BitShifted's RecursiveBC (Java)

Author:
BitShifted

Language:
Java

Upload:
2025-09-13 11:04

Download

Platform:
Multiplatform

Difficulty:
3.5

Quality:
4.5

Arch:
java

Downloads:
17

Size:
38.07 KB

Writeups:
1

Comments:
6

Description

Fun basic java crackme I designed to explore virtualization, and "bytecode in bytecode" or as I like to call it "Recursive Bytecode" Args: N/A (java -jar RecursiveBC.jar) Goal: enter the correct key. the program will print correct/incorrect Features: - Custom VM - Light Obfuscation (basic string and control flow, easy to reverse) - Simple Anti Tamper - Neat JVM-specific loading tricks Have fun!

Comments (6)
Writeups (1)

You must be logged in to post a comment

trendcrusher on 2025-09-27 20:05: [Click to reveal]

BitShifted on 2025-09-27 21:13: @trendcrusher, damn dude! both this and my python one you found a key that wasn't the intended one. What is your methodology? Are you just brute forcing these? I don't see another way you would derive that key...

trendcrusher on 2025-09-27 21:25: Not brute forcing. I extracted and emulated your hidden VM from the JAR, verified its 32-bit instruction semantics, and only then ran a tiny constrained search over printable chars to satisfy the final 0x6A3B7FF5 check.

BitShifted on 2025-09-27 21:26: Got it. So you basically reversed it far enough to find the final hashing algo, then bruteforced that. Fair enough man, good job! Did the VM semantics trip you up at all? I put a lot of work into making them misleading.

trendcrusher on 2025-09-27 21:45: Yeah a couple of the VM semantics did trip me up - the bogus flag updates on some ALU ops and a quirky byte order on loads made my first emulator wrong. I fixed it by difftesting tiny bytecode snippets against the real VM until every opcode matched. After that the hash collapsed to a small round function, and I did a constrained search over printable bytes not blind brute force. Nice touch with the misleading dispatch and opaque predicates, that kept it fun

BitShifted on 2025-09-27 21:48: @trendcrusher, glad you had fun doing it man, not many people seem to want to do java crackmes... You should make a writeup if you want to, id love to see your reimplementation of the VM

Show all spoilers

You must be logged in to submit a writeup

Solution by trendcrusher on 2025-09-27 20:45:

Download

crackmes.one

crackmes.one

BitShifted's RecursiveBC (Java)