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Ariyaan Shaikh
Ariyaan Shaikh

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One Time Pad Cipher

In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a one-time pre-shared key the same size as, or longer than, the message being sent. In this technique, a plaintext is paired with a random secret key (also referred to as a one-time pad). Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad using modular addition.
One-time features −

  1. It is an unbreakable cipher.
  2. The key is exactly same as the length of message which is encrypted.
  3. The key is made up of random symbols.
  4. As the name suggests, key is used one time only and never used again for any other message to be encrypted.

Due to this, encrypted message will be vulnerable to attack for a cryptanalyst. The key used for a one-time pad cipher is called pad, as it is printed on pads of paper.

The resulting cipher text will be impossible to decrypt or break if the following four conditions are met:

  1. The key must be truly random.
  2. The key must be at least as long as the plaintext.
  3. The key must never be reused in whole or in part
  4. The key must be kept completely secret.

It has also been proven that any cipher with the property of perfect secrecy must use keys with effectively the same requirements as OTP keys. Digital versions of one-time pad ciphers have been used by nations for critical diplomatic and military communication, but the problems of secure key distribution have made them impractical for most applications.

Encryption

To encrypt a letter, a user needs to write a key underneath the plaintext. The plaintext letter is placed on the top and the key letter on the left. The cross section achieved between two letters is the plain text.
In this example, the technique is to combine the key and the message using modular addition. The numerical values of corresponding message and key letters are added together, modulo 26. So, if key material begins with "XMCKL" and the message is "HELLO", then the coding would be done as follows:

  H       E       L       L       O  message
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7 (H) 4 (E) 11 (L) 11 (L) 14 (O) message

  • 23 (X) 12 (M) 2 (C) 10 (K) 11 (L) key = 30 16 13 21 25 message + key = 4 (E) 16 (Q) 13 (N) 21 (V) 25 (Z) (message + key) mod 26 E Q N V Z → ciphertext

If a number is larger than 25, then the remainder after subtraction of 26 is taken in modular arithmetic fashion. This simply means that if the computations "go past" Z, the sequence starts again at A.

Decryption

To decrypt a letter, user takes the key letter on the left and finds cipher text letter in that row. The plain text letter is placed at the top of the column where the user can find the cipher text letter.
To obtain the plaintext. Here the key is subtracted from the ciphertext, again using modular arithmetic:

 E       Q       N       V       Z  ciphertext
4 (E)  16 (Q)  13 (N)  21 (V)  25 (Z) ciphertext
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  • 23 (X) 12 (M) 2 (C) 10 (K) 11 (L) key = -19 4 11 11 14 ciphertext – key = 7 (H) 4 (E) 11 (L) 11 (L) 14 (O) ciphertext – key (mod 26) H E L L O → message

Similar to the above, if a number is negative, then 26 is added to make the number zero or higher.

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