某车联网App 通讯协议加密分析(四) Trace Code

一、目标

之前我们已经通过Trace Block 来比对了Unidbg和App跑的结果。现在他们运行的流程都差不多了，但是结果还是不对，今天我们就要通过Trace Code进行更细致的对比。

v6.1.0

二、步骤

缩小Trace的范围

Trace Code那么好使，我们为什么不一上来就Trace一遍？

因为Trace Code的粒度太细了，一上来就搞，跑出几百万行结果，根本没法看。

我们通过Trace Block已经在逐步缩小范围了。

JNIEnv->GetStringUtfChars("FlK6XicivmCwPSE3sk6b71m9WbWd/gYZtlajqGXhEXXjmWEZziR51rVWSEDwUUi4UN9RnoCGbLNmqI80Fiog4Sw==") was called from RX@0x4002b744[libencrypt.so]0x2b744
  sub_2b8b8
  sub_2b800
  sub_2b800
  sub_2b800
  sub_2b800
  sub_2b828
  sub_ab6c
  sub_a528
  sub_ab9c
  sub_a848
  sub_a7e8
  sub_a7c4
  sub_8ee4
  sub_7334
  sub_8f40
  sub_95e0
 ........
  sub_a7c4
  sub_8ee4
  sub_7334
  sub_8f40
  sub_96bc
  sub_9a2c
  sub_9268
  sub_a818
  sub_a90c
  sub_a928
  sub_2b854
JNIEnv->ReleaseStringUTFChars("FlK6XicivmCwPSE3sk6b71m9WbWd/gYZtlajqGXhEXXjmWEZziR51rVWSEDwUUi4UN9RnoCGbLNmqI80Fiog4Sw==") was called from RX@0x4002b86c[libencrypt.so]0x2b86c

我们的目标大概率是在 0x7000 → 0xa000 这个地址范围之类。

定位Trace Code目标

翻一翻IDA里面的导出表

1:ida

比较合眼缘的就这三个了，0x7184 ,0x77A4 貌似都没有被Trace Block命中，感觉幕后大boss应该就是这个 0x8EE4

Tip: 

可以在怀疑的几个函数上加个hook，看看是否命中。

Unidbg Trace Code

emulator.attach().addBreakPoint(module.base + 0x8EE4  , new BreakPointCallback() {
    @Override
    public boolean onHit(Emulator<?> emulator, long address) {
        System.out.println(" ====== traceCode ====== ");

        UnidbgPointer pX = UnidbgPointer.register(emulator, Arm64Const.UC_ARM64_REG_X1);

        byte[] bData = pX.getByteArray(0,16);
        String strLabel = String.format("x0值 0x%08x", pX.peer);
        Inspector.inspect(bData,strLabel);

        try {
            emulator.traceCode(module.base + 0x8EE4, module.base + 0x9C0C).setRedirect(new PrintStream(new File("traceCodeCar.log")));
        } catch (IOException e) {
            throw new IllegalStateException(e);
        }

        return true;
    });

}

先在 0x8EE4 加个断点，然后开始Trace Code。 参数是Trace的起始地址和结束地址，已经保存的结果文件。

从这个Trace结果来看，每16个字节去做解密。典型的AES。

Frida Trace Code

Stalker的好处很多，最大的功能是可以过反调试，所以基于Stalker去写Trace Code,写了好几个版本，一直不大满意。

前几天发现了

github.com/IIIImmmyyy/…

比较帅，拿来改了改。效果很好。(只支持Arm64)

let moduleBase;
let isFirstIn = true;
let pre_regs;
let infoMap = new Map();
let detailInsMap = new Map();

function parserNextAddr(ins) {
    let s = JSON.stringify(ins);
    let address = ins.address;
    // log("address:"+address)
    let offset = address - moduleBase;
    let s1 = (offset).toString(16);
    let entity = {};
    entity.address = offset;
    return s1;
}

const byteToHex = [];
for (let n = 0; n <= 0xff; ++n) {
    const hexOctet = n.toString(16).padStart(2, "0");
    byteToHex.push(hexOctet);
}

