libprotobuf-mutator-asn1 is a library that uses https://github.com/google/libprotobuf-mutator to generate inputs that can exercise ASN.1 parsers, particularly DER parsers. By using a structurally-aware approach that is able to encode valid lengths, inputs are able to more quickly exercise interesting code paths.
In addition to structurally- and syntactically-valid DER, it supports generating BER inputs as well as generating random data, ensuring coverage is not lost compared to classic fuzzers.
In addition to the generic ASN.1 fuzzer, this library can also generate X.509 certificates in a structure-aware fashion. This is to facilitate deeper fuzzing of certificate chain parsing, building, and validation engines, by creating certificates and certificate chains that are structurally-valid, but which can contain fuzzed data (e.g. arbitrary extensions)
This project is still a work-in-progress to separate as a standalone library. Until that work is completed, example usage within OSS-fuzz can be seen at https://github.com/google/oss-fuzz/commit/e15b72d833dd5c30f4cd4aed2ba06e64011ce73d
This is not an officially supported Google product