Secure Password Database Writeup

Here is the exact writeup of how I solved it, step by step, including the tools I used.

Files and tools used

• File: system.out
• Tools:
• file
• checksec
• strings
• nm
• objdump
• python3
• nc

1) Inspect binary and symbols

file system.out
checksec --file=system.out
strings -n 3 system.out
nm -n system.out

Interesting functions:

• hash
• make_secret
• main

2) Reverse auth logic

objdump -d -Mintel --start-address=0x1309 --stop-address=0x13d0 system.out
objdump -d -Mintel --start-address=0x13d0 --stop-address=0x1754 system.out
objdump -s --start-address=0x2000 --stop-address=0x2200 system.out

Observed bug:

• program asks your password and hash
• but compares input hash against hash of internal decoded secret, not your password

3) Recover internal secret

make_secret XOR-decodes obfuscated bytes with 0xAA.

Obfuscated bytes:

• c3 ff c8 c2 92 9b 8b c0 80 c2 c4 8b

Decode:

python3 - << 'PY'
obf = bytes.fromhex('c3ffc8c2929b8bc080c2c48b')
print(bytes([b ^ 0xAA for b in obf]).decode())
PY

Recovered secret:

• iUbh81!j*hn!

4) Reproduce hash function

From reverse:

• seed = 0x1505
• loop = h = h*33 + byte (64-bit wrap)

python3 - << 'PY'
h = 0x1505
for b in b'iUbh81!j*hn!':
    h = (h*33 + b) & 0xffffffffffffffff
print(h)
PY

Computed hash:

• 15237662580160011234

5) Authenticate remotely

nc candy-mountain.picoctf.net 54587

Use any password text, then provide hash:

• 15237662580160011234

Final flag

picoCTF{<redacted>}