【Android逆向】破解看雪 test1.apk

1. 获取apk，并安装至手机

apk 获取地址： https://www.kanxue.com/work-task_read-800624.htm

adb install -t test1.apk
# 这个apk必须加-t ,否则会报错

2. 只有一个输入框，随便输入内容，提示壮士继续加油

3. 将apk拖入到jadx中观察

public class MainActivity extends AppCompatActivity {
    TextView message_tv;
    EditText password_et;
    EditText username_et;

    public static native String doMath(byte[] bArr);

    static {
        System.loadLibrary("roysue");
    }

    /* JADX INFO: Access modifiers changed from: protected */
    @Override // androidx.appcompat.app.AppCompatActivity, androidx.fragment.app.FragmentActivity, androidx.activity.ComponentActivity, androidx.core.app.ComponentActivity, android.app.Activity
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        this.username_et = (EditText) findViewById(R.id.editText);
        this.message_tv = (TextView) findViewById(R.id.textView);
        findViewById(R.id.button).setOnClickListener(new View.OnClickListener() { // from class: com.roysue.easyso1.MainActivity.1
            @Override // android.view.View.OnClickListener
            public void onClick(View v) {
                String res = MainActivity.doMath(MainActivity.this.username_et.getText().toString().getBytes());
                if (res.compareTo("cjB5c3VlbGwwdmV5MHVzMG11Y2g=\n") == 0) {
                    MainActivity.this.message_tv.setText("恭喜你！");
                } else {
                    MainActivity.this.message_tv.setText("壮士请继续加油！");
                }
            }
        });
    }
}

4. 从代码中疑似感觉这个字符串是个base64加密的字符串，看看native中的逻辑确定一下猜想

5. 加压apk将libroysue.so拖入IDA中进行观察

【1】导出表中搜索 doMath函数，发现没有，说明是动态注册，搜索JNI_OnLoad

jint JNI_OnLoad(JavaVM *vm, void *reserved)
{
  JNIEnv *env; // [sp+10h] [bp-10h] BYREF

  env = 0;
  if ( _JavaVM::GetEnv(vm, (void **)&env, 65542) )
    return -1;
  if ( !env )
    _assert2(
      "/root/Desktop/202104test/easyso1/app/src/main/cpp/roysue.cpp",
      60,
      "jint JNI_OnLoad(JavaVM *, void *)",
      "env != nullptr");
  if ( registerMethods(env, "com/roysue/easyso1/MainActivity", method_table, 1) )
    return 65542;
  else
    return -1;
}

说明关联的函数在 method_table中，点击观察 method_table

.data:00004000                               ; ===========================================================================
.data:00004000
.data:00004000                               ; Segment type: Pure data
.data:00004000                               AREA .data, DATA
.data:00004000                               ; ORG 0x4000
.data:00004000                               ; JNINativeMethod method_table[1]
.data:00004000 09 26 00 00 10 26 00 00 B1 09+_ZL12method_table JNINativeMethod <aDomath, aBLjavaLangStri, _Z3mmmP7_JNIEnvP7_jclassP11_jbyteArray+1>
.data:00004000 00 00                                                                 ; DATA XREF: JNI_OnLoad+56↑o
.data:00004000                                                                       ; JNI_OnLoad+58↑o
.data:00004000                                                                       ; .text:off_B74↑o
.data:00004000                                                                       ; mmm(_JNIEnv *,_jclass *,_jbyteArray *) ...
.data:0000400C                               ; unw_addr_space_t unw_local_addr_space
.data:0000400C 10 40 00 00                   unw_local_addr_space DCD _ZN9libunwind17LocalAddressSpace17sThisAddressSpaceE ; libunwind::LocalAddressSpace::sThisAddressSpace
.data:0000400C                               ; .data ends
.data:0000400C

可以看到 Domath 关联的函数叫mmm(aDomath,aBLjavaLangStri,_Z3mmmP7_JNIEnvP7_jclassP11_jbyteArray+1)
点击_Z3mmmP7_JNIEnvP7_jclassP11_jbyteArray，观察

jobject __fastcall mmm(JNIEnv *env, jclass clazz, jbyteArray bytearray)
{
  _jmethodID *StaticMethodID; // r0
  _jclass *clazza; // [sp+10h] [bp-18h]

  clazza = _JNIEnv::FindClass(env, "android/util/Base64");
  StaticMethodID = _JNIEnv::GetStaticMethodID(env, clazza, "encodeToString", "([BI)Ljava/lang/String;");
  return _JNIEnv::CallStaticObjectMethod(env, clazza, StaticMethodID, bytearray, 0);
}

由此可得，确实是对传入的字符串进行base64处理，然后对结果进行比对，
于是解码可得flag为r0ysuell0vey0us0much

adb shell input text "r0ysuell0vey0us0much" 提交通过