefit of
not requiring any change on the example applications, but introducing
the risk of mismatching API calls and compile-time configuration,
in case a user doesn't correctly keep track of which parts of the
configuration have been fixed at compile-time, and which haven't.
Write-access for the purpose of setting defaults is simply omitted.
If `MBEDTLS_SSL_KEEP_PEER_CERTIFICATE` is not set, `mbedtls_ssl_session`
contains the digest of the peer's certificate for the sole purpose of
detecting a CRT change on renegotiation. Hence, it is not needed if
renegotiation is disabled.
This commit removes the `peer_cert_digest` fields (and friends) from
`mbedtls_ssl_session` if
`!MBEDTLS_SSL_KEEP_PEER_CERTIFICATE + !MBEDTLS_SSL_RENEGOTIATION`,
which is a sensible configuration for constrained devices.
Apart from straightforward replacements of
`if !defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE)`
by
`if !defined(MBEDTLS_SSL_KEEP_PEER_CERTIFICATE) && \
defined(MBEDTLS_SSL_RENEGOTIATION)`,
there's one notable change: On the server-side, the CertificateVerify
parsing function is a no-op if the client hasn't sent a certificate.
So far, this was determined by either looking at the peer CRT or the
peer CRT digest in the SSL session structure (depending on the setting
of `MBEDTLS_SSL_KEEP_PEER_CERTIFICATE`), which now no longer works if
`MBEDTLS_SSL_KEEP_PEER_CERTIFICATE` is unset. Instead, this function
now checks whether the temporary copy of the peer's public key within
the handshake structure is initialized or not (which is also a
beneficial simplification in its own right, because the pubkey is
all the function needs anyway).
The server expects a CertificateVerify message only if it has
previously received a Certificate from the client.
So far, this was detected by looking at the `peer_cert` field
in the current session. Preparing to remove the latter, this
commit changes this to instead determine the presence of a peer
certificate by checking the new `peer_cert_digest` pointer.
We must dispatch between the peer's public key stored as part of
the peer's CRT in the current session structure (situation until
now, and future behaviour if MBEDTLS_SSL_KEEP_PEER_CERTIFICATE is
enabled), and the sole public key stored in the handshake structure
(new, if MBEDTLS_SSL_KEEP_PEER_CERTIFICATE is disabled).
This commit simplifies the client-side code for outgoing CertificateVerify
messages, and server-side code for outgoing CertificateRequest messages and
incoming CertificateVerify messages, through the use of the macro
`MBEDTLS_KEY_EXCHANGE__CERT_REQ_ALLOWED__ENABLED`
indicating whether a ciphersuite allowing CertificateRequest messages
is enabled in the configuration, as well as the helper function
`mbedtls_ssl_ciphersuite_cert_req_allowed()`
indicating whether a particular ciphersuite allows CertificateRequest
messages.
These were already used in the client-side code to simplify the
parsing functions for CertificateRequest messages.
This commit handles occurrences of case 2 and 3 in the following list:
1. Some DTLS-specific code with no TLS-specific code (most frequent)
2. Some specific code for each protocol
3. Some TLS-specific code with no DTLS-specific code (least frequent)
Case 3 previously had a weird structure in that the TLS-specific code was
always present, but the if structure was conditional on DTLS being enabled.
This is changed by this commit to a more logical structure where both the code
and the test are conditional on TLS being enabled.
Case 2 doesn't require any change in the code structure in general. However,
there is one occurrence where the if/else structure is simplified to assigning
the result of a boolean operation, and one occurrence where I also noticed a
useless use of `ssl_ep_len()` in a TLS-specific branch, that I turned to the
constant 0 as it makes more sense.
Case 1 will be handled in the next commit, as it can easily be handled in an
automated way - only cases 2 and 3 (sometimes) required manual intervention.
The list of occurrences for cases 2 and 3 was established manually by looking
for occurrences of '= MBEDTLS_SSL_TRANSPORT_' in the code and manually
checking if there was a TLS-specific branch.
