171 lines
4.3 KiB
Plaintext
171 lines
4.3 KiB
Plaintext
Computer Ciphering
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Steve Harrison
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San Diego IBM PC User SIG
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Let's start by looking at some simple
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ciphers which do not require anything
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more than a little "brainstorming".
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Consider a form that often appears in
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children's puzzles or on television
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game shows where,
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REPSALON PEROMCUT (1)
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represents a jumbled letter (cipher)
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version of the plain text words
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PERSONAL COMPUTER. (2)
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When enciphering, it is customary to
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arrange the cipher text in groups of
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five letters to eliminate clues given
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by retaining the plain text word
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structure. Thus, (1) would appear as
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the cipher:
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REPSA LONPE ROMCU T (3)
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The method of `jumbling' the letters
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of the plain text message to form the
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cipher text should be known by the
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person to whom the cipher message is
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addressed. Single word `jumble
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ciphers' used as a word puzzle are
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usually formed by a random
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distribution of the original plain
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text letters. Deciphering, or
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solving the word puzzle, is usually
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done by trial-and-error or by knowing
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the subject context.
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If the jumbled letter cipher consists
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of more one word or word pairs, the
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cipher becomes more difficult to
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decipher. For example, who would
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think that the headline type cipher
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message
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BIG MANS CRUISE DIPS EGO (4)
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actually is the cipher text version
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of a plain text message? Not quite
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kid stuff, is it? To `unjumble' (4),
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consider these clues:
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1. The plain text message of (4) is
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related to computers.
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2. You have seen all of the words in
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the plain text in this article.
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3. There are six words in the plain
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text.
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4. The plain text contains the name
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of a city.
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Give (4) a try at deciphering. [Note
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: If you get stuck you will find the
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answer at the end of this article].
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The `jumbled letter' puzzle is
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actually one type of cipher in which
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all the letters of the plain text
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message are retained in the cipher
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text, but they are arranged by one of
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many methods of "letter jumbling".
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For convenience, let's call this
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group the Transposition Cipher group.
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Now let's look at some of the methods
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used to encipher plain text messages
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by transposing letters.
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The Rail Fence:
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The rail fence cipher is a popular
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and well known simple example of
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transposition ciphers. For example,
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the plain text message
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HAVE FUN WITH COMPUTERS (5)
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can be arranged as a `rail fence'
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H N O R
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A U W C M E S
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V F I H P T
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E T U (6)
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The cipher text is then formed from
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(6) by taking the letters row by row
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in sequence to obtain the sequence
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HNORAUWCMESVFIHPTETU (7)
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By grouping, the cipher text becomes
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HNORA UWCME SVFIH PTETU
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Not a difficult way to encipher or
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decipher, is it?
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The Columnar Tramps:
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This is a columnar transposition
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cipher more advanced than those
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described previously. It uses a
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keyword and a prescribed method of
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arranging letters, rows or columns in
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the plain text. The addition of a
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key increases cipher security.
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Consider a plain text message whose
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letters are arranged in a 5 by 5
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array, as shown in the following
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example for the plain text message;
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"CIPHERS CAN KEEP FILE SECRETS"
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C I P H E
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R S C A N
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K E E P F
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I L E S E
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C R E T S (9)
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using the keyword "PCSIG". The
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transposition is done by using the
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[alphabetical] order of the letters
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in the keyword to select columns for
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the cipher text:
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4 1 5 3 2
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P C S I G (10)
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Thus, the letters in columns 1,2,3,4
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and 5 when written out in a line
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appear as
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ISELR ENFES HAPST CRKIC PCEEE. (11)
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When the decoder receives the cipher,
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he counts the number of letters in
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it. Knowing the keyword is PCSIG, he
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divides this number by the number of
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letters in the keyword to determine
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the number of letters in each column.
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All array columns are equal in length
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which may require the insertion of
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miscellaneous letters or numbers in
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the plain text to fill out the last
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row of the array.
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Now that you know the columnar
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`tramp', try deciphering this short
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cipher using IBMPC as the keyword:
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OTIRA SCATN UOGLN (12)
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oh, by the way - the jumbled cipher
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(4) is the plain text
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SAN DIEGO IBM PC USER SIG (13)
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