Renn Cypher

Further reading on scylates

The Renn Cypher is a type of code typically used by the Order of Erdantia. Its name comes from its origins in the City of Renn, Erdantia. It is composed of the hidden message itself and a number - diameter, as well as relying on information passed before the message was sent.
The Renn Cypher itself is designed for use with a scylate. A scylate is a tool designed for use with a transposition cypher, where a piece of cloth or paper carrying text reveals its information when wrapped around a rod of fixed diameter - a scylate. While with an ordinary cypher using a scylate the rod would be of a predetermined diameter, the scylate used in a Renn Cypher is of variable diameter. This is so that if the scylate is captured, the message still cannot be translated. The receiver of the message will agree on a length beforehand. The message sent will then include a number to add or take away from the list. This means that, unlike with an ordinary Renn Cypher, merely capturing the message and the scylate is not enough. Only the receiver knows the original preset length.
As well as encryption, it doubles a message authentication, as only people who know the preset length can create a comprehensible sentence.
With the Order of Erdantia, the Renn Cypher is one of the more commonly utilised cyphers. In this case, the mission handler (person responsible for operating the mission from abroad), is the one who agrees the preset length with the one receiving the code. In most cases, they are the only one to know this, with the code only available to others in the Order on a need-to-know basis.

Variants

Several variants of the Renn Cypher exist
The Chained Renn Cypher has the preset length replace with the length change of the previous message. This means that the length change will always have to be positive. For example, the previous message having a length of +3 centimetres and the current message having a length of +2 centimetres means the scylate is set to 5 centimetres. If the next message has a length change of +1, then the length will be set to 4 (3 + 1).
In the Dual Renn Cypher, the preset length is in a second message instead of being agreed upon beforehand.