[1] Extending the principles of Delastelle's earlier bifid cipher, it combines the techniques of fractionation and transposition to achieve a certain amount of confusion and diffusion: each letter of the ciphertext depends on three letters of the plaintext and up to three letters of the key.
Delastelle notes that the most practical system uses three symbols for the trigrams:[3]In order to split letters into three parts, it is necessary to represent them by a group of three signs or numbers.
[4] A traditional method for constructing a mixed alphabet from a key word or phrase is to write out the unique letters of the key in order, followed by the remaining letters of the alphabet in the usual order.
[5] For example, the key FELIX MARIE DELASTELLE yields the mixed alphabet FELIXMARDSTBCGHJKNOPQUVWYZ+.
He describes the encryption step as follows:[7]We start by writing vertically under each letter, the numerical trigram that corresponds to it in the enciphering alphabet: then proceeding horizontally as if the numbers were written on a single line, we take groups of three numbers, look them up in the deciphering alphabet, and write the result under each column.For example, if the message is aide-toi, le ciel t'aidera, and the group size is 5, then encryption proceeds as follows: In this table the periods delimit the trigrams as they are read horizontally in each group, thus in the first group we have 111 = F, 123 = M, 231 = J, and so on.