The Whisperers' Cypher

The League Of Whispers deals in secrets, so any time there's even the suggestion that two members of the organization are speaking in private, the meeting is guaranteed to draw a crowd of would-be eavesdroppers hoping to overhear some dark secret or juicy gossip. Even though the Whisperers have developed their own cant to conceal the meaning of such conversations, communicating names and other details in a way that another Whisperer will understand but an eavesdropper won't is tricky at best. In order to keep their information secure, the Whisperers rely on the written word to supplement spoken conversations and communicate that which is best left unspoken to fellow League members.
Even though literacy is not particularly common among Khezvarans, the Whisperers would never dare commit their secrets to paper in a form that any half-smart bureaucrat could potentially understand, so they've developed a code to use when sending messages to one another. The Whisperers routinely destroy these coded messages once they've been read, but a handful of them have somehow been recovered or intercepted--and subsequently sold for princely sums to those with an interest in decoding the Whisperers' secret language--but the messages have yet to be decoded.
Since Whisperer messages are written in an alphabet of their own devising, most would-be code breakers assume that its letters correspond to those of some known alphabet. By that logic, one should be able to crack the code by looking for patterns representing common words and then reverse-engineer the alphabet form there. This so far hasn't worked, though, because the code is more intricate than that.
In order to write a missive in the Whisperers' Cypher, the Whisperer must first know the correct order of the characters used in the cypher. Next, the writer must decide upon an offset, which is a number between 0 and 29 (the number of characters in both the Imperial and the Whisperer alphabet). To decode a character, the reader finds the corresponding character in the Imperial alphabet and moves a number of places equal to the offset. So if the first letter in the coded message is the 3rd letter of the Whisperer alphabet and the offset is 5, the first letter is the 8th letter of the Imperial alphabet. The second letter, however, breaks the pattern, since alternating letters require the reader to move backwards in the Imperial alphabet. So if the second letter if the 15th character of the Whisperer alphabet, the second letter is the 10th character of the Imperial alphabet. The offset continues to alternate direction throughout the message. If the beginning or end of the alphabet is reached, the reader continues counting from the opposite end.
Someone who knows how the cypher works may be able to decode a message written in it without knowing the offset, but doing so would be extremely time consuming. To keep the cypher secure, however, the offset is also encoded, nearly always in a separate message delivered by a different messenger. This message will include a word that appears exactly once in a 60-line (and quite absurd) poem that every Whisperer must memorize in order to make use of the code. The next line of the poem contains another word that is unique within the poem and corresponds to an offset number.
Since decoding a message written in the Whisperers' cypher can be a time-consuming process even for someone who has all the necessary information at hand, the League are confident that their code is practically unbreakable, especially given the small number of samples that potential code breakers have to work with.