Reverse Engineering easy

nimrod

ImaginaryCTF 2025 · 2025 · Jul 5, 2025

Stripped Nim binary that XORs input with a keystream derived from a hard seed. Extract the keystream at runtime with gdb and XOR out the flag.

#nim #xor #gdb #keystream
source on GitHub

Category: Reversing · Difficulty: Easy · Author: Eth007

“And Cush begat Nimrod: he began to be a mighty one in the earth.”

Recon

nimrod: ELF 64-bit LSB pie executable, x86-64, dynamically linked,
        for GNU/Linux 3.2.0, not stripped

A Nim-compiled ELF. Running it:

$ ./nimrod
Enter the flag:

Any random input prints Incorrect..

Finding the check

In Ghidra, main boils down to:

uVar1 = xorEncrypt__nimrod_46(userInput, 0x13371337);
cVar2 = eqeq___nimrod_69(uVar1, encryptedFlag__nimrod_10);
if (cVar2 == '\0') echoBinSafe(..., "Incorrect.");
else              echoBinSafe(..., "Correct!");
  • Input is XOR-encrypted with seed 0x13371337.
  • Result is compared against a global encryptedFlag__nimrod_10 in .rodata.

Inspecting the global:

0x116e0: len=0x22   cap=...
0x116f0: 28 f8 3e e6 3e 2f 43 0c ...

Ciphertext is 0x22 = 34 bytes.

xorEncrypt__nimrod_46 calls keystream__nimrod_20(0x13371337, len) and XORs the result with the input.

Strategy

No need to re-implement keystream. Just let the program run, break at that function, and read the produced keystream straight out of memory — then XOR with the ciphertext.

Nim seq layout: header at the returned pointer, data at header + 0x10.

GDB

$ gdb ./nimrod
(gdb) b keystream__nimrod_20
(gdb) run
Enter the flag:
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa        # 34 chars, matches ciphertext
(gdb) p/x $rsi                            # length arg
$1 = 0x22
(gdb) finish                              # $rax = returned seq header

Now dump both the global ciphertext and the keystream, XOR them together:

(gdb) python
import gdb
inf = gdb.selected_inferior()
N = 0x22

enc_ptr = int(gdb.parse_and_eval("&encryptedFlag__nimrod_10"))
enc_hdr = int.from_bytes(inf.read_memory(enc_ptr, 8).tobytes(), "little")
enc = inf.read_memory(enc_hdr + 0x10, N).tobytes()

ks_hdr = int(gdb.parse_and_eval("$rax"))
ks = inf.read_memory(ks_hdr + 0x10, N).tobytes()

flag = bytes([e ^ k for e, k in zip(enc, ks)])
print(flag.decode())
end

Flag

ictf{a_mighty_hunter_bfc16cce9dc8}