Decrypting substitution-type ciphers requires that we have information on the frequencies of n-grams in the ciphertext. “M-x decipher” in emacs will give you the letter and bi-gram frequencies, but we’ll need frequencies of three and more letter patterns in the ciphertext.
I will try to start you on the elements of this program, but I hope that you (as a group) will be able to take the design of the program into your own hands.
For consistency, name the program “ngramFT.py” (so I can find it in your subdirectory when you “git push” your work to the github.
Copy the example program from page 14 of SRS (your Cryptography book). BUT NOT THE VARIABLE “ciphertet”. Instead, use your knowledge of reading files to set that variable to the CONTENTS of a file that you specify as the sys.argv[1] argument on a command-line call to ngramFT.py.
In other words, you will have some file, let’s call it “deleteme.txt” for now, and you will get the letter frequencies of that file by calling:
$ python ngramFT.py deleteme.txt
and the output should tell you how many times each letter shows up in the file “deleteme.txt”
Expand the program so that it will also give you the number of times each 2-letter pattern shows up.
Warning: the technique that SRS uses on page 14 may not scale up nicely!
Be ready to show your results &/| ideas next time we meet.