garble/internal/literals/swap.go

// Copyright (c) 2020, The Garble Authors.
// See LICENSE for licensing information.

package literals

import (
	"go/ast"
	"go/token"
	"math"
	mathrand "math/rand"

	ah "mvdan.cc/garble/internal/asthelper"
)

type swap struct{}

// check that the obfuscator interface is implemented
var _ obfuscator = swap{}

func getIndexType(dataLen int64) string {
	switch {
	case dataLen <= math.MaxUint8:
		return "byte"
	case dataLen <= math.MaxUint16:
		return "uint16"
	case dataLen <= math.MaxUint32:
		return "uint32"
	default:
		return "uint64"
	}
}

func positionsToSlice(data []int) *ast.CompositeLit {
	arr := &ast.CompositeLit{
		Type: &ast.ArrayType{
			Len: &ast.Ellipsis{}, // Performance optimization
			Elt: ast.NewIdent(getIndexType(int64(len(data)))),
		},
		Elts: []ast.Expr{},
	}
	for _, data := range data {
		arr.Elts = append(arr.Elts, ah.IntLit(data))
	}
	return arr
}

// Generates a random even swap count based on the length of data
func generateSwapCount(obfRand *mathrand.Rand, dataLen int) int {
	swapCount := dataLen

	maxExtraPositions := dataLen / 2 // Limit the number of extra positions to half the data length
	if maxExtraPositions > 1 {
		swapCount += obfRand.Intn(maxExtraPositions)
	}
	if swapCount%2 != 0 { // Swap count must be even
		swapCount++
	}
	return swapCount
}

func (swap) obfuscate(rand *mathrand.Rand, data []byte, extKeys []*externalKey) *ast.BlockStmt {
	swapCount := generateSwapCount(rand, len(data))
	shiftKey := byte(rand.Uint32())

	op := randOperator(rand)

	positions := genRandIntSlice(rand, len(data), swapCount)
	for i := len(positions) - 2; i >= 0; i -= 2 {
		// Generate local key for xor based on random key and byte position
		localKey := byte(i) + byte(positions[i]^positions[i+1]) + shiftKey
		// Swap bytes from i+1 to i and encrypt using operator and local key
		data[positions[i]], data[positions[i+1]] = evalOperator(op, data[positions[i+1]], localKey), evalOperator(op, data[positions[i]], localKey)
	}

	return ah.BlockStmt(
		&ast.AssignStmt{
			Lhs: []ast.Expr{ast.NewIdent("data")},
			Tok: token.DEFINE,
			Rhs: []ast.Expr{dataToByteSliceWithExtKeys(rand, data, extKeys)},
		},
		&ast.AssignStmt{
			Lhs: []ast.Expr{ast.NewIdent("positions")},
			Tok: token.DEFINE,
			Rhs: []ast.Expr{positionsToSlice(positions)},
		},
		&ast.ForStmt{
			Init: &ast.AssignStmt{
				Lhs: []ast.Expr{ast.NewIdent("i")},
				Tok: token.DEFINE,
				Rhs: []ast.Expr{ah.IntLit(0)},
			},
			Cond: &ast.BinaryExpr{
				X:  ast.NewIdent("i"),
				Op: token.LSS,
				Y:  ah.IntLit(len(positions)),
			},
			Post: &ast.AssignStmt{
				Lhs: []ast.Expr{ast.NewIdent("i")},
				Tok: token.ADD_ASSIGN,
				Rhs: []ast.Expr{ah.IntLit(2)},
			},
			Body: ah.BlockStmt(
				&ast.AssignStmt{
					Lhs: []ast.Expr{ast.NewIdent("localKey")},
					Tok: token.DEFINE,
					Rhs: []ast.Expr{&ast.BinaryExpr{
						X: &ast.BinaryExpr{
							X:  ah.CallExpr(ast.NewIdent("byte"), ast.NewIdent("i")),
							Op: token.ADD,
							Y: ah.CallExpr(ast.NewIdent("byte"), &ast.BinaryExpr{
								X:  ah.IndexExpr("positions", ast.NewIdent("i")),
								Op: token.XOR,
								Y: ah.IndexExpr("positions", &ast.BinaryExpr{
									X:  ast.NewIdent("i"),
									Op: token.ADD,
									Y:  ah.IntLit(1),
								}),
							}),
						},
						Op: token.ADD,
						Y:  byteLitWithExtKey(rand, shiftKey, extKeys, highProb),
					}},
				},
				&ast.AssignStmt{
					Lhs: []ast.Expr{
						ah.IndexExpr("data", ah.IndexExpr("positions", ast.NewIdent("i"))),
						ah.IndexExpr("data", ah.IndexExpr("positions", &ast.BinaryExpr{
							X:  ast.NewIdent("i"),
							Op: token.ADD,
							Y:  ah.IntLit(1),
						})),
					},
					Tok: token.ASSIGN,
					Rhs: []ast.Expr{
						operatorToReversedBinaryExpr(
							op,
							ah.IndexExpr("data",
								ah.IndexExpr("positions", &ast.BinaryExpr{
									X:  ast.NewIdent("i"),
									Op: token.ADD,
									Y:  ah.IntLit(1),
								}),
							),
							ast.NewIdent("localKey"),
						),
						operatorToReversedBinaryExpr(
							op,
							ah.IndexExpr("data", ah.IndexExpr("positions", ast.NewIdent("i"))),
							ast.NewIdent("localKey"),
						),
					},
				},
			),
		},
	)
}