Krypton2

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Level Info

ROT13 is a simple substitution cipher.

Substitution ciphers are a simple replacement algorithm. In this example of a substitution cipher, we will explore a ‘monoalphebetic’ cipher. Monoalphebetic means, literally, “one alphabet” and you will see why.

This level contains an old form of cipher called a ‘Caesar Cipher’. A Caesar cipher shifts the alphabet by a set number. For example:

plain:  a b c d e f g h i j k ...
cipher: G H I J K L M N O P Q ...

In this example, the letter ‘a’ in plaintext is replaced by a ‘G’ in the ciphertext so, for example, the plaintext ‘bad’ becomes ‘HGJ’ in ciphertext.

The password for level 3 is in the file krypton3. It is in 5 letter group ciphertext. It is encrypted with a Caesar Cipher. Without any further information, this cipher text may be difficult to break. You do not have direct access to the key, however you do have access to a program that will encrypt anything you wish to give it using the key. If you think logically, this is completely easy.

One shot can solve it!

Have fun.

Additional Information:

The encrypt binary will look for the keyfile in your current working directory. Therefore, it might be best to create a working direcory in /tmp and in there a link to the keyfile. As the encrypt binary runs setuid krypton3, you also need to give krypton3 access to your working directory.

Here is an example:

krypton2@melinda:~$ mktemp -d
/tmp/tmp.Wf2OnCpCDQ
krypton2@melinda:~$ cd /tmp/tmp.Wf2OnCpCDQ
krypton2@melinda:/tmp/tmp.Wf2OnCpCDQ$ ln -s /krypton/krypton2/keyfile.dat
krypton2@melinda:/tmp/tmp.Wf2OnCpCDQ$ ls
keyfile.dat
krypton2@melinda:/tmp/tmp.Wf2OnCpCDQ$ chmod 777 .
krypton2@melinda:/tmp/tmp.Wf2OnCpCDQ$ /krypton/krypton2/encrypt /etc/issue
krypton2@melinda:/tmp/tmp.Wf2OnCpCDQ$ ls
ciphertext  keyfile.dat

Study guide

Level map: logged in as krypton2, decrypt the file that holds the password for krypton3.

What this level teaches

A chosen-plaintext attack: if you can make a system encrypt text of your choice, you can learn its key.

Key ideas

• Caesar cipher. A substitution cipher where the alphabet is shifted by a fixed amount, here with an unknown shift.
• Encryption oracle. The encrypt program encrypts anything you give it using the secret key.
• Chosen plaintext. Encrypting the full alphabet once shows exactly how every letter is transformed, which is enough to decrypt anything.
• Practical setup. The program looks for its key file in your current directory and runs as the next user, so it needs a working directory under /tmp with a link to the key and permissive directory permissions. The page walks you through an example.

How to approach it

1. Follow the setup in the page text to be able to run encrypt.
2. Think about the smallest, most informative input to encrypt.
3. Use what you learn to build the reverse mapping and apply it to the password file.

Why it matters

Any system that lets attackers encrypt, decrypt or sign arbitrary data is an oracle. Many real attacks, from padding oracles to the ECB cut-and-paste in Natas, use this idea.