Codes and Ciphers :: One-time Pad

The one-time pad is a long sequence of random letters. These letters are combined with the plaintext message to produce the ciphertext. To decipher the message, a person must have a copy of the one-time pad to reverse the process. A one-time pad should be used only once (hence the name) and then destroyed. This is the first and only encryption algorithm that has been proven to be unbreakable.

To encipher a message, you take the first letter in the plaintext message and add it to the first random letter from the one-time pad. For example, suppose you are enciphering the letter S (the 19th letter of the alphabet) and the one-time pad gives you C (3rd letter of the alphabet). You add the two letters and subtract 1. When you add S and C and subtract 1, you get 21 which is U. Each letter is enciphered in this method, with the alphabet wrapping around to the begining if the addition results in a number beyond 26 (Z).

To decipher a message, you take the first letter of the ciphertext and subtract the first random letter from the one-time pad. If the number is negative you wrap around to the end of the alphabet.

plaintext   : SECRETMESSAGE
one-time pad: CIJTHUUHMLFRU
ciphertext  : UMLKLNGLEDFXY

• Monoalphabetic Cipher
• Caesar Cipher
• Atbash Cipher
• Keyword Cipher
• Pigpen / Masonic Cipher
• Polybius Square
• Polyalphabetic Cipher
• Vigenère Cipher
• Beaufort Cipher
• Autokey Cipher
• Running Key Cipher

• Transposition Ciphers
• Rail Fence
• Route Cipher
• Columnar Transposition
• Polygraphic Substitution
• Playfair Cipher
• Bifid Cipher
• Trifid Cipher
• Four-square cipher

• Other Ciphers and Codes
• Book Cipher
• Beale Cipher
• Morse Code
• Tap Code
• One-time Pad
• Scytale
• Semaphore
• ASCII
• Steganography
• Techniques
• Frequency Analysis