Ok, I’m no cryptography expert, but I’ll chime in with some basics. This is obviously a substitution cipher. Any look at two or three letter pairs will reveal this. XI appears a bunch of times and without even deciphering the test, I’ll bet you dollars to donuts that “x” is O and “i” is “F,” since the cipher “XII” appears, and along with the two letter “XI”, the only letter combination I can think that would satisfy both these ciphers is “OFF” and “OF.” The other possibility is “TOO” and “TO.”
From there, I’d suggest a simple frequency analysis.
In fact, if you do a simple search on the web for “cryptogram solver” you’ll be able to find an applet to solve this for you. But that’s not the point. The point is figuring out how to do it. You should have learned how to identify simple substitution ciphers, and you should be able on a cursory glance to note that this is a substitution cipher from the clues I have mentioned. The repeating letter combinations are a big give away.
As for the Vigenere cipher, in certain ways, I find that easier than a simple substitution cipher. You need not know anything about the keyword to solve a Vigener cipher. You use the index of coincidence principle to figure out the keyword length.
Here’s how it works. You take your cipher and you write a program (or do it on paper). Write your original cipher on one line, and below it, the cipher with one letter shifted over. Mark how many times the cipher text and the shifted cipher text coincide (ie. are the same letter). Next rewrite the cipher text, and this time write the cipher shifted over two letters. Mark the coincidences. Then three shifted over. Four shifted over. Five shifted over, etc. When you reach your key length, you will notice a marked jump in the amount of letters that coincide. I’m going from memory here, but the index of coincidence will normally be around 3-4%, but once you hit your keylength, it’ll be 8-12%. Once you know your keylength, you know which sets of letters to do a frequency analysis on. Since a Vigenere cipher is based on a simple shift, you will find a clump of adjacent letters which should correspond to the R-S-T alphabet sequence (the most statistically probably sequence.) And from there, it’s easy. Just slot in the rest of the letters, and you’re set.
There’s also another way of solving Vigeneres which does not require the index of coincidence. In the other method, you find letter pairs or triplets that recur throughout your cipher text. You then figure out the spacing between the recurring pairs (or triplets or quadruplets). Usually, you’ll come up with numbers like 4, 12, 16, 32, etc… find the lowest common denominator and that’s your key length.
