Vernam Cipher

May 6, 2022 - Reading time: 2 minutes

The Uncrackable Cipher

One-time pad (Vernam Cipher)

The people always wonder, is there any way to design uncrackable cipher? Short answer is yes. But in practice, there is no way to use them.


We need to an array which contains random characters and also you need give the key to who is you communicating.

  1. Create an array (One-time pad) which contains random characters.
  2. Plain text XORed with one-time pad .

Key Value
Plain Text 010101111001001
One-time Pad (key) 101111010110101
Encrpyted Text 111010101111110

  • The security related by randomized characters. So one-time pad need to be random.
  • One-time pad can be used just one time.
  • Perfect privacy (Encrypted text won't give any clue about the plain text)
  • They call it one-time pad cause usually they write it to notepad with very small fonts.
  • Can be used alphanumeric characters against bits.
  • One-time pads are creator of stream ciphers.

  • Key is so long.
  • Disturbiting the key is problem as it self.
  • Psudo-randomness is a big problem.
  • Wrong usage or abuse of one-time pad.

  • Same key usage is gonna leak two plain texts.

If the attacker can get two crypted texts (C1, C2) which them encrypt with the same key, they he/she can use the fallowing method to decrypt the message:

C1 ⊕ C2 = (P1 ⊕ K) ⊕ (P2 ⊕ K) = P1 ⊕ P2

Later, frequency analysis or repetition in language can possible used to crack all or biggest part of P1 and P2.

⊕: Symbol of XOR


  • Venona Project: At the World War II, the United States was tried to crack Soviet encryption. They succeeded. Because they used the same or non-random keys for messages. So they get thousend of plain texts from crypted messages.

  • In the past Washington and Moscow had transferred the one-time pads to each other. They will use them if the current chipers are cracked.

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