TLS Internals#

Overview#

In CCF, the TLS layer is implemented using OpenSSL 1.1.1 from Open Enclave (OE). However, the original implementation was using OE’s MbedTLS library, which the current implementation replaced. During the transition period, the OpenSSL implementation had to emulate the previous MbedTLS one and the remaining code isn’t particularly suited to OpenSSL.

In addition, we’ll soon be adding QUIC support to CCF, which has its own usage of TLS (supporting OpenSSL, but from its own tree).

This document is an attempt to describe how that works, so that we can plan a suited implementation for both TCP-based and QUIC-based endpoints using TLS.

Enclave Endpoints#

CCF handles RPC requests by exposing an HTTP endpoint for each registered REST node. The HTTP endpoint receives information from the ring buffer and pass it down to the TLS implementation that tries to decrypt on read (after a successful handshake).

If there isn’t enough information to decrypt on read, the TLS layer responds with a ‘WANTS_READ’ message, meaning more packages are needed to complete the message, so the endpoint tried to get more data from the ring buffer.

Once all incoming data is decrypted, the HTTP endpoint passes the plain text data to the application, which process it and returns a plain text response. The endpoint then sends this back to the client, through the TLS layer, which encrypts it and pushes it back to the ring buffer.

TLS Implementation#

The TLS implementation in ‘src/tls’ has three main components:

  • A ‘Certificate Authority’ (CA), which has a root certificate that can sign other certificates in the network. This CA certificate is generated by the network outside of the TLS implementation and is imported when clients and servers are created.

  • A ‘Certificate’ (Cert), which is the server/client’s own certificate, signed by the CA and unique to this server or client.

  • A ‘Context’, which is the common implementation of both ‘Client’ and ‘Server’ and “know” how to read&decrypt and encrypt&write through endpoints ring buffer and its triggers. Clients and servers are basically just a Context with a certificate.

Both CA and Cert have internal logic to validate their certificates and private keys, and the Context has logic to complete a TLS handshake, read and write using encryption and to query the peer’s certificate.

The next layer up is the ‘TLSEndpoint’ (of which ‘HTTPEndpoint’ is a sub-class) that holds the ring buffer, the pending read and write buffers, and the call backs for sending and receiving data.

Receiving Messages#

When a REST node message is received through the ring buffer, the ‘HTTPEndpoint’ recv_() method is called (as it was registered as a callback). That calls TLSEndpoint::read() which in turn calls tls::Context::read().

However, when creating the TLS ‘Context’, the ‘TLSEndpoint’ had to register some callbacks of its own, too. This is one of the parts that was specific to MbedTLS and that was (unnaturally) replicated to OpenSSL.

Those callbacks are implemented in ‘TLSEndpoint’ because they need access to the pending_read buffers to read from and the ‘ring buffer’ to write to. But the TLS context (‘SSL’ and ‘SSL_CTX’ objects) need to encrypt/decrypt data, and they can only do that from ‘BIO’s in OpenSSL, so the callback has a mix of ring buffer and BIO handling that tricks both sides to take the appropriate steps at the right time in this complex dance.

In the ‘read’ case, the message has arrived into the pending_read buffer via a (previously triggered) ring buffer callback, and that data is written to TLS’s ‘read’ BIO (via BIO_write_ex) _before_ TLS itself reads it. That data is still encrypted, so when the TLS reads it with SSL_read_ex, it tries to decrypt and on success, returns a plain text buffer. On error, it emits the error or a ‘WANTS_READ’ status, so the endpoint can try to extract more information from the pending_read buffer again.

Upon success, the read BIO is flushed and new data can be read again. The plain text result is returned all the way back to HTTPEndpoint::recv_() that then sends it to the application to process.

Sending Messages#

When the application finishes, it returns a plain text response. That message is sent back to the client via the send() call. This in turn calls TLSEndpoint::send_raw() that via a series of asynchronous dispatches ends up filling the pending_write buffer and calling write_some() which itself calls tls::Context::write().

This is the same process as for reads: a callback in ‘TLSEndpoint’ was registered for the ‘write’ BIO, with access to the pending_write and the ring buffer. But in this case, the callback is only executed _after_ the TLS layer has encrypted the data and written to the ‘write’ BIO.

Now, we take that (encrypted) data, flush the BIO ourselves (to avoid clogging the pipes with the next message) and send it through the ring buffer with the RINGBUFFER_TRY_WRITE_MESSAGE(tls_outbound,...) macro.

