702.97c The card with cipher remains encoded on the chosen creature as long as the card with cipher remains exiled and the creature remains on the battlefield. The card remains encoded on that object even if it changes controller or stops being a creature, as long as it remains on the battlefield.

Ja, je kan een Keyrune Cipheren.
Ja, de Keyrune behoud de Cipher, mits de Cipher exiled blijft en de Keyrune op het slagveld.