ctf-rev

CTF reverse engineering methodology

1. Triage

file ./bin
strings ./bin | head -50
strings ./bin | grep -i "flag\|ctf{\|password\|key"
xxd ./bin | head -20                       # magic bytes
checksec --file=./bin                      # protections

2. Detect packing

# Entropy check (>7.5 = packed/encrypted)
ent ./bin   # or: python3 -c "from collections import Counter; ..."

# UPX / known packers
upx -t ./bin            # tests + identifies UPX
detect-it-easy-cli ./bin
diec ./bin

If UPX-packed: upx -d ./bin -o unpacked. For custom packers: dump from memory after unpacking stub runs (gdb / x64dbg).

3. Pick the decompiler

Binary type	Best tool
ELF / PE / Mach-O	Ghidra (registry), IDA (commercial), Binary Ninja
Stripped ELF	Ghidra + recover symbols via FunctionID / Lumen
.NET (DLL/EXE)	dnSpyEx, ilspycmd, dotPeek
Java JAR	jadx, cfr, procyon
Java class	javap -c -p
Android APK	jadx-gui, apktool d then jadx on dex
iOS / Mach-O	Hopper, Ghidra
Go binary	redress, GoReSym, Ghidra + Go plugin
Rust	Ghidra + rustfilt for symbols
WASM	wabt (wasm-decompile), wasmer for run
Python .pyc	uncompyle6, decompyle3, pycdc
PyInstaller .exe	pyinstxtractor then pycdc on .pyc
Compiled Lua	unluac, luadec

4. Dynamic analysis

# Trace
ltrace ./bin
strace ./bin
strace -f -e trace=read,write,open ./bin

# Debugger
gdb-multiarch ./bin
# pwndbg or gef extensions are loaded by default

# Fault injection / branch flipping
gdb> set $eax = 1     # change return value to bypass check

For Android: frida for runtime instrumentation, objection on top.

5. Anti-debug / anti-VM

Common checks:

• ptrace(PTRACE_TRACEME) returns -1 if already debugged → patch with nop
• /proc/self/status TracerPid: 0 check → LD_PRELOAD a fake or patch
• timing checks (rdtsc) → patch
• IsDebuggerPresent() (Windows) → patch the call site

Tooling: ScyllaHide (Windows), gdb scripts to auto-bypass, frida to hook.

6. Custom VMs

If you see a big switch dispatcher reading a "bytecode" buffer:

1. Identify the opcode handler table
2. Map opcodes → operations
3. Disassemble the bytecode manually (write a small Python disasm)
4. Reverse the program logic in the decoded bytecode

This is common in harder rev challenges. Don't fight the VM — disassemble it.

7. Symbolic execution / SMT

For challenges where input → boolean check, and check is complex but pure:

• angr — Python symbolic execution. Find path to "win" basic block.
• manticore — alternative.
• z3 directly — when constraints are explicit (e.g., "input[i] ^ key[i] == ...").

Pwntools venv has z3-solver already.

8. Don't waste time on

• Reading every disassembly line top-to-bottom — find main, focus there
• Reimplementing strings, objdump, readelf — use the tools
• Manual hex editing for big patches — use radare2 or Ghidra's patch instructions

After solve

Use the writeup-template skill. Include the key reverse-engineered algorithm or VM disassembly.