Every letter of the message becomes the row and column numbers of its cell in a 5×5 grid.
substitution cipher
trivial to break
Classical Greece, 2nd century BC
Polybius' torch-signalling scheme, later the basis of tap code and fractionation
encrypt · decrypt · cryptanalysis
Each pair of digits names a cell of the unknown square: assign letters by frequency, pairs run 11–55.
Probable word (crib):