Cryptology - Unofficial St. Mary's College of California Web Site

10.7. BREAKING THE COLUMNAR TRANSPOSITION CIPHER 199
frequency letters. For example, a q in the message must be followed by a u;
j, v and z are almost always followed by a vowel; x is generally preceded by a
consonant.
Counting the contacts will help determine which strips fit together the best.
For each pair of columns that may possibly fit together, sum the values from
Figure 10.3 for that pair. Often the pair of columns with the highest sum will
be neighbors in the plaintext.
Finally, look for words. Decrypting a transposition cipher is very similar
to the final step in decrypting a complicated monoalphabetic ciphers in that
you must just work hard, carefully sort through the possibilities, and let your
English intuition solve it for you.
Examples: Decrypt the following transposition ciphers.
(1) EERHE LARGE GNEDH IWIDD OERET AIYTT SSERT.
This has 35 fairly standard letters, so must be a transposition cipher, and
is either 5 × 7 or 7 × 5. Let’s guess 5 rows. Then our columns are exactly
those 5 letter groupings given.
1 2 3 4 5 6 7
1 E L G I O A S
2 E A N W E I S
3 R R E I R Y E
4 H G D D E T R
5 E E H D T T T
First, there are seven letters per row, and 40% of 7 is about 3, so we’d
expect each row to contain about 3 vowels. Rows 1, 2 and 3 all have
exactly 3 vowels, and row 5 has 2. Only row 4, with 1 vowel, hints that
this rectangle is the wrong one. On the other hand, there are only 12
vowels among these 35 letters, a percentage closer to 33% than 40% so
row 4 by itself is probably not a reason to discard this arrangement.
Second, column 6 has a y, a very uncommon letter. In English y appears
almost exclusively as the final letter of a word, so we should not worry
about what letter follows it. Using Figure 10.3, y is mostly preceded by one
of the letters NBETRAL. (These are they only significant non-zero entries in
the .Y column of Figure 10.2.) Can we make any of these pairs Assuming
that y is not the first letter of the row we can make three ry’s, with the
column-pairs 16, 26 and 56, and two ey’s, with the column-pairs 36 and
76. (This means five pairs to check, out of a possible (7 × 6)/2 = 21.)
Next we count the total number of contacts we have for each pair of