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golang binary build code-obfuscator obfuscation

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Daniel Martí e2f06cce94 set positions when using cursor.Replace The regular obfuscation process simply modifies some simple nodes, such as identifiers and strings. In those cases, we modify the nodes in-place, meaning that their positions remain the same. This hasn't caused any problems. Literal obfuscation is trickier. Since we replace one expression with an entirely different one, we use cursor.Replace. The new expression is entirely made up on the spot, so it lacks position information. This was causing problems. For example, in the added test input: > garble -literals build [stderr] # test/main dgcm4t6w.go:3: misplaced compiler directive dgcm4t6w.go:4: misplaced compiler directive dgcm4t6w.go:3: misplaced compiler directive dgcm4t6w.go:6: misplaced compiler directive dgcm4t6w.go:7: misplaced compiler directive dgcm4t6w.go:3: misplaced compiler directive dgcm4t6w.go:9: misplaced compiler directive dgcm4t6w.go:3: misplaced compiler directive dgcm4t6w.go:3: too many errors The build errors are because we'd move the compiler directives, which makes the compiler unhappy as they must be directly followed by a function declaration. The root cause there seems to be that, since the replacement nodes lack position information, go/printer would try to estimate its printing position by adding to the last known position. Since -literals adds code, this would result in the printer position increasing rapidly, and potentially printing directive comments earlier than needed. For now, making the replacement nodes have the same position as the original node seems to stop go/printer from making this mistake. It's possible that this workaround won't be bulletproof forever, but it works well for now, and I don't see a simpler workaround right now. It would be possible to use fancier mechanisms like go/ast.CommentMap or dave/dst, but those are a significant amount of added complexity as well. Fixes #285.		3 years ago
.github	all: drop support for Go 1.15.x (#265 )	3 years ago
internal	set positions when using cursor.Replace	3 years ago
scripts	update the list of runtime-related packages for 1.16 (#246 )	3 years ago
testdata	set positions when using cursor.Replace	3 years ago
.gitattributes	start testing on GitHub Actions	5 years ago
.gitignore	skip literals used in constant expressions	4 years ago
AUTHORS	set up an AUTHORS file to attribute copyright	4 years ago
CHANGELOG.md	CHANGELOG: start drafting for v0.2.0	3 years ago
CONTRIBUTING.md	CONTRIBUTING: include some basic terminology (#188 )	4 years ago
LICENSE	set up an AUTHORS file to attribute copyright	4 years ago
README.md	README: refactor and include more useful information	3 years ago
bench_test.go	rework the build benchmarks	3 years ago
go.mod	all: drop support for Go 1.15.x (#265 )	3 years ago
go.sum	all: drop support for Go 1.15.x (#265 )	3 years ago
hash.go	make flags like -literals and GOPRIVATE affect hashing (#288 )	3 years ago
main.go	obfuscate literals as part of transformGo (#299 )	3 years ago
main_test.go	handle unknown flags in reverse (#290 )	3 years ago
position.go	set positions when using cursor.Replace	3 years ago
reverse.go	make reverse error if no changes were made	3 years ago
runtime_strip.go	all: drop support for Go 1.15.x (#265 )	3 years ago
shared.go	make flags like -literals and GOPRIVATE affect hashing (#288 )	3 years ago

README.md

garble

GO111MODULE=on go get mvdan.cc/garble

Obfuscate Go code by wrapping the Go toolchain. Requires Go 1.16 or later.

garble build [build flags] [packages]

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 deobfuscate 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 and module information
Strip filenames and shuffle position information
Strip debugging information and symbol tables via -ldflags="-w -s"
Obfuscate literals, if the -literals flag is given
Remove extra information, if the -tiny flag is given

By default, the tool obfuscates the packages under the current module. If not running in module mode, then only the main package is obfuscated. To specify what packages to obfuscate, set GOPRIVATE, documented at go help private.

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 and set up $PATH with them. For example, for Go 1.16.1:

$ go get golang.org/dl/go1.16.1
$ go1.16.1 download
$ PATH=$(go1.16.1 env GOROOT)/bin:${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.

Note: if -tiny is passed, no panics, fatal 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.

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 finally the obfuscated build.

Go's build cache is fully supported; if a first garble build run is slow, a second run should be significantly faster. This should offset the cost of the double builds, as incremental builds in Go are fast.

Determinism and seeds

Just like Go, garble builds are deterministic and reproducible if the inputs remain the same: the version of Go, the version of Garble, and the input code. This has significant benefits, such as caching builds or being able to use garble reverse to de-obfuscate stack traces.

However, it also means that an input package will be obfuscated in exactly the same way if none of those inputs change. If you want two builds of your program to be entirely different, you can use -seed to provide a new seed for the entire build, which will cause a full rebuild.

If any open source packages are being obfuscated, providing a custom seed can also provide extra protection. It could be possible to guess the versions of Go and garble given how a public package was obfuscated without a seed.

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 and reflection. This area is a work in progress.
It can be hard for garble to know what types will be used with reflection, including JSON encoding or decoding. If your program breaks because a type's names are obfuscated when they should not be, you can add an explicit hint:
```
type Message struct {
	Command string
	Args    string
}

// Never obfuscate the Message type.
var _ = reflect.TypeOf(Message{})
```
Go plugins are not currently supported; see #87.

Contributing

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