Highly Controlled, Fine-grained Delegation of Signing Capabilities

Delegation of signing rights is a central problem in security. Whereas delegating by givingpower of attorney is well studied and digitally realized via delegatable anonymous credentials,directly delegating signing possibilities without the need for an external logic, can be done viamalleable signature schemes. However, the existing schemes do not allow for privacy preserving,fine-grained malleability and they do not allow for a controlled way of further delegating themalleability. We bridge this gap by introducing delegatable functional signatures (DFS).We present the first construction of a DFS scheme. This construction is based on stan-dard cryptographic primitives and shows that our strong unforgeability and privacy notions areachievable for arbitrary efficiently computable forms of malleability and delegatability.