This is what actually sends the message back to the client, so when this callback returns, the remaining stack just returns the status of that write. Different errors are treated at different levels (TLS errors in Context, ring buffer errors in TLSEndpoint and HTTP errors in HTTPEndpoint).

Why OpenSSL?#

The main reasons why we moved to OpenSSL are:

  • We already use OpenSSL for our crypto library (‘src/crypto’).

  • We wanted TLS 1.3 support and MbedTLS doesn’t have it.

  • We want to support QUIC, that doesn’t work with MbedTLS.

By now we have removed any traces of MbedTLS, but we are still using OE’s OpenSSL library, which doesn’t have QUIC support.

To implement the last point above, we need to build QUIC with its own OpenSSL and use that as our library for the remaining crypto, but OE has its own modifications to OpenSSL as well, and we can’t have both.

So, for now, the only way to have QUIC support is in ‘virtual’ mode, retaining OE’s OpenSSL for ‘sgx’ mode. Once we have other types of enclaves that don’t require Open Enclave we can use QUIC’s version, too.

MbedTLS vs OpenSSL#

As stated above, the current OpenSSL implementation is _emulating_ the previous MbedTLS one, so some oddities are observed.

First, MbedTLS returns errors as negative values and amount of data handled as positive values. OpenSSL concurs on positive values but returns 0 (or -1 in previous versions) for all errors, using SSL_get_error to then classify which error and what to do. The error values are also positive.

To simulate this, we implement the error handling at each invocation and, on error, we negate the value of the error so that we can retain the old behavior of checking for negative values.

Second, MbedTLS keeps all its context (configuration, connection info, read and write buffers) in a single large structure, while OpenSSL has separate structures for each and uses ‘BIO’ objects for buffers. Reads and writes in MbedTLS is done exclusively via callbacks.

OpenSSL callbacks, however, are very different from MbedTLS ones. They are called twice for each action, one before the actual action and another after.

To simulate this we had to implement a read callback _before_ the BIO read (so we could fill it up with the contents of the ring buffer) and the write callback _after_ the BIO write (so we could pick up its contents and send it into the ring buffer).

There is a complex dance of return values in OpenSSL’s callbacks. If any returns errors the action is canceled immediately. On reads, because the BIO was empty, the initial return value is an error, so we must make sure that, if there is anything in the pending_read buffer, we have to change the status to the amount of bytes read, so it can continue.

Third, the handshake in MbedTLS had various types of errors, which we had to emulate by making the appropriate SSL_* calls, check the peer certificate, etc. to get the same types of responses for the same situations.

Finally, in MbedTLS, the configuration and session objects were setup at the same time, while in OpenSSL they’re separate. We ended up duplicating every single configuration, but this is unnecessary, because once the config object is correct, any session object created from it has the same properties.

But the TLS Context doesn’t handle more than one session per configuration, so we could set either of them once and ignore the other. The simplest thing would be to setup just the session, but if we end up having more than one session later, we’d have to refactor that.

Pure OpenSSL Implementation#

With MbedTLS gone from the code base, we can now think of a pure OpenSSL implementation.

The considerations are:

  • We don’t need to handle errors inside the calls to read/write, but can leave for each caller to handle IFF there is an error by calling SSL_get_error. This also means we don’t need to negate error values, as they’re in different domains.

  • We can simplify the SSL configuration on startup, handshake handling and peer certificate handling.

However, getting rid of the callbacks and using BIOs directly is going to be hard.

First, the current callback is in ‘TLSEndpoint’ because it has access to both pending buffers and the ring buffer. The TLS Context does not have access to it nor it would be wise to pass references to it, as that’d make the Context exclusive to the TLSEndpoint.

Second, both endpoint and TLS have a need to read and write asynchronously. Data arrives from the ring buffer at any time and the TLS implementation can request reads and writes (for example, during handshake) that the endpoint didn’t request itself.

So if SSL_handshake, SSL_read_ex and SSL_write_ex don’t have direct access to read and write from the ring buffers without direct requests from the endpoints, it won’t be able to conclude the asynchronous handshake and start the connection.

One possible way out of it is to create a BIO pair for each read/write action between the ‘TLSEndpoint’ and the TLS ‘Context’, driven by two asynchronous tasks in ‘TLSEndpoint’ that just poll the BIOs and buffers and pass data across. This removes as callback, but introduces polling, which is not an actual improvement.