OverTheWire offer a multitude of wargames. Wargames are a series of security related challenges that vary in difficulty.
This blog post will cover the game that is recommended to play after Bandit, namely Krypton. Krypton is a game about cryptography, ranging from very simple, to somewhat more advanced. I highly recommend all to try it!
Welcome to Krypton! The first level is easy. The following string encodes the password using Base64:
S1JZUFRPTklTR1JFQVQ= Use this password to log in to krypton.labs.overthewire.org with username krypton1 using SSH. You can find the files for other levels in /krypton/
To get the password we use for the ssh, we base64-decode it:
❯ echo S1JZUFRPTklTR1JFQVQ= | base64 -d - KRYPTONISGREAT%
And connecting to the server by entering the password
Level 0 ➡ Level 1
The password for level 2 is in the file ‘krypton2’. It is ‘encrypted’ using a simple rotation. It is also in non-standard ciphertext format. When using alpha characters for cipher text it is normal to group the letters into 5 letter clusters, regardless of word boundaries. This helps obfuscate any patterns. This file has kept the plain text word boundaries and carried them to the cipher text. Enjoy!
If we view the README file in /krypton/krypton1:
krypton1@melinda:/krypton/krypton1$ cat /krypton/krypton1/README Welcome to Krypton! --- SOME LINES OMITTED FOR BREVITY --- The first level is easy. The password for level 2 is in the file 'krypton2'. It is 'encrypted' using a simple rotation called ROT13. It is also in non-standard ciphertext format. When using alpha characters for cipher text it is normal to group the letters into 5 letter clusters, regardless of word boundaries. This helps obfuscate any patterns. This file has kept the plain text word boundaries and carried them to the cipher text. Enjoy!
This mean that we can use the great tr utility to perform the rot13 to decode the string:
krypton1@melinda:/krypton/krypton1$ tr "a-zA-Z" "n-za-mN-ZA-M" < krypton2 LEVEL TWO PASSWORD ROTTEN
Yielding the password
ROTTEN for the next level.
Level 1 ➡ Level 2
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!
Let’s first create a working directory, make the keyfile accessible and our working directory accessible for krypton3, which is the owner of the encrypt binary.
krypton2@melinda:~$ cd `mktemp -d` krypton2@melinda:/tmp/tmp.1Bvm8qkbrF$ ln -s /krypton/krypton2/keyfile.dat krypton2@melinda:/tmp/tmp.1Bvm8qkbrF$ chmod 777 .
Then, let’s create a known plaintext file that we can use to crack the key:
rypton2@melinda:/tmp/tmp.1Bvm8qkbrF$ echo "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ" > plain krypton2@melinda:/tmp/tmp.1Bvm8qkbrF$ /krypton/krypton2/encrypt plain krypton2@melinda:/tmp/tmp.1Bvm8qkbrF$ cat plain ciphertext abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ MNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKL
And again, we can use tr, first for verification, and then for decryption:
krypton2@melinda:/tmp/tmp.1Bvm8qkbrF$ tr "M-ZA-L" "A-NO-Z" < ciphertext ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLMNOPQRSTUVWXYZ krypton2@melinda:/tmp/tmp.1Bvm8qkbrF$ tr "M-ZA-L" "A-NO-Z" < /krypton/krypton2/krypton3 CAESARISEASY
And we can acces krypton3 by using
CAESARISEASY as the password.
Level 2 ➡ Level 3