This week we learn about the basics of cryptography, using one of the oldest and simplest ciphers.
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Unlike the Caesar salad, the Caesar cipher is named after the Caesar you’d expect. A cipher is simply a reversible technique for encrypting a message. The message is obscured in some way, so that if it is intercepted during communication, it cannot be read by the interceptor. Every cipher has a key, some piece of information that allows the intended recipient to reverse the encryption and decipher the message. A good cipher is only decipherable using the key, even if the interceptor knows how the cipher works. In fact, cryptographers generally follow Kerckhoffs’ principle, which states that the cipher should be secure even if the method of encryption is public knowledge.
The Caesar cipher is essentially the method of encryption a child with a decoder ring would use. Each letter of the alphabet is assigned a replacement letter, which is always a fixed number of places before or after it in the alphabet. This assignment is cyclical, so that if every letter is replaced by the letter 3 places after it in the alphabet (a left shift of 3) then the letter B in the message would be replaced by the letter E, and the letter Z would be replaced by the letter C. The key in a Caesar cipher is the number of places left or right that the letters of the message were shifted. The recipient uses this knowledge to shift the letters of the encrypted message back, in order to read the original message. This left shift of 3 was the key Julius Caesar himself used to convey important military messages to his generals.
As you may have guessed, the Caesar cipher is not particularly secure, because the encrypted message can be easily deciphered by brute-force: trying different keys until a sensible message results. Those familiar with cryptogram puzzles, which are encrypted using a variation of this method, will know that the frequency of letter usage can provide further assistance in deciphering the message. For example, the most common letter in the English language is E, so it is likely that the letter which appears most often in the encrypted message represents the letter E.
Caesar himself may not have been concerned with the security of this cipher, as the method was relatively unknown at the time, and most of his enemies were not highly literate. In today’s world, however, the Caesar cipher can be solved in seconds by computer programs, and therefore should never be used to encrypt sensitive information. In fact, a variation of the Caesar cipher used by mafia boss Bernardo Provenzano led to his arrest in 2006 when police intercepted and immediately deciphered one of his messages. Although useless by itself, the Caesar cipher still plays a role in cryptography as part of more complex and less easily cracked ciphers, such as the Vigenere cipher. And of course, due to its very simplicity, the Caesar cipher remains vital in teaching children about puzzles and codes and in teaching students about the basics of cryptography.