Mod 26 — picoCTF

Challenge

Cryptography can be easy, do you know what ROT13 is?

cvpbPGS{arkg_gvzr_V'yy_qb_zber_gunaa_ebg13_Ncualgvf}

The title literally says "Mod 26", and the hint specifies ROT-13. ROT-13 is just Caesar with shift 13, which is its own inverse modulo 26.

Solve

Any of these works:

# tr
echo "cvpbPGS{arkg_gvzr_V'yy_qb_zber_gunaa_ebg13_Ncualgvf}" \
  | tr 'A-Za-z' 'N-ZA-Mn-za-m'
 
# python
python3 -c "import codecs; print(codecs.decode(\"cvpbPGS{arkg_gvzr_V'yy_qb_zber_gunaa_ebg13_Ncualgvf}\", 'rot_13'))"

Output:

picoCTF{next_time_I'll_do_more_thann_rot13_Aphnytis}

Notes

ROT-13 only shuffles letters — punctuation, digits and braces pass through, which is why picoCTF{...} survives intact on output.

Worth keeping tr 'A-Za-z' 'N-ZA-Mn-za-m' in muscle memory: it does the round-trip without any code.