With the introduction of the CID extension, the record content type
may change during decryption; we must therefore re-consider every
record content type check that happens before decryption, and either
move or duplicate it to ensure it also applies to records whose
real content type is only revealed during decryption.
This commit does this for the silent dropping of unexpected
ApplicationData records in DTLS. Previously, this was caught
in ssl_parse_record_header(), returning
MBEDTLS_ERR_SSL_UNEXPECTED_RECORD which in ssl_get_next_record()
would lead to silent skipping of the record.
When using CID, this check wouldn't trigger e.g. when delayed
encrypted ApplicationData records come on a CID-based connection
during a renegotiation.
This commit moves the check to mbedtls_ssl_handle_message_type()
and returns MBEDTLS_ERR_SSL_NON_FATAL if it triggers, which leads
so silent skipover in the caller mbedtls_ssl_read_record().
The SSL context structure mbedtls_ssl_context contains several pointers
ssl->in_hdr, ssl->in_len, ssl->in_iv, ssl->in_msg pointing to various
parts of the record header in an incoming record, and they are setup
in the static function ssl_update_in_pointers() based on the _expected_
transform for the next incoming record.
In particular, the pointer ssl->in_msg is set to where the record plaintext
should reside after record decryption, and an assertion double-checks this
after each call to ssl_decrypt_buf().
This commit removes the dependency of ssl_update_in_pointers() on the
expected incoming transform by setting ssl->in_msg to ssl->in_iv --
the beginning of the record content (potentially including the IV) --
and adjusting ssl->in_msg after calling ssl_decrypt_buf() on a protected
record.
Care has to be taken to not load ssl->in_msg before calling
mbedtls_ssl_read_record(), then, which was previously the
case in ssl_parse_server_hello(); the commit fixes that.
If a record exhibits an invalid feature only after successful
authenticated decryption, this is a protocol violation by the
peer and should hence lead to connection failure. The previous
code, however, would silently ignore such records. This commit
fixes this.
So far, the only case to which this applies is the non-acceptance
of empty non-AD records in TLS 1.2. With the present commit, such
records lead to connection failure, while previously, they were
silently ignored.
With the introduction of the Connection ID extension (or TLS 1.3),
this will also apply to records whose real content type -- which
is only revealed during authenticated decryption -- is invalid.
In contrast to other aspects of the Connection ID extension,
the CID-based additional data for MAC computations differs from
the non-CID case even if the CID length is 0, because it
includes the CID length.
Quoting the CID draft 04:
- Block Ciphers:
MAC(MAC_write_key, seq_num +
tls12_cid + // New input
DTLSPlaintext.version +
cid + // New input
cid_length + // New input
length_of_DTLSInnerPlaintext + // New input
DTLSInnerPlaintext.content + // New input
DTLSInnerPlaintext.real_type + // New input
DTLSInnerPlaintext.zeros // New input
)
And similar for AEAD and Encrypt-then-MAC.
This commit temporarily comments the copying of the negotiated CIDs
into the established ::mbedtls_ssl_transform in mbedtls_ssl_derive_keys()
until the CID feature has been fully implemented.
While mbedtls_ssl_decrypt_buf() and mbedtls_ssl_encrypt_buf() do
support CID-based record protection by now and can be unit tested,
the following two changes in the rest of the stack are still missing
before CID-based record protection can be integrated:
- Parsing of CIDs in incoming records.
- Allowing the new CID record content type for incoming records.
- Dealing with a change of record content type during record
decryption.
Further, since mbedtls_ssl_get_peer_cid() judges the use of CIDs by
the CID fields in the currently transforms, this change also requires
temporarily disabling some grepping for ssl_client2 / ssl_server2
debug output in ssl-opt.sh.
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 tests exercising mutually inverse pairs of
record encryption and decryption transformations for the various
transformation types allowed in TLS: Stream, CBC, and AEAD.
The hash contexts `ssl_transform->md_ctx_{enc/dec}` are not used if
only AEAD ciphersuites are enabled. This commit removes them from the
`ssl_transform` struct in this case, saving a few bytes.