function hex(arrayBuffer) {
    const buff = new Uint8Array(arrayBuffer);
    const hexOctets = [];
    for (let i = 0; i < buff.length; ++i)
        hexOctets.push(byteToHex[buff[i]]);
    return hexOctets.join("");
}

function formatArm64Regs(context) {
    let regs = [];
    regs.push(context.x0);
    regs.push(context.x1);
    regs.push(context.x2);
    regs.push(context.x3);
    regs.push(context.x4);
    regs.push(context.x5);
    regs.push(context.x6);
    regs.push(context.x7);
    regs.push(context.x8);
    regs.push(context.x9);
    regs.push(context.x10);
    regs.push(context.x11);
    regs.push(context.x12);
    regs.push(context.x13);
    regs.push(context.x14);
    regs.push(context.x15);
    regs.push(context.x16);
    regs.push(context.x17);
    regs.push(context.x18);
    regs.push(context.x19);
    regs.push(context.x20);
    regs.push(context.x21);
    regs.push(context.x22);
    regs.push(context.x23);
    regs.push(context.x24);
    regs.push(context.x25);
    regs.push(context.x26);
    regs.push(context.x27);
    regs.push(context.x28);
    regs.push(context.fp);
    regs.push(context.lr);
    regs.push(context.sp);
    regs.push(context.pc);
    return regs;
}
function getPcReg(regs) {
    return regs[32];
}
function isRegsChange(context, ins) {
    let currentRegs = formatArm64Regs(context);
    let logInfo = "";
    for (let i = 0; i < 32; i++) {
        if (i === 30) {
            continue;
        }
        let preReg = pre_regs[i];
        let currentReg = currentRegs[i];
        if (Number(preReg) !== Number(currentReg)) {
            if (logInfo === "") {
                //尝试读取string
                let changeString = "";
                try {
                    let nativePointer = new NativePointer(currentReg);
                    changeString = nativePointer.readCString();
                }
                catch (e) {
                    changeString = "";
                }
                if (changeString !== "") {
                    currentReg = currentReg + "   (" + changeString + ")";
                }
                logInfo = "\t " + getRegsString(i) + " = " + preReg + " --> " + currentReg;
            }
            else {
                logInfo = logInfo + "\t " + getRegsString(i) + " = " + preReg + " --> " + currentReg;
            }
        }
    }
    //打印PC寄存器
    let parse = JSON.parse(ins);
    let mnemonic = parse.mnemonic; //补充str
    if (mnemonic === "str") {
        let strParams = getStrParams(parse, currentRegs);
        logInfo = logInfo + strParams;
    }
    else if (mnemonic === "cmp") {
        let cmpParams = getCmpParams(parse, currentRegs);
        logInfo = logInfo + cmpParams;
    }
    else if (mnemonic === "b.gt" || mnemonic === "b.le" || mnemonic === "b.eq" || mnemonic === "b.ne" || mnemonic === "b") {
        // log(ins)
        let bgtAddr = getbgtAddr(parse, currentRegs);
        logInfo = logInfo + bgtAddr;
    }
    let entity = {};
    entity.info = logInfo;
    let address = parse.address;
    if (lastAddr === undefined) {
        entity.color = getColor();
        lastAddr = address;
    }
    else {
        let number = address - lastAddr;
        if (number === 0x4) {
            entity.color = getColor();
        }
        else {
            currentIndex++;
            entity.color = getColor();
        }
        lastAddr = address;
    }
    pre_regs = currentRegs;
    return entity;
}
let lastAddr = undefined;
let currentIndex = 0;
function getColor() {
        return "";

