Encrypting Radio

There is a type of rare ruby known to crystalist casters of Essence which, when charged with power, twinkles from within with a dim light. These gems are sometimes used as decoration for the clothing of the crystalist caster or amongst the nobility.
Technomages know the ruby as something else entirely. The luminescent flashes of the charged gemstone are completely random. They do not correlate to any environmental factors, only to the power held in their crystal. If such a ruby is charged with power and then carefully split, the shards of the original crystal continue their flashes in unison. Thus they are a source of synchronized chaos, totally correlated between themselves and totally unpredictable otherwise. If the flashing of the crystals is sampled and used as an encryption key an essentially unbreakable code results.

Sapphire Crystal

A matched set of radios can be built, each with an internal chamber to hold a shard of a charged ruby crystal. The chamber is lined with photodiodes to detect and measure the flashes of the gem which drive the encryption of the communications channel. The only known way to break this encryption code is the use of divination magic to predict the flashing of the crystal. The crystal shard used in the radios is quite small, charged only weakly with essence, and completely sealed inside the mechanism, so focusing divination magic on it would be very difficult.

The only thing which can disturb the synchronized chaos of the shards is if an individual shard is charged by a source of power which the other shards do not experience. Detuning a shard renders it useless for encrypted communications with its brethren. This can be done deliberately by a crystalist caster of essence, who must touch the shard to do so. It can also happen by stray radiation from intense sources of magic, as follows:

Whenever a person carrying a synchronized shard is hit by an elemental attack there is a 1/2% chance per level of the spell that the crystal will be detuned.
Each day spent in a lesser earth node has a 5% chance of detuning the crystal shard.
Each day spent in a greater earth node has a 15% chance of detuning the crystal shard.

Weight: the shard weight is negligible.