vishwaCTF 2021: Rotations | szymanski.ninja

Rotations

Category: Reverse Engineering

450 points

Rotations make you dizzy…don’t they?

file: mm

Solution

Attached file is a binary.

file mm

mm: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, 
interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=9d1420344c3a7c70c70b68b947ef2ec8ae498eb1,
for GNU/Linux 3.2.0, not stripped

./mm
bla bla bla
EWWWW DUMBBB

Decompiled the binary with Ghidra. ghidra

As it is Little Endian, 0x434445464748494a equals to JIHGFEDC and 0x41414141414142 equals to BAAAAAA.

At this moment I skipped flag() investigation (it was not necessary).

./mm
JIHGFEDCBAAAAAA
ivfujnPGS{s1Nt_1f_e0g4gRq_Ol_!3}

To sort out this roted flag I wrote python script.

from string import ascii_letters, digits
flag_encoded = 'ivfujnPGS{s1Nt_1f_e0g4gRq_Ol_!3}'
flag_decoded = ''
for c in flag_encoded:
    if c not in '{}_' + digits:
        if chr(ord(c) + 13) in ascii_letters:
            flag_decoded += chr(ord(c) + 13)
        elif chr(ord(c) - 13) in ascii_letters:
            flag_decoded += chr(ord(c) - 13)
        else:
            flag_decoded += c
    else:
        flag_decoded += c
print(flag_decoded)

Flag

vishwaCTF{f1Ag_1s_r0t4tEd_By_!3}