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The Vigenère cipher is based on a multiple version of the Caesar cipher.
The Caesar cipher encrypts a message by shifting every letter forward a specific number of places. For example, every letter could be shifted forward one place, so that 'a' is encrypted as 'B', 'b' as 'C', 'c' as 'D', and so on. As there are 26 letters in the alphabet, you can chose a shift of anything up to 25 places. (A shift of 26 takes each letter back to its original position, thus leaving each one unencrypted.)
All the possible Caesar shifts are represented by one of the numbered rows in a cipher table. To encrypt a message by applying a Caesar shift of one, for example, we would look up each letter in the top row and replace it with the corresponding letter in row 1. So the message 'defend east side', for example, becomes 'EFGFOE FBTU TJEF'.
The Caesar cipher is easy to break by analysing the frequency with which letters occur in the code and comparing the result with the average frequencies for letters in the plain alphabet. For example, 'e' is generally the most common letter in English, so whatever letter replaces it will be the most common letter in the encrypted text. The Vigenère cipher, however, cannot be broken by frequency analysis.
This is because the Vigenère cipher employs not just one Caesar shift but several. For example, we can encrypt the same message ('defend east side') using alternate letters from rows 1 and 2, which turns the message into 'EGGGOF FCTV TKEG'.
Now the letter 'e' is encrypted in two ways, as 'F' and 'G'. This severely affects the power of frequency analysis as a code-breaking technique. Worse still, the 'G' in the encrypted text represents two possible letters in the original message, namely f and e.
The cipher table is generally known as a Vigenère square. A message can be encrypted using any number of rows in any order. The order of rows can be defined by a keyword: the keyword 'JOHN' would mean cycling through the rows J, O, H, N, J, O, H, N (that is, rows 9, 14, 7, 13, 9, 14, 7, 13) and so on. As with all codes, the person receiving the message needs to know the recipe for decryption, which is also defined by the keyword.
The Caesar cipher is known as a monoalphabetic substitution cipher, because each letter in the message is encrypted in only one way. The Vigenère cipher is known as a polyalphabetic substitution cipher, because each letter in the message can be encrypted in different ways, depending on which row of the Vigenère cipher is used.
Vigenère and Caesar
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