    if (currentIndex > 1) {
        currentIndex = 0;
    }
    if (currentIndex === 0) {
        return "C35";  // logger_1.LogColor.C35;
    }
    else if (currentIndex === 1) {
        return "C97";  // logger_1.LogColor.C97;
    }
    else if (currentIndex === 2) {
        return "C97";  // logger_1.LogColor.C97;
    }
}
function getRegsString(index) {
    let reg;
    if (index === 31) {
        reg = "sp";
    }
    else {
        reg = "x" + index;
    }
    return reg;
}
function getbgtAddr(parser, currentRegs) {
    let bgtAddr = "";
    let operands = parser.operands;
    for (let i = 0; i < operands.length; i++) {
        let operand = operands[i];
        if (operand.type === "imm") {
            let value = operand.value;
            let number = value - moduleBase;
            bgtAddr = "\t block addr:" + number.toString(16);
            break;
        }
    }
    return bgtAddr;
}
function getStrParams(parser, currentRegs) {
    let operands = parser.operands;
    for (let i = 0; i < operands.length; i++) {
        let operand = operands[i];
        if (operand.type === "reg") {
            //获取value
            let value = operand.value;
            if (value === "wzr") {
                return "\t " + "str = 0";
            }
            else {
                let replace = value.replace("w", "");
                let index = replace.replace("x", "");
                let index_reg = currentRegs[index];
                let changeString = "";
                try {
                    let nativePointer = new NativePointer(index_reg);
                    changeString = nativePointer.readCString();
                }
                catch (e) {
                    changeString = "";
                }
                //读取值
                if (changeString !== "") {
                    index_reg = index_reg + "   (" + changeString + ")";
                }
                return "\t " + "str = " + index_reg;
            }
        }
    }
}
function getCmpParams(parser, currentRegs) {
    let operands = parser.operands;
    let cmpInfo = "";
    for (let i = 0; i < operands.length; i++) {
        let operand = operands[i];
        if (operand.type === "reg") {
            let value = operand.value;
            let replace = value.replace("w", "");
            let index = replace.replace("x", "");
            let index_reg = currentRegs[index];
            let changeString = "";
            try {
                let nativePointer = new NativePointer(index_reg);
                changeString = nativePointer.readCString();
            }
            catch (e) {
                changeString = "";
            }
            //读取值
            if (changeString !== "") {
                index_reg = index_reg + "   (" + changeString + ")";
            }
            cmpInfo = cmpInfo + "\t " + value + " = " + index_reg;
        }
    }
    return cmpInfo;
}

function ZY_unTraceAddrEnd(){
    var tid = Process.getCurrentThreadId();
    Stalker.unfollow(tid);
    Stalker.garbageCollect();
    console.log(TAG + " ======== unTraceAddr_End");
}

function ZY_trace_Stalker_begin(soname, addr, size) {
    let module = Process.findModuleByName(soname);
    moduleBase = module.base;
    console.log(TAG + JSON.stringify(module));
        console.log(TAG + "addr = " + addr);
        console.log(TAG + "size = " + size);

    Interceptor.attach(moduleBase.add(addr), {
        onEnter: function (args) {
            this.pid = Process.getCurrentThreadId();
                        // console.log(TAG + " ==== ZY_trace_Stalker_begin ==== ");

            //看下结构体的值
            Stalker.follow(this.pid, {
                events:{
                    call:false,
                    ret:false,
                    exec:false,

                    block:false,
                    compile:false
                },
                onReceive:function(events){
                },

                transform: function (iterator) {
                    let lastInfo;
                    const instruction = iterator.next();
                    let startAddress = instruction.address;
                    // console.log(TAG + "startAddress:" + startAddress + " base:" + module.base );

                    if (size === 0) {
                        size = module.size;
                        addr = 0;
                    }

                    const isModuleCode = startAddress.compare(moduleBase.add(addr)) >= 0 &&
                                                             startAddress.compare(moduleBase.add(addr).add(size)) < 0;
                    do {
                        if (isModuleCode) {
                                                        // console.log(TAG + instruction.address + ":" + instruction);

