  Blockchain-based cryptocurrencies, facilitating the convenience of payment by
providing a decentralized online solution, have not been widely adopted so far
due to slow confirmation of transactions. Offline delegation offers an
efficient way to exchange coins. However, in such an approach, the coins that
have been delegated confront the risk of being spent twice since the
delegator's behaviour cannot be restricted easily on account of the absence of
effective supervision. Even if a third party can be regarded as a judge between
the delegator and delegatee to secure transactions, she still faces the threat
of being compromised or providing misleading assure. Moreover, the approach
equipped with a third party contradicts the real intention of decentralized
cryptocurrency systems. In this paper, we propose \textit{DelegaCoin}, an
offline delegatable cryptocurrency system to mitigate such an issue. We exploit
trusted execution environments (TEEs) as decentralized "virtual agents" to
prevent malicious delegation. In DelegaCoin, an owner can delegate his coins
through offline-transactions without interacting with the blockchain network. A
formal model and analysis, prototype implementation, and further evaluation
demonstrate that our scheme is provably secure and practically feasible.
