The Vernam Cipher is an algorithm invented in 1917 to encrypt teletype (TTY) messages.
So named for Gilbert Sandford Vernam, it is a symmetric cipher patented July 22, 1919. The Vernam Cipher combines plaintext (the original message) with pseudo-random series of polyalphabetic characters to form the ciphertext using an “exclusive or” (XOR) function. US Army Captain Joseph Mauborgne soon discovered that the cipher could be made much stronger by using truly random numbers printed on pads of paper. Streams of paper with the random numbers in that fashion became a process known as “one-time pad”. The Vernam using one-time pad is regarded as unbreakable.
On background, a teletype is a character printer connected to a telegraph that provides a user interface for people to communicate over various communications protocols such as dedicated or public wires, radio, or microwave.
“The Vernam Cipher with one-time pad is said to be an unbreakable symmetric encryption algorithm in part because its key-exchange process uses true random number generation and secure key distribution.”