This commit guards code specific to AEAD, CBC and stream cipher modes
in `ssl_derive_keys` by the respective configuration flags, analogous
to the guards that are already in place in the record decryption and
encryption functions `ssl_decrypt_buf` resp. `ssl_decrypt_buf`.
Analogous to the previous commit, but concerning the record decryption
routine `ssl_decrypt_buf`.
An important change regards the checking of CBC padding:
Prior to this commit, the CBC padding check always read 256 bytes at
the end of the internal record buffer, almost always going past the
boundaries of the record under consideration. In order to stay within
the bounds of the given record, this commit changes this behavior by
always reading the last min(256, plaintext_len) bytes of the record
plaintext buffer and taking into consideration the last `padlen` of
these for the padding check. With this change, the memory access
pattern and runtime of the padding check is entirely determined by
the size of the encrypted record, in particular not giving away
any information on the validity of the padding.
The following depicts the different behaviors:
1) Previous CBC padding check
1.a) Claimed padding length <= plaintext length
+----------------------------------------+----+
| Record plaintext buffer | | PL |
+----------------------------------------+----+
\__ PL __/
+------------------------------------...
| read for padding check ...
+------------------------------------...
|
contents discarded
from here
1.b) Claimed padding length > plaintext length
+----------------------------------------+----+
| Record plaintext buffer | PL |
+----------------------------------------+----+
+-------------------------...
| read for padding check ...
+-------------------------...
|
contents discarded
from here
2) New CBC padding check
+----------------------------------------+----+
| Record plaintext buffer | | PL |
+----------------------------------------+----+
\__ PL __/
+---------------------------------------+
| read for padding check |
+---------------------------------------+
|
contents discarded
until here
The previous version of the record encryption function
`ssl_encrypt_buf` takes the entire SSL context as an argument,
while intuitively, it should only depend on the current security
parameters and the record buffer.
Analyzing the exact dependencies, it turned out that in addition
to the currently active `ssl_transform` instance and the record
information, the encryption function needs access to
- the negotiated protocol version, and
- the status of the encrypt-then-MAC extension.
This commit moves these two fields into `ssl_transform` and
changes the signature of `ssl_encrypt_buf` to only use an instance
of `ssl_transform` and an instance of the new `ssl_record` type.
The `ssl_context` instance is *solely* kept for the debugging macros
which need an SSL context instance.
The benefit of the change is twofold:
1) It avoids the need of the MPS to deal with instances of
`ssl_context`. The MPS should only work with records and
opaque security parameters, which is what the change in
this commit makes progress towards.
2) It significantly eases testing of the encryption function:
independent of any SSL context, the encryption function can
be passed some record buffer to encrypt alongside some arbitrary
choice of parameters, and e.g. be checked to not overflow the
provided memory.
The macro constant `MBEDTLS_SSL_MAC_ADD` defined in `ssl_internal.h`
defines an upper bound for the amount of space needed for the record
authentication tag. Its definition distinguishes between the
presence of an ARC4 or CBC ciphersuite suite, in which case the maximum
size of an enabled SHA digest is used; otherwise, `MBEDTLS_SSL_MAC_ADD`
is set to 16 to accomodate AEAD authentication tags.
This assignment has a flaw in the situation where confidentiality is
not needed and the NULL cipher is in use. In this case, the
authentication tag also uses a SHA digest, but the definition of
`MBEDTLS_SSL_MAC_ADD` doesn't guarantee enough space.
The present commit fixes this by distinguishing between the presence
of *some* ciphersuite using a MAC, including those using a NULL cipher.
For that, the previously internal macro `SSL_SOME_MODES_USE_MAC` from
`ssl_tls.c` is renamed and moved to the public macro
`MBEDTLS_SOME_MODES_USE_MAC` defined in `ssl_internal.h`.
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_transform` security parameter structure contains opaque
cipher contexts for use by the record encryption/decryption functions
`ssl_decrypt_buf`/`ssl_encrypt_buf`, while the underlying key material
is configured once in `ssl_derive_keys` and is not explicitly dealt with
anymore afterwards. In particular, the key length is not needed
explicitly by the encryption/decryption functions but is nonetheless
stored in an explicit yet superfluous `keylen` field in `ssl_transform`.
This commit removes this field.
