Fuzzing The Library
============================

Botan comes with a set of fuzzing endpoints which can be used to test
the library.

.. highlight:: shell

Fuzzing with libFuzzer
------------------------

To fuzz with libFuzzer (http://llvm.org/docs/LibFuzzer.html), you'll first
need to compile libFuzzer::

  $ svn co http://llvm.org/svn/llvm-project/compiler-rt/trunk/lib/fuzzer libFuzzer
  $ cd libFuzzer && clang -c -g -O2 -std=c++11 *.cpp
  $ ar cr libFuzzer.a libFuzzer/*.o

Then build the fuzzers::

  $ ./configure.py --with-santitizers --build-fuzzer=libfuzzer --unsafe-fuzzer-mode
  $ make fuzzers

Using `--with-sanitizers` is optional but highly useful.

The fuzzer binaries will be in `build/fuzzer`. Simply pick one and run it, optionally
also passing a directory containing corpus inputs.

Using `libfuzzer` build mode implicitly assumes the fuzzers need to
link with `libFuzzer`; if another library is needed (for example in
OSS-Fuzz, which uses `libFuzzingEngine`), use the flag
`--with-fuzzer-lib` to specify the desired name.

Fuzzing with AFL
--------------------

To fuzz with AFL (http://lcamtuf.coredump.cx/afl/)::

  $ ./configure.py --with-sanitizers --build-fuzzer=afl --unsafe-fuzzer-mode --cc-bin=afl-g++
  $ make fuzzers

You can also use `afl-clang-fast++` or `afl-clang++`.

The fuzzer binaries will be in `build/fuzzer`. To run them you need to
run under `afl-fuzz`::

  $ afl-fuzz -i corpus_path -o output_path ./build/fuzzer/binary

Input Corpus
-----------------------

AFL requires an input corpus, and libFuzzer can certainly make good
use of it.

Some crypto corpus repositories include

* https://github.com/randombit/crypto-corpus
* https://github.com/mozilla/nss-fuzzing-corpus
* https://github.com/google/boringssl/tree/master/fuzz
* https://github.com/openssl/openssl/tree/master/fuzz/corpora

Adding new fuzzers
---------------------

New fuzzers are created by adding a source file to `src/fuzzers` which
have the signature:

``void fuzz(const uint8_t in[], size_t len)``

After adding your fuzzer, rerun `./configure.py` and build.