garble/README.md

# garble

	go install mvdan.cc/garble@latest

Obfuscate Go code by wrapping the Go toolchain. Requires Go 1.17 or later.

	garble build [build flags] [packages]

The tool also supports `garble test` to run tests with obfuscated code,
and `garble reverse` to de-obfuscate text such as stack traces.
See `garble -h` for up to date usage information.

### Purpose

Produce a binary that works as well as a regular build, but that has as little
information about the original source code as possible.

The tool is designed to be:

* Coupled with `cmd/go`, to support modules and build caching
* Deterministic and reproducible, given the same initial source code
* Reversible given the original source, to de-obfuscate panic stack traces

### Mechanism

The tool wraps calls to the Go compiler and linker to transform the Go build, in
order to:

* Replace as many useful identifiers as possible with short base64 hashes
* Replace package paths with short base64 hashes
* Remove all [build](https://go.dev/pkg/runtime/#Version) and [module](https://go.dev/pkg/runtime/debug/#ReadBuildInfo) information
* Strip filenames and shuffle position information
* Strip debugging information and symbol tables via `-ldflags="-w -s"`
* [Obfuscate literals](#literal-obfuscation), if the `-literals` flag is given
* Remove [extra information](#tiny-mode), if the `-tiny` flag is given

The tool obfuscates the packages matching `GOGARBLE`, a comma-separated list of
glob patterns of module path prefixes, as documented in `go help private`.
When `GOGARBLE` is empty, it assumes the value of `GOPRIVATE`.
When `GOPRIVATE` is also empty, then `GOGARBLE` assumes the value of the current
module path, to obfuscate all packages under the current module.

Note that commands like `garble build` will use the `go` version found in your
`$PATH`. To use different versions of Go, you can
[install them](https://go.dev/doc/manage-install#installing-multiple)
and set up `$PATH` with them. For example, for Go 1.17.1:

```sh
$ go install golang.org/dl/go1.17.1@latest
$ go1.17.1 download
$ PATH=$(go1.17.1 env GOROOT)/bin:${PATH} garble build
```

### Literal obfuscation

Using the `-literals` flag causes literal expressions such as strings to be
replaced with more complex variants, resolving to the same value at run-time.
This feature is opt-in, as it can cause slow-downs depending on the input code.

Literal expressions used as constants cannot be obfuscated, since they are
resolved at compile time. This includes any expressions part of a `const`
declaration.

### Tiny mode

When the `-tiny` flag is passed, extra information is stripped from the resulting
Go binary. This includes line numbers, filenames, and code in the runtime that
prints panics, fatal errors, and trace/debug info. All in all this can make binaries
2-5% smaller in our testing, as well as prevent extracting some more information.

With this flag, no panics or fatal runtime errors will ever be printed, but they
can still be handled internally with `recover` as normal. In addition, the
`GODEBUG` environmental variable will be ignored.

Note that this flag can make debugging crashes harder, as a panic will simply
exit the entire program without printing a stack trace, and all source code
positions are set to line 1. Similarly, `garble reverse` is generally not useful
in this mode.

### Speed

`garble build` should take about twice as long as `go build`, as it needs to
complete two builds. The original build, to be able to load and type-check the
input code, and then the obfuscated build.

Garble obfuscates one package at a time, mirroring how Go compiles one package
at a time. This allows Garble to fully support Go's build cache; incremental
`garble build` calls should only re-build and re-obfuscate modified code.

Note that the first call to `garble build` may be comparatively slow,
as it has to obfuscate each package for the first time. This is akin to clearing
`GOCACHE` with `go clean -cache` and running a `go build` from scratch.

### Determinism and seeds

Just like Go, garble builds are deterministic and reproducible in nature.
This has significant benefits, such as caching builds and being able to use
`garble reverse` to de-obfuscate stack traces.

By default, garble will obfuscate each package in a unique way,
which will change if its build input changes: the version of garble, the version
of Go, the package's source code, or any build parameter such as GOOS or -tags.
This is a reasonable default since guessing those inputs is very hard.

However, providing your own obfuscation seed via `-seed` brings some advantages.
For example, builds sharing the same seed will produce the same obfuscation,
even if any of the build parameters or versions vary.
It can also make reverse-engineering harder, as an end user could guess what
version of Go or garble you're using.

Note that extra care should be taken when using custom seeds.
If a seed used to build a binary gets lost, `garble reverse` will not work.
Rotating the seeds can also help against reverse-engineering in the long run,
as otherwise some bits of code may be obfuscated the same way over time.

An alternative approach is `-seed=random`, where each build is entirely different.

### Caveats

Most of these can improve with time and effort. The purpose of this section is
to document the current shortcomings of this tool.

* Exported methods are never obfuscated at the moment, since they could
  be required by interfaces. This area is a work in progress; see
  [#3](https://github.com/burrowers/garble/issues/3).

* Garble aims to automatically detect which Go types are used with reflection,
  as obfuscating those types might break your program.
  Note that Garble obfuscates [one package at a time](#speed),
  so if your reflection code inspects a type from an imported package,
  and your program broke, you may need to add a "hint" in the imported package:
   ```go
   type Message struct {
       Command string
       Args    string
   }

   // Never obfuscate the Message type.
   var _ = reflect.TypeOf(Message{})
   ```

* Go declarations exported for cgo via `//export` are not obfuscated.

* Go plugins are not currently supported; see [#87](https://github.com/burrowers/garble/issues/87).

### Contributing

We welcome new contributors. If you would like to contribute, see
[CONTRIBUTING.md](CONTRIBUTING.md) as a starting point.