  In current blockchain systems, full nodes that perform all of the available
functionalities need to store the entire blockchain. In addition to the
blockchain, full nodes also store a blockchain-summary, called the
\emph{state}, which is used to efficiently verify transactions. With the size
of popular blockchains and their states growing rapidly, full nodes require
massive storage resources in order to keep up with the scaling. This leads to a
tug-of-war between scaling and decentralization since fewer entities can afford
expensive resources. We present \emph{hybrid nodes} for proof-of-work (PoW)
cryptocurrencies which can validate transactions, validate blocks, validate
states, mine, select the main chain, bootstrap new hybrid nodes, and verify
payment proofs. With the use of a protocol called \emph{trimming}, hybrid nodes
only retain polylogarithmic number of blocks in the chain length in order to
represent the proof-of-work of the blockchain. Hybrid nodes are also optimized
for the storage of the state with the use of \emph{stateless blockchain}
protocols. The lowered storage requirements should enable more entities to join
as hybrid nodes and improve the decentralization of the system. We define novel
theoretical security models for hybrid nodes and show that they are provably
secure. We also show that the storage requirement of hybrid nodes is
near-optimal with respect to our security definitions.
