This is the first in a series of "Intro to Cryptanalysis" Challenges to teach some mad skills in ciphers and decrypting for those that have little to no experience with them. I am hopping these are fun and informative and help to get others involved in crypto challenges. In this series we will be visiting the 3 General Categories ciphers fall into:
1. Concealment Cipher
2. Transposition Cipher
3. Substitution Cipher
Concealment Cipher - Although some might argue that this does not fall into a true cipher we will cover it since you may see this in challenges, or CTF events. In Concealment cipher the message is Hidden in plain sight, and intended to pass without even being detected as a secret communication. This may also be referred to as Null Cipher, Open Letter Cipher, conventional writing, dissimulated writing, or Steganography There are many interesting examples of these ciphers just Google for some of them, for example one called puncture cipher where any printed text is used (news paper, book) and a pin hole is put under the letters to spell out the secret message. An interesting use of this was demonstrated in England when postage on letters was high but it was free to mail newspapers. Individuals would put pin holes in articles and send to friends or family to get around having to pay the postage on a normal letter. The receiving individual (knowing the pin hole cipher was used) could easily decrypt the message by matching the letters to the pin holes.
We are going to look at a Type of Null Cipher you could see in a challange which would try to conceal a message in plain text.
Our example will be the phrase
"Another two twins after contacting keith and took down another winning number"
As you can see right off it might not make much sense as a complete sentence. Knowing this phrase may contain a Null Cipher it could be fairly easy to solve. In our case we could line up the words vertically and easily see that the first letters spell out a Message
Using the first letters of the words the phrase spells "Attack at dawn". This is one of the weaknesses with Null Ciphers in once you know the "key" they are very easy to decipher. The keys could be the first letter of the word, the last letter, or even a sequence like 1,2,3 (first letter of first word, second letter of second word,third letter of third word, first letter of fourth word...etc.etc) and repeat the pattern throughout the paragraph or message.
Lets take the following phrase and see if you can decipher on your own:
Send me the decripted message via email to flightgod at gmail dot com or message @fg in seckc slack to solve the puzzle.
What other ways can you think of to hide a message using Concealment. Do additional research and look up some of these different types:
Lord Bacons Cipher
Trtheme (Trithemius;Trittemius) Alphabet