Gilbert Vernam

The combining function Vernam specified in U.S. patent 1,310,719, issued July 22, 1919, is the XOR operation, applied to the individual impulses or bits used to encode the characters in the Baudot code.

[1] Shortly thereafter, Joseph Mauborgne, at that time a captain in the US Army Signal Corps, proposed, in addition, that the paper tape key contain random information.

Claude Shannon, also at Bell Labs, proved that the one-time pad, properly implemented, is unbreakable in his World War II research that was later published in October 1949.

An operator's mistake of this sort famously allowed the Cryptanalysis of the Lorenz cipher by the British at Bletchley Park during World War II.

They diagnosed how the keystream was generated, worked out how to break the cipher, and read vast quantities of high-level messages to and from German high command without ever seeing an actual Lorenz machine.