The Vernam Cipher, also known as the One-Time Pad, is a symmetric encryption technique that uses a random key that is at least as long as the plaintext. Each letter of the plaintext is combined with the corresponding letter of the key using modular addition. This results in a ciphertext that is statistically indistinguishable from random noise, and the key can only be used once to ensure security. It was invented by Gilbert Vernam in 1917 and is still considered unbreakable if used correctly.