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Basic Android RE 1
Link to the Room : https://ctflearn.com/challenge/962

Date Start : 5/11/2023 Date Finish : 5/11/2023

Reviewed any writeup : N/A

Tools :
[1] BlueStacks - For opening apk
[2] JADX-GUI - For reversing apk
[3] https://hashes.com/en/tools/hash_identifier - For identifying the hash
[4] https://hashes.com/en/decrypt/hash - For decrypting MD5

OS : Windows 10

Procedure :

  1. Download the apk (and all the Tools if you do not have)

2) Use Tool [2] and review source code directly into "com.example.secondapp" > "MainActivity"

3a) You will see Flag string at the bottom (line 26) ((TextView) findViewById(R.id.textView)).setText("Success! CTFlearn{" + editText.getText().toString() + "_is_not_secure!}");

3b) in upper line you will see hash code (line 25) if (DigestUtils.md5Hex(editText.getText().toString()).equalsIgnoreCase("b74dec4f39d35b6a2e6c48e637c8aedb")) {

4) use Tools [3] to identfy the hash and find out it is MD5

5) use Tools [4] to decrypt MD5 code in 3b)

6) You will get the password
7) use Tools [1] to open the apk
8) enter the password and you will get the flag