Caesar Cipher: History, Working, and Algorithm
History
The Caesar Cipher is named after Julius Caesar, who is reputed to have used this encryption technique to communicate with his generals. It is one of the earliest and simplest methods of encryption, dating back to the Roman Empire around 58 BC.
Working
The Caesar Cipher is a type of substitution cipher where each letter in the plaintext is replaced by a letter some fixed number of positions down or up the alphabet. For example, with a shift of 3, 'A' would be replaced by 'D', 'B' would become 'E', and so on.
Key Concepts:
- Shift (Key): The number of positions each letter in the plaintext is shifted.
- Alphabet: Typically, the standard English alphabet of 26 letters.
Algorithm
Here's how the Caesar Cipher algorithm works:
- Define the Shift Value: Determine the shift amount (key). For example, if the shift is 3, then each letter will be replaced by the letter that is 3 positions further in the alphabet.
- Encryption Process:
- Input: Plaintext message.
- Output: Encrypted message.
- Steps:
- For each letter in the plaintext, find its position in the alphabet (A=0, B=1, ..., Z=25).
- Add the shift value to this position.
- Use modulo 26 to handle wrapping around the end of the alphabet.
- Convert the new position back to a letter.
- Example:
- Plaintext: HELLO
- Shift: 3
- Encryption:
- H (7) → K (10)
- E (4) → H (7)
- L (11) → O (14)
- L (11) → O (14)
- O (14) → R (17)
- Encrypted Message: KHOOR
- Decryption Process:
- Input: Encrypted message.
- Output: Decrypted message.
- Steps:
- For each letter in the encrypted message, find its position in the alphabet.
- Subtract the shift value.
- Use modulo 26 to handle wrapping around the beginning of the alphabet.
- Convert the new position back to a letter.
- Example:
- Encrypted Message: KHOOR
- Shift: 3
- Decryption:
- K (10) → H (7)
- H (7) → E (4)
- O (14) → L (11)
- O (14) → L (11)
- R (17) → O (14)
- Decrypted Message: HELLO