New sizes (see previous commit for the measuring script):
```
both
text data bss dec hex filename
1820 0 4 1824 720 debug.o (ex library/libmbedtls.a)
0 0 0 0 0 net_sockets.o (ex library/libmbedtls.a)
548 0 0 548 224 ssl_cache.o (ex library/libmbedtls.a)
11155 0 596 11751 2de7 ssl_ciphersuites.o (ex library/libmbedtls.a)
17156 0 0 17156 4304 ssl_cli.o (ex library/libmbedtls.a)
460 0 0 460 1cc ssl_cookie.o (ex library/libmbedtls.a)
17649 0 0 17649 44f1 ssl_srv.o (ex library/libmbedtls.a)
800 0 0 800 320 ssl_ticket.o (ex library/libmbedtls.a)
39286 60 0 39346 99b2 ssl_tls.o (ex library/libmbedtls.a)
88874 60 600 89534 15dbe (TOTALS)
DTLS-only
text data bss dec hex filename
1820 0 4 1824 720 debug.o (ex library/libmbedtls.a)
0 0 0 0 0 net_sockets.o (ex library/libmbedtls.a)
548 0 0 548 224 ssl_cache.o (ex library/libmbedtls.a)
11155 0 596 11751 2de7 ssl_ciphersuites.o (ex library/libmbedtls.a)
17068 0 0 17068 42ac ssl_cli.o (ex library/libmbedtls.a)
460 0 0 460 1cc ssl_cookie.o (ex library/libmbedtls.a)
17553 0 0 17553 4491 ssl_srv.o (ex library/libmbedtls.a)
800 0 0 800 320 ssl_ticket.o (ex library/libmbedtls.a)
38499 60 0 38559 969f ssl_tls.o (ex library/libmbedtls.a)
87903 60 600 88563 159f3 (TOTALS)
TLS-only
text data bss dec hex filename
1820 0 4 1824 720 debug.o (ex library/libmbedtls.a)
0 0 0 0 0 net_sockets.o (ex library/libmbedtls.a)
548 0 0 548 224 ssl_cache.o (ex library/libmbedtls.a)
11155 0 596 11751 2de7 ssl_ciphersuites.o (ex library/libmbedtls.a)
14912 0 0 14912 3a40 ssl_cli.o (ex library/libmbedtls.a)
460 0 0 460 1cc ssl_cookie.o (ex library/libmbedtls.a)
15868 0 0 15868 3dfc ssl_srv.o (ex library/libmbedtls.a)
800 0 0 800 320 ssl_ticket.o (ex library/libmbedtls.a)
27619 60 0 27679 6c1f ssl_tls.o (ex library/libmbedtls.a)
73182 60 600 73842 12072 (TOTALS)
```
The function mbedtls_ssl_hdr_len() returns the length of the record
header (so far: always 13 Bytes for DTLS, and always 5 Bytes for TLS).
With the introduction of the CID extension, the lengths of record
headers depends on whether the records are incoming or outgoing,
and also on the current transform.
Preparing for this, this commit splits mbedtls_ssl_hdr_len() in two
-- so far unmodified -- functions mbedtls_ssl_in_hdr_len() and
mbedtls_ssl_out_hdr_len() and replaces the uses of mbedtls_ssl_hdr_len()
according to whether they are about incoming or outgoing records.
There is no need to change the signature of mbedtls_ssl_{in/out}_hdr_len()
in preparation for its dependency on the currently active transform,
since the SSL context is passed as an argument, and the currently
active transform is referenced from that.
Prior to this commit, the security parameter struct `ssl_transform`
contained a `ciphersuite_info` field pointing to the information
structure for the negotiated ciphersuite. However, the only
information extracted from that structure that was used in the core
encryption and decryption functions `ssl_encrypt_buf`/`ssl_decrypt_buf`
was the authentication tag length in case of an AEAD cipher.
The present commit removes the `ciphersuite_info` field from the
`ssl_transform` structure and adds an explicit `taglen` field
for AEAD authentication tag length.
This is in accordance with the principle that the `ssl_transform`
structure should contain the raw parameters needed for the record
encryption and decryption functions to work, but not the higher-level
information that gave rise to them. For example, the `ssl_transform`
structure implicitly contains the encryption/decryption keys within
their cipher contexts, but it doesn't contain the SSL master or
premaster secrets. Likewise, it contains an explicit `maclen`, while
the status of the 'Truncated HMAC' extension -- which determines the
value of `maclen` when the `ssl_transform` structure is created in
`ssl_derive_keys` -- is not contained in `ssl_transform`.
The `ciphersuite_info` pointer was used in other places outside
the encryption/decryption functions during the handshake, and for
these functions to work, this commit adds a `ciphersuite_info` pointer
field to the handshake-local `ssl_handshake_params` structure.
