解题思路

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导入jadx查看manifest.xml
查看主函数并未发现有价值的东西，于是查看manifest.xml中主函数下一个<activity>
截取FileDataActivity代码

package com.tencent.testvuln;

import android.os.Bundle;
import android.widget.TextView;
import com.tencent.testvuln.c.Encryto;

public class FileDataActivity extends a {
    private TextView c;

    /* access modifiers changed from: protected */
    @Override // com.tencent.testvuln.a
    public void onCreate(Bundle bundle) {
        super.onCreate(bundle);
        setContentView(R.layout.activity_main3);
        this.c = (TextView) findViewById(R.id.textView1);
        this.c.setText(Encryto.decode(this, "9YuQ2dk8CSaCe7DTAmaqAA=="));
    }
}

这里有一个Encryto.decode函数，查看声明

package com.tencent.testvuln.p003c;

/* renamed from: com.tencent.testvuln.c.Encryto */
public class Encryto {
    public static native int checkSignature(Object obj);

    public static native String decode(Object obj, String str);

    public static native String doRawData(Object obj, String str);

    public static native String encode(Object obj, String str);

    public native String HelloLoad();

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

看到JNI就说明调用外部的库文件了，查看资源文件
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放入IDA中分析，shift + F12查看字符串
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双击跳转，再次双击，跳转到汇编代码，按F5转换伪代码

int __cdecl decode(int a1, int a2, int a3, int a4)
{
  char *v4; // esi
  int result; // eax
  char *v6; // esi
  size_t v7; // eax
  int v8; // [esp+0h] [ebp-2Ch]
  int (__cdecl *v9)(int, char *, size_t); // [esp+0h] [ebp-2Ch]
  char v10[20]; // [esp+4h] [ebp-28h] BYREF
  unsigned int v11; // [esp+18h] [ebp-14h]

  v11 = __readgsdword(0x14u);
  if ( checkSignature(a1, a2, a3) == 1 )
  {
    strcpy(v10, "thisisatestkey==");
    v4 = (char *)(*(int (__cdecl **)(int, int, _DWORD))(*(_DWORD *)a1 + 676))(a1, a4, 0);
    v8 = AES_128_ECB_PKCS5Padding_Decrypt(v4, (int)v10);
    (*(void (__cdecl **)(int, int, char *))(*(_DWORD *)a1 + 680))(a1, a4, v4);
    result = (*(int (__cdecl **)(int, int))(*(_DWORD *)a1 + 668))(a1, v8);
  }
  else
  {
    v6 = UNSIGNATURE[0];
    v9 = *(int (__cdecl **)(int, char *, size_t))(*(_DWORD *)a1 + 652);
    v7 = strlen(UNSIGNATURE[0]);
    result = v9(a1, v6, v7);
  }
  return result;
}