                            let s = parserNextAddr(instruction);
                            let address = instruction.address;
                            let offset = address - moduleBase;
                            let lastInfo = s.toString(16) + "\t\t" + instruction;
                            detailInsMap.set(offset, JSON.stringify(instruction));
                            infoMap.set(offset, lastInfo);
                            iterator.putCallout(function (context) {
                                let regs = JSON.stringify(context);
                                if (isFirstIn) {
                                    isFirstIn = false;
                                    //保存寄存器
                                    pre_regs = formatArm64Regs(context);
                                }
                                else {
                                    //打印的实际是上一次的 这样延迟一次可以打印出寄存器变化
                                    let pcReg = getPcReg(pre_regs);
                                    let offset = Number(pcReg) - moduleBase;
                                    let logInfo = infoMap.get(offset);
                                    let detailIns = detailInsMap.get(offset);
                                    // log("detailIns:"+detailIns)
                                    let entity = isRegsChange(context, detailIns);
                                    console.log(TAG + logInfo + " ; " + entity.info, entity.color);
                                }
                            });
                        }
                        iterator.keep();
                    } while (iterator.next() != null);
                },
            });
        },
        onLeave: function (ret) {
            // libtprt.saveStringMapTofile();
            Stalker.unfollow(this.pid);
            console.log(TAG + "ret:" + ret);
        }
    });
}

调用方法

ZY_trace_Stalker_begin('libencrypt.so',0x8EF4, 0x9C0C - 0x8EF4);

Tip: 

Trace之前可以先匹配入参，只Trace指定的密文。然后unidbg 去 Trace 同样的密文，这样有利于比对。

对比结果

前戏铺垫的太长了，总算拿到Trace Code的结果了。

1:main

这里有个很奇怪的地方，

9608: "ldr w8, [x9, x8, lsl #2]" x9=0x40147510 x8=0x9094 => w8=0xf54de125

0x40147510 这个地址是在so的数据段。 这段代码的意思是 从 0x40147510+(0x9094<<2) = 0x4016B760 这个地址取数据

m0x4016B760

>-----------------------------------------------------------------------------<
[11:57:20 551]RW@0x4016b760[libencrypt.so]0x16b760, md5=cc2b1f1f88429f40f84599a613cf3143, hex=25e14df5a6ea8219a148998cebd3d9b5e55522c3109f9bd0347f27854da2729b83f3a390ce0c318645a588f14b339626692918f7119819acb36b0f92cd0dc4d596a74b1bc8f1263f2841c72757b265b4b1692c56a24a9c8f528a41e0d1d95149e25a0120a9e884147d223f7323eb4ff2
size: 112
0000: 25 E1 4D F5 A6 EA 82 19 A1 48 99 8C EB D3 D9 B5    %.M......H......
0010: E5 55 22 C3 10 9F 9B D0 34 7F 27 85 4D A2 72 9B    .U".....4.'.M.r.

从unidbg里面打印一下，没毛病。 就是  25 E1 4D F5

但是对应的App的结果就有点意思了

9608                ldr w8, [x9, x8, lsl #2] ;          x8 = 0x90f7 --> 0x73b55372

0x40147510+(0x90f7<<2) = 0x4016B8EC

m0x4016B8EC

>-----------------------------------------------------------------------------<
[12:00:52 810]RW@0x4016b8ec[libencrypt.so]0x16b8ec, md5=cd71edc06f5f49b56a700c1f6e541610, hex=e35324c7bb9b61e3b06ee0f4f571fb062a44630bc501d369b49e3e2cee56eaeb463ab0c57e5c3b12a8abf8f81264047dd98a871bc1b84e9d807db83241a8ee1d0be8aeecd8e1328a7311b4a73e2c9b925f24946cc35eacb6953472a0b4cdc9baebce5c65701ab7afa0f48251f1e5afab
size: 112
0000: E3 53 24 C7 BB 9B 61 E3 B0 6E E0 F4 F5 71 FB 06    .S$...a..n...q..
0010: 2A 44 63 0B C5 01 D3 69 B4 9E 3E 2C EE 56 EA EB    *Dc....i..>,.V..