The SSL module accesses ECDH context members directly. This can't work
with the new context, where we can't make any assumption about the
implementation of the context.
This commit makes use of the new functions to avoid accessing ECDH
members directly. The only members that are still accessed directly are
the group ID and the point format and they are independent from the
implementation.
Previously, mbedtls_ssl_read_record() always updated the handshake
checksum in case a handshake record was received. While desirable
most of the time, for the CertificateVerify message the checksum
update must only happen after the message has been fully processed,
because the validation requires the handshake digest up to but
excluding the CertificateVerify itself. As a remedy, the bulk
of mbedtls_ssl_read_record() was previously duplicated within
ssl_parse_certificate_verify(), hardening maintenance in case
mbedtls_ssl_read_record() is subject to changes.
This commit adds a boolean parameter to mbedtls_ssl_read_record()
indicating whether the checksum should be updated in case of a
handshake message or not. This allows using it also for
ssl_parse_certificate_verify(), manually updating the checksum
after the message has been processed.
This commit is another step towards supporting the packing of
multiple records within a single datagram.
Previously, the incremental outgoing record sequence number was
statically stored within the record buffer, at its final place
within the record header. This slightly increased efficiency
as it was not necessary to copy the sequence number when writing
outgoing records.
When allowing multiple records within a single datagram, it is
necessary to allow the position of the current record within the
datagram buffer to be flexible; in particular, there is no static
address for the record sequence number field within the record header.
This commit introduces an additional field `cur_out_ctr` within
the main SSL context structure `mbedtls_ssl_context` to keep track
of the outgoing record sequence number independent of the buffer used
for the current record / datagram. Whenever a new record is written,
this sequence number is copied to the the address `out_ctr` of the
sequence number header field within the current outgoing record.
This will allow fragmentation to always happen in the same place, always from
a buffer distinct from ssl->out_msg, and with the same way of resuming after
returning WANT_WRITE
For the situation where the mbedTLS device has limited RAM, but the
other end of the connection doesn't support the max_fragment_length
extension. To be spec-compliant, mbedTLS has to keep a 16384 byte
incoming buffer. However the outgoing buffer can be made smaller without
breaking spec compliance, and we save some RAM.
See comments in include/mbedtls/config.h for some more details.
(The lower limit of outgoing buffer size is the buffer size used during
handshake/cert negotiation. As the handshake is half-duplex it might
even be possible to store this data in the "incoming" buffer during the
handshake, which would save even more RAM - but it would also be a lot
hackier and error-prone. I didn't really explore this possibility, but
thought I'd mention it here in case someone sees this later on a mission
to jam mbedTLS into an even tinier RAM footprint.)
Fix compilation warnings with IAR toolchain, on 32 bit platform.
Reported by rahmanih in #683
This is based on work by Ron Eldor in PR #750, some of which was independently
fixed by Azim Khan and already merged in PR #1646.
In ssl_parse_encrypted_pms, some operational failures from
ssl_decrypt_encrypted_pms lead to diff being set to a value that
depended on some uninitialized unsigned char and size_t values. This didn't
affect the behavior of the program (assuming an implementation with no
trap values for size_t) because all that matters is whether diff is 0,
but Valgrind rightfully complained about the use of uninitialized
memory. Behave nicely and initialize the offending memory.
Rename to mbedtls_ssl_get_async_operation_data and
mbedtls_ssl_set_async_operation_data so that they're about
"async operation data" and not about some not-obvious "data".
When a handshake step starts an asynchronous operation, the
application needs to know which SSL connection the operation is for,
so that when the operation completes, the application can wake that
connection up. Therefore the async start callbacks need to take the
SSL context as an argument. It isn't enough to let them set a cookie
in the SSL connection, the application needs to be able to find the
right SSL connection later.
Also pass the SSL context to the other callbacks for consistency. Add
a new field to the handshake that the application can use to store a
per-connection context. This new field replaces the former
context (operation_ctx) that was created by the start function and
passed to the resume function.
Add a boolean flag to the handshake structure to track whether an
asynchronous operation is in progress. This is more robust than
relying on the application to set a non-null application context.
In the refactoring of ssl_parse_encrypted_pms, I advertently broke the
case when decryption signalled an error, with the variable ret getting
overwritten before calculating diff. Move the calculation of diff
immediately after getting the return code to make the connection more
obvious. Also move the calculation of mask immediately after the
calculation of diff, which doesn't change the behavior, because I find
the code clearer that way.
