  Time-Lock Puzzles (TLPs) are cryptographic protocols that enable a client to
lock a message in such a way that a server can only unlock it after a specific
time period. However, existing TLPs have certain limitations: (i) they assume
that both the client and server always possess sufficient computational
resources and (ii) they solely focus on the lower time bound for finding a
solution, disregarding the upper bound that guarantees a regular server can
find a solution within a certain time frame. Additionally, existing TLPs
designed to handle multiple puzzles either (a) entail high verification costs
or (b) lack generality, requiring identical time intervals between consecutive
solutions. To address these limitations, this paper introduces, for the first
time, the concept of a "Delegated Time-Lock Puzzle" and presents a protocol
called "Efficient Delegated Time-Lock Puzzle" (ED-TLP) that realises this
concept. ED-TLP allows the client and server to delegate their
resource-demanding tasks to third-party helpers. It facilitates real-time
verification of solution correctness and efficiently handles multiple puzzles
with varying time intervals. ED-TLP ensures the delivery of solutions within
predefined time limits by incorporating both an upper bound and a fair payment
algorithm. We have implemented ED-TLP and conducted a comprehensive analysis of
its overheads, demonstrating the efficiency of the construction.
