This chapter covers a broad range of fractionation ciphers where the plaintext groups and/or the ciphertext groups have variable lengths. These include monom-binom (section 10.2), Huffman substitution (section 10.4) and Post tag systems (section 10.5).
In section 4.4 I illustrated the concept of fractionation by describing two versions of M. E. Ohaver’s Fractionated Morse cipher. Fractionated Morse is an example of variable-length fractionation because it uses 1-, 3- and 4-symbol Morse groups. Let me begin the broader discussion of variable-length fractionation with a different form of Morse fractionation that resembles the trifid cipher described in section 9.9. Let’s call it Morse3.
Morse3 is a cipher that operates in 4 steps. (1) Replace the letters of the messages by Morse code groups. You can use either the standard Morse code, or a mixed Morse alphabet like the one in section 4.4. (2) Separate the Morse groups using a / symbol. Use a double // to separate words and to mark the end of the message. (3) Divide the symbols into groups of 3. Append an extra · or ·· if needed to complete the last group of 3 symbols. The recipient will ignore these extra dots following the last //. (4) Substitute a letter for each group of 3 symbols using a second mixed alphabet.