Informatio

The challenge required decrypting passwords in order to form AES key to decrypt the flag bytes. On decompiling flair.apk file we can see four classes for which we need to decrypt passwords. 1.Michael Class  It was pretty simple password comparison.The password is  MYPRSHE__FTW . 2. Brian Class Password is formed as shown below. String.format("%s_%s%x_%s!", new Object[]{t, y, Integer.valueOf(p), c}); t = (ImageView) findViewById(R.id.pfdu).getTag().toString() y = getApplicationContext().getPackageManager().getApplicationInfo(getApplicationContext().getPackageName(), 128).metaData.getString("vdf") p = (TextView) findViewById(R.id.vcxv).getCurrentTextColor() & SupportMenu.USER_MASK; c = (TextView) findViewById(R.id.vcxv).getText().toString().split(" ")[4]; The Password is  hashtag_covfefe_Fajitas! . 3.Milton Class The password is formed by decrypting a string and taking SHA1 of the decrypted string. Decryption algorithm

The challenge is about selecting correct coordinates on to the map and advancing to the next stage to get flag. As we advance to next stage, the game print some metadata. After debugging the binary, the logic to calculate co-ordinate can be rewritten. Below is the python implementation of calculating co-ordinate and decrypting metadata for each stage. import binascii key = 0x3B1EE5F6B3D99FF7                #initial key to decrypt metadata. offset = 0x50E0                         #offset of metadata in binary f = open('pewpewboat.exe','rb') for i in range(0,11):     stage = i     v = ((i << 3) + i) << 6     f.seek(offset + v)     mask = '0x'     temp = '0x'     res = []     metadata = []     for i in range(0,0x240):         key = ((key * 0x41c64e6d) + 0x3039) & 0xFFFFFFFFFFFFFFFF         c = binascii.hexlify(f.read(1))         c = int(c,16)         c = c ^ (key & 0xFF)         metadata.append(chr(c))      

 The CTF is over but i really enjoyed solving the challenge (https://sctf.codeground.org/ctf/prob).The challenge is to decrypt flag.enc. This is another APK + JNI (Java Native Interface) kind of challenge similar to the one in Google CTF 2017 (http://shasaurabh.blogspot.com/2017/07/google-ctf-2017-android-re-challenge.html). Challenge can be divided into three parts. 1.Android application (application) loading Native Library 2.Native Library decrypts DEX file in memory. 3.Using Reflection to call methods of decrypted DEX file in step 2. 1. Android application loading Native Library OnClick() method of MainActivity class calls constructor of Crypt class. Calling constructor of Crypt class results in loading of Native library ( libegg.so ) which is ELF for ARM. Constructor of Crypt class calls crackEgg method implemented in native library. 2.Native Library decrypts DEX file in memory. The native library reads the encrypted DEX file stored in asset fo