Hopper is an Open-Source Mixer for Mobile-friendly private transfers on Ethereum. It allows the private transfer of value from one Ethereum account to another, via an iOS client. Users can deposit notes of 1 ETH into a mixer smart contract and withdraw them later to a different account by only providing a Zero-Knowledge proof (zkSNARK) that they previously deposited a note into the mixer, without revealing from which account that note was sent. Relayers are used to post transactions to the blockchain so that the recipient of a private transfer can withdraw a private note from the mixer without needing any prior ether.