soal

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

solve

# work dir
WORKDIR=$(mktemp -d)
cd $WORKDIR

# link keyfile.dat
ln -s /krypton/krypton2/keyfile.dat
chmod 777 .
/krypton/krypton2/encrypt /krypton/krypton2/krypton3
cat ciphertext 
# AYCQYPGQCYQW

cat /krypton/krypton2/krypton3 
# OMQEMDUEQMEK

# solve with cryptii
## go to https://cryptii.com/pipes/caesar-cipher
## change the shift=12 if u want decrypt "OMQEMDUEQMEK"
## change the shift=24 if u want decrypt "AYCQYPGQCYQW"

# solve with my tool
wget https://raw.githubusercontent.com/ariafatah0711/ctf_aria/refs/heads/main/tool/caesar
chmod +x caesar
./caesar "OMQEMDUEQMEK"
./caesar "AYCQYPGQCYQW"

flag

CAESARISEASY