We talked about various ciphers used during early American history and then continued talking about regular transposition ciphers. We'll continue with irregular transposition ciphers (with crib). The book has no material on transposition ciphers in general, but check out Helen Gaines' book Cryptanalysis (published by Dover).
After some more talk about transposition systems, we will move on to the mechanization of cryptology up to the time of WWI.
1. (Reading Assignment) As background for next week's topic read the first two sections of chapter 3 (up to page 115), covering the ADGVX cipher.
2. (Simple regular Transposition Cipher, 25pt)
You suspect the following text to have been the result of a regular transposition cipher.
TIEUT FKTST ADTAA ETIEH OUOHT HSEUO EEINS XRTRC ISVAS LFOAH EBIER TMMGE HSNSR MUHTI LSUDT ASOAN TAFO
a) Determine all possible key lengths (given that it is a regular cipher) in the range 5 - 13.
b) Test all possible key lengths from a (using the vowel test: at least half the rows should have at least 40% vowels and the other rows shouldn't be far off, count Y as a vowel). Show results of your counts. Based on the test, order the key-lengths in the most likely order.
c) Try key lengths in the order suggested by b) and decrypt the ciphertext.
3. (Transposition Cipher with crib, 25pt)
You suspect the following text to have been the result of a transposition cipher, and you also suspect that the word “principle” appeared in the plaintext.
ETCIT UTYED RESSF FPISY SFOLY EATOO TRILH UNORA UHIET LIRPT OTOUF OTSPN OHSNE AMOLS NAEER O
a) Determine the likely length of the key (making use of the crib).
b) Reconstruct the plaintext.
4. (Extra Credit, 5pt) Here's an anagramming game in the style of "Every Word": Given some letters you need to find all words that can be made with those letters, e.g. given letters ESENVI you can form ENVIES (6 letters), VINES, VEINS, SIEVE, SEVEN, SEINE, EVENS (5 letters), and 9 four-letter and 7 three-letter words. Start with the letters RELCRUS. How many words of length at least 4 can you find using just those letters? Extra extra credit for whoever finds the most words.