Here’s one for you, from the forgotten wearables files: in the late 90s, Sun Microsystems created the Java Ring. The gaudy stainless steel ring is a wearable computer with “a built-in Java Virtual Machine, non-volatile storage, and an serial interface for data transfer.” Here’s more from Hackaday:
Made of of stainless steel and waterproof grommets, this thing is built to be indestructible. The batteries were rated for a ten-year life, and the ring itself for one million hot contacts with Blue Dot receptors.
This thing has several types of encryption going for it, including 1024-bit RSA public-key encryption, which acts like a PGP key. There’s a random number generator and a real-time clock to disallow backdating transactions. And the processor is driven by an unstabilized ring oscillator, so it constantly varies its clock speed between 10 and 20 MHz. This way, the speed can’t be detected externally.