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10.5. TRANSPOSITION VS. SUBSTITUTION 195
Examples:
(1) Encipher some spots are simply skipped using the keyword START with
the interruptions as given:
S T A R T
∗
∗
∗
∗
∗
Enciphering is almost the same as before: put the message into the array,
skipping the designated spots. Then when pulling out the columns,
remember to forget to write down the ∗’s.
(2) Decipher STPAR STEIA GPSTC NUTMN RNRIU SEENH IRSID EOUET. The keyword
is EXPLODE and the interruption pattern is
∗
∗
∗
∗
∗
∗
∗
∗
∗
7
⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄ ⋄
10.5 Transposition vs. Substitution
How do we recognize that a cipher is possibly from a columnar transposition,
rather than from some form of substitution A message enciphered with a any
transposition consists of exactly the same letters as the plaintext message. A
frequency analysis of the ciphertext will thus result in a chart that matches the
a-e-i, no, rst, and uvwxyz patterns exactly, and in their natural positions. So it
is easy to detect a transposition cipher: its frequency chart will look like one from
common English but the text won’t be readable. 8 Conversely, a substitution
7 (1) MPAIS ORMKE STSLP OSSYP EEPID. (2) Missed interruptions cause anger eruptions.
8 Since position and not identity is now what is important, we probably want to make any
nulls we use to be similar to the other letters in the plaintext, in the hope that the enemy
will be confused as to which letters are meaningful. So, while in a substitution cipher we use
letters like jkxqz as nulls to try to make frequency analysis harder, in a transposition cipher
we use etaoinshr letters to add insignificant significant-looking letters to the ciphertext.