Pico CTF 2018: Caesar Cipher 2 Writeup

Points : 250

Cipher Text : 4-’3evh?’c)7%t#e-r,g6u#.9uv#%tg2v#7g’w6gA

The Pico CTF was held this month with lots of sec challenges in it.One challenge that was really solved from scratch by me was this

Caesar Cipher 2 .

A string was given which was supposed to be a ciphertext …..wait not just a normal cipher text but a cipher text involving special characters as well
The reason I had to solve it from scratch is because I couldnt find any online service to decipher it with ROTation of more than 25 …LEL …

Hint : ASCII

of course the Cipher text contained special characters so ASCII table was used for enciphering the flag.

As seen in the cipher text we do not see any characters from ASCII range 1 to 32 i.e. NULL etc.

So the best guess is it is using characters from

! (Exclaimation) whose ASCII value is 33


~ (Tilde) whose ASCII value is 126

That is the total characters used are 94

Here is a simple program to try all possible shifts with the cipher text until we get the flag.

Caesar Cipher with ROT more than 26

After running it we can see the 60th Rotation has the flag in it . :)

Output of the program

Hence the Flag was : picoCTF{cAesaR_CiPhErS_juST_aREnT_sEcUrE}

Thank you for reading.

Follow me on Twitter : @vk_tushar



Security Enthusiast | A Web App Developer Sometimes

Love podcasts or audiobooks? Learn on the go with our new app.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store
Tushar Kulkarni

Tushar Kulkarni

Security Enthusiast | A Web App Developer Sometimes