use hashWith for obfuscation position information

Position information was obfuscated with math/rand manually, which meant
that the resulting positions were pretty small like "x.go:34", but they
were also very hard to reverse due to their short length and difficulty
to reproduce.

We now hash them with hashWith and the package's GarbleActionID:

	"main.go:203" hashed with 933ad1c700755b7c3a9913c55cade1 to "mwu1xuNz.go"

The input to the hash is the base filename and the byte offset of the
declaration within the file, meaning that it's unique within a package.
The output filename is long enough to allow easy reversal.

The line number is always 1, since the information needed for reversing
is contained entirely within the filename. It doesn't really matter if
we encode data in the filename or line number, but it's easier for us to
use a string.

For #5.

line_obfuscator.go

@@ -6,7 +6,6 @@ package main
 import (
 	"fmt"
 	"go/ast"
-	mathrand "math/rand"
 	"strings"
 )
 
@@ -92,9 +91,9 @@ func clearNodeComments(node ast.Node) {
 
 // transformLineInfo removes the comment except go directives and build tags. Converts comments to the node view.
 // It returns comments not attached to declarations and names of declarations which cannot be renamed.
-func (tf *transformer) transformLineInfo(file *ast.File, name string) (detachedComments []string, f *ast.File) {
+func (tf *transformer) transformLineInfo(file *ast.File, filename string) (detachedComments []string, f *ast.File) {
 	prefix := ""
-	if strings.HasPrefix(name, "_cgo_") {
+	if strings.HasPrefix(filename, "_cgo_") {
 		prefix = "_cgo_"
 	}
 
@@ -114,9 +113,7 @@ func (tf *transformer) transformLineInfo(file *ast.File, name string) (detachedC
 		return true
 	})
 
-	newLines := mathrand.Perm(len(file.Decls))
-
-	for i, decl := range file.Decls {
+	for _, decl := range file.Decls {
 		var doc **ast.CommentGroup
 		switch decl := decl.(type) {
 		case *ast.FuncDecl:
@@ -124,14 +121,13 @@ func (tf *transformer) transformLineInfo(file *ast.File, name string) (detachedC
 		case *ast.GenDecl:
 			doc = &decl.Doc
 		}
-		newPos := prefix + ":1"
+		newName := ""
 		if !opts.Tiny {
-			newPos = fmt.Sprintf("%s%c.go:%d",
-				prefix,
-				nameCharset[mathrand.Intn(len(nameCharset))],
-				PosMin+newLines[i],
-			)
+			origPos := fmt.Sprintf("%s:%d", filename, fset.Position(decl.Pos()).Offset)
+			newName = hashWith(curPkg.GarbleActionID, origPos) + ".go"
+			// log.Printf("%q hashed with %x to %q", origPos, curPkg.GarbleActionID, newName)
 		}
+		newPos := fmt.Sprintf("%s%s:1", prefix, newName)
 
 		comment := &ast.Comment{Text: "//line " + newPos}
 		*doc = prependComment(*doc, comment)

main.go

@@ -548,6 +548,7 @@ func transformCompile(args []string) ([]string, error) {
 		files = append(files, file)
 	}
 
+	// Literal obfuscation uses math/rand, so seed it deterministically.
 	randSeed := opts.Seed
 	if len(randSeed) == 0 {
 		randSeed = curPkg.GarbleActionID

testdata/scripts/position.txt

@@ -11,8 +11,8 @@ exec ./main
 stdout 'main.go'
 stdout 'other_file_name'
 stdout ':19: main'
-stdout 'initLines is sorted'
-stdout 'varLines is sorted'
+stdout 'initPositions is sorted'
+stdout 'varPositions is sorted'
 
 -- go.mod --
 module test/main
@@ -43,17 +43,17 @@ func main() {
 	funcDecl()
 	funcVar()
 
-	// initLines is filled by ten consecutive funcs.
+	// initPositions is filled by ten consecutive funcs.
 	// If we are not shuffling or obfuscating line numbers,
 	// this list will be sorted.
 	// If we are, it's extremely unlikely it would remain sorted.
-	if sort.IsSorted(sort.IntSlice(initLines)) {
-		fmt.Println("initLines is sorted")
+	if sort.IsSorted(sort.StringSlice(initPositions)) {
+		fmt.Println("initPositions is sorted")
 	}
 
 	// Same as the above, but with vars.
-	if sort.IsSorted(sort.IntSlice(varLines)) {
-		fmt.Println("varLines is sorted")
+	if sort.IsSorted(sort.StringSlice(varPositions)) {
+		fmt.Println("varPositions is sorted")
 	}
 }
 -- other_file_name.go --
@@ -74,36 +74,36 @@ var funcVar = func() {
 	fmt.Printf("%s:%d: func var\n", file, line)
 }
 
-var initLines []int
+var initPositions []string
 
-func curLine() int {
-	_, _, line, _ := runtime.Caller(1)
-	return line
+func curPos() string {
+	_, file, line, _ := runtime.Caller(1)
+	return fmt.Sprintf("%s:%d", file, line)
 }
 
-func init() { initLines = append(initLines, curLine()) }
-func init() { initLines = append(initLines, curLine()) }
-func init() { initLines = append(initLines, curLine()) }
-func init() { initLines = append(initLines, curLine()) }
-func init() { initLines = append(initLines, curLine()) }
-func init() { initLines = append(initLines, curLine()) }
-func init() { initLines = append(initLines, curLine()) }
-func init() { initLines = append(initLines, curLine()) }
-func init() { initLines = append(initLines, curLine()) }
-func init() { initLines = append(initLines, curLine()) }
-
-var varLine0 = curLine()
-var varLine1 = curLine()
-var varLine2 = curLine()
-var varLine3 = curLine()
-var varLine4 = curLine()
-var varLine5 = curLine()
-var varLine6 = curLine()
-var varLine7 = curLine()
-var varLine8 = curLine()
-var varLine9 = curLine()
-
-var varLines = []int{
+func init() { initPositions = append(initPositions, curPos()) }
+func init() { initPositions = append(initPositions, curPos()) }
+func init() { initPositions = append(initPositions, curPos()) }
+func init() { initPositions = append(initPositions, curPos()) }
+func init() { initPositions = append(initPositions, curPos()) }
+func init() { initPositions = append(initPositions, curPos()) }
+func init() { initPositions = append(initPositions, curPos()) }
+func init() { initPositions = append(initPositions, curPos()) }
+func init() { initPositions = append(initPositions, curPos()) }
+func init() { initPositions = append(initPositions, curPos()) }
+
+var varLine0 = curPos()
+var varLine1 = curPos()
+var varLine2 = curPos()
+var varLine3 = curPos()
+var varLine4 = curPos()
+var varLine5 = curPos()
+var varLine6 = curPos()
+var varLine7 = curPos()
+var varLine8 = curPos()
+var varLine9 = curPos()
+
+var varPositions = []string{
 	varLine0, varLine1, varLine2, varLine3, varLine4,
 	varLine5, varLine6, varLine7, varLine8, varLine9,
 }

testdata/scripts/tiny.txt

@@ -20,7 +20,7 @@ stderr '^recovered: ya like jazz?'
 env GODEBUG=
 garble build
 ! exec ./main$exe
-stderr '^caller: \w\.go [1-9]'
+stderr '^caller: [0-9a-zA-Z_]+\.go [1-9]'
 stderr '^recovered: ya like jazz?'
 stderr 'panic: oh noes'