OverTheWire Krypton Level 5 → 6 tutorial!!

Login

Log in as krypton5 using the password from Level 4 → 5.

ssh krypton5@krypton.labs.overthewire.org -p 2231
# password: CLEARTEXT

Why? Each level is a different user. To solve 5 → 6, you must be krypton5.

Task

This level is another Vigenère cipher, but this time the key length is unknown. The password for the next level is in krypton6.

A little bit of Theory

• In a Vigenère cipher:

  • If key length is unknown, you can use Kasiski or Friedman test to guess it.
  • Online solvers can do both automatically: detect key length and recover the key.

• Once the key is found, decryption is direct.

Further reading:

• Kasiski examination
• dCode Vigenère solver

Solution

1. Explore the directory

   cd /krypton/krypton5
   ls -la
   cat README
   

   Output (excerpt):

   Frequency analysis can break a known key length as well.
   Lets try one last polyalphabetic cipher, but this time the key length is unknown.
   

   

2. Get a long ciphertext and run automatic decryption

   • Copy the content of found1 (or found2) into dCode – Vigenère solver.
   • Select Automatic Decryption (no need to provide key length).
   • The solver proposes a key.

   Example result:

   Key: KEYLENGTH
   

   

3. Decrypt the target file krypton6

   • Content of krypton6 is a short ciphertext: BELOS Z.
   • Back on dCode, enter BELOS Z as ciphertext.
   • Choose KNOWING THE KEY/PASSWORD and set it to KEYLENGTH.
   • Decrypt → result:

   RANDOM
   

   

4. Log into the next level

   ssh krypton6@krypton.labs.overthewire.org -p 2231
   # password: RANDOM
   

Password

RANDOM

Troubleshooting

• Wrong output → Ensure you used Automatic Decryption on a long ciphertext (found1, found2, found3).
• Multiple candidate keys → Pick the one producing the most readable English.
• Offline route → Use kasiski or freq analysis manually to estimate key length, then crack each Caesar column.

Copy-paste quick run

ssh krypton5@krypton.labs.overthewire.org -p 2231
# password: CLEARTEXT

cd /krypton/krypton5
# Paste found1 into dCode -> auto decrypt -> key KEYLENGTH
# Decrypt krypton6 with key KEYLENGTH -> RANDOM

ssh krypton6@krypton.labs.overthewire.org -p 2231
# password: RANDOM

Congrats 🎉 You just broke a Vigenère cipher without knowing the key length, and advanced to krypton6!

Thanks for reading!

Until next time — Otsumachi!! 💖☄️✨