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