What is tlspuffin?

The tlspuffin fuzzer is the reference implementation for the Dolev-Yao fuzzing approach. The fuzzer mostly fuzzes TLS implementations like OpenSSL, LibreSSL, or wolfSSL, but can also fuzz other cryptographic protocols like SSH. Internally, puffin uses LibAFL to drive the fuzzing loop.

Fuzzing Loop

The following image shows the terminology and the flow of test cases through the fuzzer. We typically start the fuzzing with some "happy protocol flows" in the corpus.

• State: Comprising "Corpus" and "Objectives," this stores all test cases with their traces and metadata, with the Objectives focusing specifically on cases that have triggered security violations.
• Scheduler: This selects and schedules test cases from the Corpus for mutation and re-testing, based on various strategic criteria.
• Mutational Stage: The "Mutator" alters a trace from a scheduled test case to create a mutated test case, which is then sent to the harness.
• Harness: The harness executes the mutated test case in the Program Under Test (PUT) and observes the execution.
• Feedback: This component evaluates the observed outcomes of the test case execution, adding interesting cases to the Corpus for further testing.
• Objective Oracle: It checks test cases for violations of security policies, adding those that do violate to the Objectives for focused analysis.

Implementation

From an implementation perspective, several modules exist to make the fuzzer reusable:

• puffin - Core fuzzing engine which is protocol agnostic.
• tlspuffin - TLS fuzzer which uses puffin and is implementation agnostic.
• sshpuffin (WIP) - SSH fuzzer which uses puffin and is implementation agnostic.
• puts - Linkable Programs Under Test that can be linked with tlspuffin or sshpuffin.

The interfaces between the modules are defined by the following Rust traits which define what a protocol is and what a Put is.

Protocol

pub trait ProtocolBehavior: 'static {
    type Claim: Claim;
    type SecurityViolationPolicy: SecurityViolationPolicy<Self::Claim>;

    type ProtocolMessage: ProtocolMessage<Self::OpaqueProtocolMessage>;
    type OpaqueProtocolMessage: OpaqueProtocolMessage;

    type Matcher: Matcher
        + for<'a> TryFrom<&'a MessageResult<Self::ProtocolMessage, Self::OpaqueProtocolMessage>>;

    /// Get the signature that is used in the protocol
    fn signature() -> &'static Signature;

    /// Creates a sane initial seed corpus.
    fn create_corpus() -> Vec<(Trace<Self::Matcher>, &'static str)>;
}

pub struct MessageResult<M: ProtocolMessage<O>, O: OpaqueProtocolMessage>(pub Option<M>, pub O);

/// A structured message. This type defines how all possible messages of a protocol.
/// Usually this is implemented using an `enum`.
pub trait ProtocolMessage<O: OpaqueProtocolMessage>: Clone + Debug {
    fn create_opaque(&self) -> O;
    fn debug(&self, info: &str);
    fn extract_knowledge(&self) -> Result<Vec<Box<dyn VariableData>>, Error>;
}

/// A non-structured version of [`ProtocolMessage`]. This can be used for example for encrypted messages
/// which do not have a structure.
pub trait OpaqueProtocolMessage: Clone + Debug + Codec {
    fn debug(&self, info: &str);

    fn extract_knowledge(&self) -> Result<Vec<Box<dyn VariableData>>, Error>;
}

Put

pub trait Put<PB: ProtocolBehavior>:
    Stream<PB::ProtocolMessage, PB::OpaqueProtocolMessage> + 'static
{
    /// Process incoming buffer, internal progress, can fill in the output buffer
    fn progress(&mut self) -> Result<(), Error>;

    /// In-place reset of the state
    fn reset(&mut self, new_name: AgentName) -> Result<(), Error>;

    fn descriptor(&self) -> &AgentDescriptor;

    /// Returns a textual representation of the state in which self is
    fn describe_state(&self) -> &str;

    /// Checks whether the Put is in a good state
    fn is_state_successful(&self) -> bool;

    /// Shut down the PUT by consuming it and returning a string that summarizes the execution.
    fn shutdown(&mut self) -> String;

    /// Returns a textual representation of the version of the PUT used by self
    fn version() -> String
    where
        Self: Sized;
}