I've seen a few puzzles based on a type of cipher in which letters in the plaintext are substituted with groups of characters in the ciphertext. The ciphertext only uses a handful of unique characters. For example:
a -> 154 b -> 53 c -> 12 d -> 41 e -> 153 ...
In this example,
bead is encoded as
Since not all letters are encrypted to have the same length, I can't simply break the message up into pairs of numbers and run frequency analysis. Once the key is known, however, then the character boundaries are unambiguous.
Difficulties in cryptanalysis also arise when one letter's encryption completely contains another letter's encryption, such as how
e 153 contains
b 53. For example, the following is unambiguous given knowledge of the key, yet might not be easy to for an attacker notice:
4153 -> db 154153 -> ae
Let's say that I have a ciphertext, and I am reasonably sure that it is encoded in this way. How would I go about cracking it?
The only thing I know to do is to look for repeating patterns, as repeating words in the plaintext also repeat in the ciphertext. I haven't successfully solved one of these kinds of puzzles, however.