Not Your MPC, Not Your Coin

A multi-party computation (MPC) wallet is being developed as a cryptocurrency wallet that enables the distribution of signing authority to multiple parties. Many vendors are developing MPC wallets based on different cryptographic protocols, which can be readily installed by users, i.e., cryptocurrency exchanges. However, for users, simply introducing the wallet system provided by vendors poses a risk of secret key shares being stolen without their knowledge. We will illustrate methods to secretly steal key shares by exploiting the communication channels with steganography or by modifying the protocol to achieve a fault injection attack. One of the methods is using the error correction capability in QR codes as a covert channel, and another method is embedding hidden bits in the legitimate communication of the underlying MPC protocol. Such attacks can be executed by sophisticated threat actors targeting the vendor’s codebase or through external software dependencies. We will propose measures that user exchanges can implement to address these risks and provide guidance for the secure use of MPC wallets.