Vernam Cipher

Introducing the Vernam cipher

The most common one-time pad is the version that applies to binary plaintexts and keys (as opposed to the versions applied to letters of the alphabet and numbers from 1 to $n$ that we have just described). This version is often called the Vernam cipher.

The Vernam cipher can be created as follows:

• Let the plaintext be a string of bits, $P_1$, $P_2$, …, $P_n$ (where $P_i$ means the $i$th bit of the plaintext).

• Randomly generate a key that consists of $n$ bits $K_1$, $K_2$, . . ., $K_n$.

• Encrypt plaintext bit $P_i$ by XORing it to key bit $K_i$ to obtain ciphertext bit $C_i$. Mathematically put:

$$C_1 = P_1 \oplus K_1$$

$$C_2 = P_2 \oplus K_2$$

$$\vdots$$

$$C_n = P_n \oplus K_n$$

This process is depicted in the figure given below: