AMIS is supporting more audited threshold signature schemes

AMIS is a financial technology company creating bonds between traditional and decentralized worlds. One of our businesses is to help clients manage their private keys (ex. Qubic Wallet). In short, clients can utilize our service to generate signatures cooperatively without revealing private keys during this process.

Threshold Signature Scheme (TSS) and Alice

More importantly, even AMIS does not know the private keys and any private information of users. This technique is contributed to “Threshold Signature Scheme” (abbrev. TSS).

AMIS has developing our core library “Alice”, which supports two protocols of TSS: “GG18” and “CCLST” in the past two years. Now, Alice supports more protocols: CGGMP and FROST and its audition has completed. With the assistance of Kudelski, we were able to fix all potential threats in the code except for the observation. More details can be found in here.

A t-n Threshold signature scheme is a cryptographic primitive to facilitate joint ownership over a private key by a set of n participants, such that a threshold number t of participants must use their secrets called “share” together to issue a signature that can be verified by a single public key (ref. FROST: Flexible Round-Optimized Schnorr Threshold Signatures). In this procedure, no participant knows the others’ shares and the private key. One main advantage of this technique is to avoid single point of failure in private key management.

CGGMP and FROST Integration

CGGMP is a multi-party computation of ECDSA-protocol. Compared with GG18, the number of rounds of generating signature is less. Meanwhile, it also offers a reliable protocol to refresh private information attaining proactive security. In a proactive threshold signature scheme, time is divided into epochs, such that at the end of each epoch the participants engage in a protocol for refreshing their keys and local states. The security guarantees that the scheme remains unforgeable as long as at most t-1 signatories are compromised within a single epoch (ref. CGGMP). FROST is a very efficient protocol that only requires two-rounds to generate a signature of EdDSA.

Hierarchical Authorization and Proactive Security

In addition, compared to other threshold signature scheme solutions, hierarchy of authorization is feasible in our implementation, each share in our library can have different “weights” in respect to the ability to generate a valid signature. For example, an important contract not only requires enough signatures, but also needs to be signed by a manager.

Despite the fact that vertical access control can be realized on the application layer and tracked by an audit log. Once a hack happens, we will have no idea about who should claim the responsibility. However, in our framework contributed to the Hierarchical Threshold Signature Scheme, through assigning different ranks of each share induces that any valid signature includes the share of the manager.

AMIS gains trust in our products by opening the core codes. Although Alice has been audited by a third-party organization, this is just the beginning! Subsequently, we welcome suggestions and comments to improve this library. Finally, the completion of this audit is supported by Coinbase Developer Grant, which allows us to contribution to the community.

ChihYun Chuang

Cryptography Researcher @ AMIS

ChihYun is a cryptography researcher specializing in Multi-Party Computation (MPC) and Threshold Signature Schemes (TSS). He is dedicated to building robust and secure infrastructure for decentralized finance.