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

208 CHAPTER 10. TRANSPOSITION CIPHERS
To find the ciphertext pull out the columns of this array in the alphabetical
order of the letters of the keyword. Deciphering is the reverse process, with the
bit of complication that the length of the ciphertext and length of the keyword
must first be used to determine how many letters each column contains.
Recognizing transposition ciphers is easy – the frequency chart of a ciphertext
appears to be that of normal English. Breaking transposition ciphers is,
obviously, a matter of putting the letters back into their proper positions. For
columnar transposition one first decides upon a guess of the size of the ciphertext
rectangle and then creates the proposed columns of the plaintext. By knowing
which letters are more likely to contact which one then proceeds by putting the
columns next to their proper neighbors.
The North used columnar transposition of words to great success during the
Civil War. Keeping the words largely prevented Morse Code-based garbles, and
the extensive use of code words, for proper names and locations, prevented the
South from having any easy entry into the messages.
10.12 Topics and Techniques
1. From what do transposition cipher receive their security
2. What is the main difference between transposition ciphers and substitution
ciphers
3. What is a route cipher What does its name mean
4. What is a geometrical cipher
5. What is a turning grille Explain how to construct one.
6. Explain how to use a keyword to set up a columnar transposition.
7. In a columnar transposition, how is the plaintext entered
8. In a columnar transposition, how is the ciphertext removed
9. Given the number of letters in the ciphertext and the length of the keyword,
how are the lengths of the columns used that transposition determined
10. How does one decipher a columnar transposition cipher
11. How does one recognize that a cipher is a transposition
12. How does one decide whether a ciphertext is from a substitution cipher or
from a transposition cipher
13. Explain how to break a columnar transposition ciphertext.