【Android逆向】破解看雪test3.apk方案一

1. test3.apk 安装到手机

2. 发现其实际逻辑和之前的test2.apk基本一致，逆向so查看到加入了一些检查逻辑

代码：

jstring __fastcall fuck(JNIEnv *env, jclass jcls, jstring str_)
{
......

  if ( !str_ )
    return 0;
  src = (unsigned __int8 *)_JNIEnv::GetStringUTFChars(env, str_, 0);
  v20 = "REAL";
  clazz = _JNIEnv::FindClass(env, "android/os/Build");
  fieldID = _JNIEnv::GetStaticFieldID(env, clazz, "FINGERPRINT", "Ljava/lang/String;");
  StaticObjectField = (_jstring *)_JNIEnv::GetStaticObjectField(env, clazz, fieldID);
  if ( function_check_tracerPID()
    || system_getproperty_check()
    || (StringUTFChars = (char *)_JNIEnv::GetStringUTFChars(env, StaticObjectField, 0), strstr(StringUTFChars, "aosp")) )
  {
    v20 = "FAKE";
  }
  strcat((char *)src, v20);

  obj = (_jobject *)j_o0OoOOOO(env, src);
  Class = _JNIEnv::FindClass(env, "java/security/MessageDigest");
  methodID = _JNIEnv::GetStaticMethodID(env, Class, "getInstance", "(Ljava/lang/String;)Ljava/security/MessageDigest;");
  v4 = j_o0OoOOOO(env, "MD5");
  v14 = _JNIEnv::CallStaticObjectMethod(env, Class, methodID, v4);
  v13 = _JNIEnv::GetMethodID(env, Class, "digest", "([B)[B");
  v12 = _JNIEnv::FindClass(env, "java/lang/String");
  v11 = _JNIEnv::GetMethodID(env, v12, "getBytes", "()[B");
  v10 = _JNIEnv::CallObjectMethod(env, obj, v11);
  array = (_jbyteArray *)_JNIEnv::CallObjectMethod(env, v14, v13, v10);
  ByteArrayElements = _JNIEnv::GetByteArrayElements(env, array, 0);
  for ( i = 0; i <= 15; ++i )
    sprintf((char *)&v27[i], "%02x", (unsigned __int8)ByteArrayElements[i]);
  v26 = (char *)j_ll11l1l1ll(src);
  strcat(v26, (const char *)v27);
  v6 = j_o0OoOOOO(env, (const unsigned __int8 *)v26);
  _JNIEnv::ReleaseStringUTFChars(env, str_, src);
  free(v26);
  return v6;
}

从这里可以看到，增加了一个检查system_getproperty_check和strstr(StringUTFChars, "aosp"),如果满足条件就会把 REAL 改为 FAKE，追加到字符串后面，那么就怎么都不会成功得到正确结果了

就不会进入到if块中；方案一是strcat的入参，让第二个参数传入时永远是REAL

方案一代码：

function main() {
    Java.perform(function () {
        hookCheck()
        var MainActivityHandler = Java.use('com.roysue.easyso1.MainActivity')

        for (var i = 87650; i <= 87700; i++) {
            var str = i + ""
            var ret = MainActivityHandler.Sign(str)
            if (i % 1000 == 0) {
                console.log("now is", str);
            }
            //console.log(ret)
            if (ret == "57fdeca2cac0509b2e9e5c52a5b573c1608a33ac1ffb9e8210d2e129557e7f1b") {
                console.log("find it : " + str)
                break
            }
        }

        console.log("end : ")
    })

}

function hookCheck() {
    var lib_hanlder = Process.findModuleByName("libroysue.so");
    console.log("lib_handler: " + lib_hanlder)

    if (lib_hanlder) {
        var addr_hook = lib_hanlder.base.add(0x00037226 + 0x1)

        Interceptor.attach(addr_hook,{
            onEnter: function(args) {
                console.log(" === hook before")
                console.log("=== r1:" + this.context.r1.readCString())
                Memory.protect(this.context.r1, 1024, 'rw-')
                this.context.r1.writeUtf8String("REAL")
                console.log("=== r1:" + this.context.r1.readCString())
            },
            onLeave:function(retVal) {
                console.log(" === hook after: ")
                //print_dump(retVal, 128)
                console.log("=== after r1:" + this.context.r1.readCString())

            }
        })
    }
}

setTimeout(main, 2000)

关键汇编代码

.text:00037220 24 91                         STR             R1, [SP,#0xD8+var_48]
.text:00037222 26 98                         LDR             R0, [SP,#0xD8+var_40]   ; char *
.text:00037224 24 99                         LDR             R1, [SP,#0xD8+var_48]   ; char *
.text:00037226 FC F7 00 ED                   BLX             strcat

这里的 0x00037226 就是 strcat 对应的偏移地址，其实也可以hook 0x00037224，结果是一样的（hook的这一行相当于是执行后进入onEnter）

日志:

 === hook before
=== r1:FAKE
=== r1:REAL
 === hook before
=== r1:REAL
=== r1:REAL
 === hook before
=== r1:REAL
=== r1:REAL
 === hook before
=== r1:REAL
=== r1:REAL
 === hook before
=== r1:REAL
=== r1:REAL
find it : 87654
end :

从日志上课 onLeave没执行，原因是这里不是一个标准的函数特征，所以没有入栈出栈逻辑，所以就onLeave没执行