Alice and Bob
Alice and Bob are placeholder names which are commonly used to talk about problems. It is used in cryptography, game theory and physics. Instead of “A sends B a message” people say “Alice sends Bob a message”. Ron Rivest seems to have used these names first, in publications about the RSA algorithm. This was 1977 and 1978.
Other names change
The most used names are Alice and Bob. Eve, Mallory, and Trent are used often too, and they have pretty well-defined uses.