这个地址对应的结果不对。 难道App会在内存中变异？

m0x4017B8EC

>-----------------------------------------------------------------------------<
[12:01:51 113]RW@0x4017b8ec[libencrypt.so]0x17b8ec, md5=09b5524828f471a42a3e75beed7b23a8, hex=7253b573f596a2d945a40787a842b3a0e46eadcc761474b12000e2a1ec0ab7053cca47f537ed81c4b823dc9866cc99a7b5b4b4adb3fffa867aa45080ea698ddb04e771872ee075f999b9ba241908bc5ac29666eb5f6a7800abee8213d52a28f19321df19b077a1716dbf3ae72a10f33f
size: 112
0000: 72 53 B5 73 F5 96 A2 D9 45 A4 07 87 A8 42 B3 A0    rS.s....E....B..
0010: E4 6E AD CC 76 14 74 B1 20 00 E2 A1 EC 0A B7 05    .n..v.t. .......

在unidbg的内存里面搜索了一下，发现加上 0x10000 之后的值恰好是App里显示的一致。

真相只有一个

pc时代过来的老同学就很敏感了。这个so并不是原始so，而是我们在内存中dump出来的。

我们观察下原始so的节表

fenfei$ greadelf -l libencrypt.so

Elf file type is DYN (Shared object file)
Entry point 0x3210
There are 5 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x000000000002f288 0x000000000002f288  R E    0x10000
  LOAD           0x000000000002fba0 0x000000000003fba0 0x000000000003fba0
                 0x0000000000194a08 0x0000000000194ab8  RW     0x10000
  DYNAMIC        0x00000000001c5b90 0x000000000003fc00 0x000000000003fc00
                 0x0000000000000210 0x0000000000000210  RW     0x8
readelf: Error: no .dynamic section in the dynamic segment
  GNU_EH_FRAME   0x000000000002cd10 0x000000000002cd10 0x000000000002cd10
                 0x000000000000048c 0x000000000000048c  R      0x4
  LOAD           0x00000000001c5078 0x00000000001d5078 0x00000000001d5078
                 0x0000000000000afc 0x0000000000000afc  R E    0x1000

文件地址 0x2fba0 映射到了 内存地址 0x3fba0

所以我们dump出来的so要修复这个映射

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
                 0x000000000002f288 0x000000000002f288  R E    0x10000
  LOAD           0x000000000003fba0 0x000000000003fba0 0x000000000003fba0
                 0x0000000000194a08 0x0000000000194ab8  RW     0x10000
  DYNAMIC        0x00000000001c5b90 0x000000000003fc00 0x000000000003fc00
                 0x0000000000000210 0x0000000000000210  RW     0x8
readelf: Error: no .dynamic section in the dynamic segment
  GNU_EH_FRAME   0x000000000002cd10 0x000000000002cd10 0x000000000002cd10
                 0x000000000000048c 0x000000000000048c  R      0x4
  LOAD           0x00000000001c5078 0x00000000001d5078 0x00000000001d5078
                 0x0000000000000afc 0x0000000000000afc  R E    0x1000

把我们dump出的so头里面的 0x2fba0 改成 0x3fba0

这次终于可以成功解密了

call decheckcode: {"code":"0","message":"success","respondData":{"serverTime":1663643024410,"timeSpan":"5","loginFlag":0}}

三、总结

忙活了老半天，其实最后只做了一下修复so文件头。严格意义上只改了3个字节。

改3个字节很简单，分析并知道如何改，再哪改，才是我们的重点。

Trace Function 、Trace Block 、Trace Code。逐渐缩小范围来定位。

还可以通过不同的入参来TraceCode，对比一下更有助于分析算法。

1:ffshow

纵浪大化中 不喜亦不惧 应尽便须尽 无复独多虑

Tip: 

: 本文的目的只有一个就是学习更多的逆向技巧和思路，如果有人利用本文技术去进行非法商业获取利益带来的法律责任都是操作者自己承担，和本文以及作者没关系，本文涉及到的代码项目可以去 奋飞的朋友们 知识星球自取，欢迎加入知识星球一起学习探讨技术。有问题可以加我wx: fenfei331 讨论下。

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