By cooperating with each other, the record layer protocol, the handshake protocol, the alert protocol, and the application data protocol create the TLS protocol.

By working together, they establish a connection that provides an integrity-protected tunnel or socket. The connection, if negotiated, also include authentication of server, or both server and client. Most connections also provide encryption of the data travelling in the tunnel.

By modifying the messages sent, tester can check if the other side establishes the connection in expected circumstances. Tester can also check if an implementation aborts the connection on protocol violations, use of unimplemented, or turned off features. Same for the integrity, authentication, and encryption, by modifying the data sent, tester can verify if the other side implements the checks necessary to provide these properties.

TLS protocols

The record layer protocol provides the multiplexing capability to exchange the data from the other TLS protocols over the same TCP connection or socket.

Record layer

The record layer protocol uses records to transfer data belonging to a given upper level protocol. It provides a stream abstraction, just like a TCP connection. That means, the upper layer protocols can’t depend on writes generating a particular number or size of records. The record layer can combine messages into a single record with other data of the same higher level protocol.

In particular, a record can have at most 2^{14} bytes of payload. To process bigger messages from higher level protocols (e.g. ClientHello) record layer fragments them and sends them in more than one record.

By extending the payload with the protocol type and size of the payload, the record layer provides multiplexing to the higher level protocols.

Record layer protects the integrity of exchanged data and, optionally, encrypts and decrypts data. It uses keys and ciphers negotiated by the handshake protocol to do that.

Handshake protocol

Handshake protocol establishes the keys used in the connection and, optionally, the identities of the server or server and client.

Similarly to record layer, handshake protocol messages also include the payload type and payload size.

Unlike the record layer, handshake protocol limits the size of messages to 2^{24}-1 bytes. Handshake protocol also forbids fragmenting or combining of the messages.

Application data protocol

Application data protocol encapsulates the data provided to TLS so that it can travel in the same connection as messages internal to TLS. It serves as a content type to the record layer protocol.

But just like other protocols travelling over record layer, it can’t depend on specific fragmentation of writes to the other side.

Alert protocol

Alert protocol provides signalling of error conditions or unmet expectations to the other side of the connection. When messaging non-fatal errors, in some cases, the connection can continue even after their exchange.

An alert message consists of two bytes.

Testing process

The basic testing scenarios focus on the so called “happy path”: verifying that everything works when nothing unexpected occurred. While testing for support of features needs to use this kind of approach, negative test cases must use malformed or unexpected messages, especially in security protocols. Correct handling of unexpected situations provides the security.

The TLS specification requires strict verification of message format from the parsers. It also describes precisely the expected contents of majority of exchanged fields—encryption or integrity protection of messages allows for one valid and correct formatting of messages or records, for a given set of keys. The specification includes also information on error handling, it describes the expected alert messages for given error conditions.

This allows the tests to send either malformed or inconsistent messages and check for specified alerts to verify if the other side of the connection performed the expected error checking.


Fuzzers generally don’t operate in this way. Typical fuzzers feed the system under test (SUT) with lots of random or semi-random inputs and check if the SUT doesn’t crash, use uninitialised memory or invokes some other undefined behaviour. While tlsfuzzer can generate this kind of tests, included scripts don’t do it—they focus on checking if the server behaves as expected, even when they use random data for it.

Checking alerts

Given that the guiding RFCs allow for not sending the alerts at all, one could argue that checking both reception of alerts and the included error codes in them to be undue carefulness.

Actually though exploitation of security vulnerabilities thanks to the different error codes returned for different errors detected has a long history. When returned errors depend on secret data, unknown to attacker, that may lead to decryption oracles or other side-channel attacks. The standards do take this into account, which makes standard-compliant behaviour the “known good” behaviour.

Consistent and standards-compliant errors also make debugging of interoperability issues easier. Alert description points to the reason of rejection: a certificate issue, a malformed message, a message inconsistent with other messages, etc.

Consistent and correct alerts also allow pushing those errors higher in the stack—if user-level application can depend on particular meaning of errors it can provide more correct and relevant errors to the user.

To confidently test for security vulnerabilities across different implementations, the implementations must behave in consistent, or at least similar ways. When they do, tlsfuzzer can reuse a single verification script to test them.

When test doesn’t have an easy insight into the process serving TLS, getting the alert instead of connection close allows for at least basic verification if the SUT didn’t crash but handled the error.

Sharing of general test suites has the same limitations as sharing of security test scripts. If different implementations exhibit the same behaviour, they can share the same test suite, in turn reducing effort necessary to develop new implementations or extend existing implementations with new features.

Last, but not least, particular way of handling errors provides a strong signal for fingerprinting (identifying) the implementation used. As alert descriptions returned by an implementation don’t depend on implementation configuration, the fingerprints don’t either, making them robust—hard to masquerade one implementation for another (with some exceptions, like in case the server doesn’t parse extensions from turned-off features).