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How to Decrypt a Shift Cipher

A Shift Cipher is also known as a Caesar Cipher. According to historical record, this code is at least as old as Julius Caesar, who was said to have used it to transmit classified messages. A Shift Cypher is a good, simple starting point for the amateur cryptographer. Even a newcomer to code breaking should have little trouble deciphering a Shift Cipher.

Instructions

- 1
  Write down the alphabet, A-Z. Number the letters 1-26: A-1, B-2, and so on, until Z-26.
- 2
  Assign a number value to every lesser in the encrypted sentence. These number values should be based on the A=1, B=2 values that you noted earlier. For example, if the first word of the code was KHOOR, you would assign it the numbers 11, 8, 15, 15, 18.
- 3
  Note the "shift" of the cypher. The shift is the difference between the original message and the encoded message. According to the book "Life of Julius Caesar," Caesar himself used a shift of three. So, for example, an "A" in the original message would be "D" in the encoded message. For the sake of example, we will be using a Shift Cipher with a shift of three.
- 4
  Subtract the shift- in our example, three- from the letter code in the message. In our example word, KHOOR, we got the numbers 11, 8, 15, 15, 18. Subtract three from each of these numbers, and we have 8, 5, 12, 12, 15. This is the decoded version of the code.
- 5
  Translate the decoded version of the code from numbers to letters, using the A=1, B=2 system that you noted earlier. In our example, 8=H, 5=E, 12=L, and 15=0. So 8, 5, 12, 12, 15 becomes "HELLO."

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