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Fizz is a TLS 1.3 implementation.

This modified version of Fizz is just a proof of concept and not intended for production. The C++ parser for all protocol messages defined in RFC8446 (TLS 1.3) is replaced by a SPARK implementation for which the absence of runtime errors can be proven. The parsing code is generated with RecordFlux based on a protocol message specification.

Fizz currently supports TLS 1.3 drafts 28, 26 (both wire-compatible with the final specification), and 23. All major handshake modes are supported, including PSK resumption, early data, client authentication, and HelloRetryRequest.

More background and details are available on the Facebook Code Blog.


Fizz largely depends on three libraries: folly, OpenSSL, and libsodium.

Additionally, RecordFlux and GNAT Community 2018 are needed to generate and build the parsing code.

Source Layout

  • fizz/crypto: Cryptographic primitive implementations (most are wrapping OpenSSL or libsodium)
  • fizz/record: TLS 1.3 record layer parsing
  • fizz/protocol: Common protocol code shared between client and server
  • fizz/client: Client protocol implementation
  • fizz/server: Server protocol implementation
  • fizz/tool: Example CLI app source


The core protocol implementations are in ClientProtocol and ServerProtocol. FizzClientContext and FizzServerContext provide configuration options. FizzClient and FizzServer (which both inherit from FizzBase) provide applications with an interface to interact with the state machine. FizzClient/FizzServer receives events from the application layer, invokes the correct event handler, and invokes the application ActionVisitor to process the actions.

AsyncFizzClient and AsyncFizzServer provide implementations of the folly AsyncTransportWrapper interface. They own an underlying transport (for example AsyncSocket) and perform the TLS handshake and encrypt/decrypt application data.


Fizz has several important features needed from a modern TLS library.


Fizz supports scatter/gather IO by default via folly's IOBufs, and will encrypt data in-place whenever possible, saving memcpys. Due to this and several other optimizations, we found in our load balancer benchmarks that Fizz has 10% higher throughput than our prior SSL library which uses folly's AsyncSSLSocket. Fizz also consumes less memory per connection than AsyncSSLSocket.

Async by default

Fizz has asynchronous APIs to be able to offload functions like certificate signing and ticket decryption. The API is based on folly's Futures for painless async programming.

TLS features

Fizz supports APIs like exported keying material as well as zero-copy APIs needed to use TLS in other protocols like QUIC.

Secure design abstractions

Fizz is built on a custom state machine which uses the power of the C++ type system to treat states and actions as types of their own. As the code changes, this allows us to catch invalid state transitions as compilation errors instead of runtime errors and helps us move fast.

Sample Applications

Fizz includes an example program that showcases the basic client/server functionality supported by Fizz. The binary is called fizz and it has similar usage to the openssl or bssl commands.

For example, to start a TLS server on port 443 with a specified cert:

fizz server -accept 443 -cert foo.pem -key foo.key

Then, on the same host, you can connect with:

fizz client -connect localhost:443

Both ends will echo whatever data they receive and send any terminal input to the peer. Hitting CTRL+D on either end will terminate the connection.

The source code for this program can be found under fizz/tool.


Ubuntu 16.04 LTS

To begin, you should install the dependencies we need for build. This largely consists of folly's dependencies, as well as libsodium.

sudo apt-get install \
    g++ \
    cmake \
    libboost-all-dev \
    libevent-dev \
    libdouble-conversion-dev \
    libgoogle-glog-dev \
    libgflags-dev \
    libiberty-dev \
    liblz4-dev \
    liblzma-dev \
    libsnappy-dev \
    make \
    zlib1g-dev \
    binutils-dev \
    libjemalloc-dev \
    libssl-dev \
    pkg-config \

Then, build and install folly:

git clone
mkdir folly/build_ && cd folly/build_
cmake ..
make -j $(nproc)
sudo make install

And lastly, build and install fizz.

cd ../..
git clone
mkdir fizz/build_ && cd fizz/build_
cmake ../fizz
make -j $(nproc)
sudo make install

Building on Mac

The following instructions were tested on MacOS High Sierra with Xcode 9.4.1. They should work with later Xcode versions as well.

Run the helper script from within the fizz subdirectory. The helper script assumes that you have homebrew installed and are using homebrew as your package manager. To install homebrew use the instructions on the homebrew website.

It will install and link the required dependencies and also build folly. This may take several minutes the first time.

cd fizz

After building, the directory out/ will contain the libraries as well as out/bin will contain the ClientSocket and ServerSocket binaries. Running it again will be faster and only rebuild fizz.

You can also install both fizz as well as folly to a custom directory using the build script, by supplying an INSTALL_PREFIX env var.

INSTALL_PREFIX=/usr/local ./

You might need to run the script as root to install to certain directories.


We'd love to have your help in making Fizz better. If you're interested, please read our guide to guide to contributing


Fizz is BSD licensed, as found in the LICENSE file.

Reporting and Fixing Security Issues

Please do not open GitHub issues or pull requests - this makes the problem immediately visible to everyone, including malicious actors. Security issues in Fizz can be safely reported via Facebook's Whitehat Bug Bounty program:

Facebook's security team will triage your report and determine whether or not is it eligible for a bounty under our program.


C++14 implementation of the TLS-1.3 standard



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