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IceCTF 2018 - Garfeld [Cryptography]

Garfeld - 1. Cryptography
Author: 5ynax

Worth: $100


At the start of the challenge you are provided with the following image:
Based on the picture, you can see that there is an encrypted message at the bottom of the picture. You will also notice that there is a number in the upper right corner of the image that looks like a date.

If you google "07271978" you will find the following Garfield cartoon strip:

This shows the date is relevant in some way shape or form. Plus, its a great example of a good old fashioned Garfield cartoon strip. 

Back to the cryptographic message, we notice that the format is already in the IceCTF{flag} format that we are looking for so we can confidently say this is the encrypted flag.

If you look at the first part of the encrypted string "IjgJUO" you will see that it is the same amount of letters as IceCTF. It is probably safe to assumed that the number of letters is not changed during the encryption process. Additionally, it is most likely that a substitution cipher is being used since the letter C is always mapped to J in "IjgJUO." Since the number in the upper right corner of the picture appears to be relevant, it is also safe to assume that is a key used in the encryption process. 

My first thought was that the cipher used was some sort of Vignere cipher, but we had a number instead of a word or phrase so I kept looking for additional ciphers that could have been used. Some quick searching in Google led me to the Gronsfeld cipher. It is similar to the Vignere cipher except is uses numbers, and it actually more closely resembles a Caesar cipher where the key being used to determine how each letter of the plain text phrase is rotated versus a standard Caesar cipher where every letter is rotated using the same key. 

Putting the encrypted phrase into a decryption routine (script or online decryption tool if you're lazy like me) and you end up decrypting the encrypted phrase to :

This specific decryption tool did not respect the case of the original flag, so when the flag is entered, you need to enter it as IceCTF{I_DONT_THINK_GRONSFELD_LIKES_MONDAYS}.


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