A Caesar cipher is one of the simplest (and easiest cracked) encryption methods. It is a Substitution Cipherthat involves replacing each letter of the secret message with a different letter of the alphabet which is a fixed number of positions further in the alphabet.
Because each letter in the message has a direct translation to another letter,Frequency Analysis can be used to decipher the message. For example, the letter E is the most commonly used letter in the English language. Thus, if the most common letter in a secret message is K, it is likely that K represents E. Additionally, common word endings such as ING, LY, and ES also give clues.
A brute-force approach of trying all 25 possible combinations would also work to decipher the message.
Example
In this example, each letter in the plaintext message has been shifted 3 letters down in the alphabet.
Plaintext: This is a secret message
Ciphertext: wklv lv d vhfuhw phvvdjh
