Blink Overview

Blink is an interactive light client for Proof-of-Work blockchains which, contrary to other existing clients (e.g., NiPoPoW, Flyclient, SPV, and more) operates by only consuming an $\mathcal{O}(k)$ amount of communication, computational, and storage resources. k is the common prefix security parameter, e.g., the conventional 6 confirmation blocks in Bitcoin. In practice, k is considered constant, making Blink the first constant-size client for PoW chains. Contrary to most zero-knowledge based clients, Blink does not assume a trusted setup.

Proof Design

In a nutshell, the Blink client connects to multiple full nodes, so that at least one of them can be assumed honest.

The client locally samples a random value $\eta$, includes it in a transaction ${\mathsf{Tx}}_{\eta }$, and sends it to the full nodes. For instance, ${\mathsf{Tx}}_{\eta }$ can be a payment to a vendor's fresh address, sampled with high entropy. Then, Blink waits for ${\mathsf{Tx}}_{\eta }$ to be included on-chain in a block and confirmed. The full nodes respond to the client with a proof $\pi$ consisting of only 2k+1 consecutive block headers, with the header of the block including ${\mathsf{Tx}}_{\eta }$ sitting in the middle.

This proof ensures that the first block in the proof is stable and, therefore, it can be considered as a checkpoint or as a new genesis.

Security

Blink is proven secure in the Bitcoin Backbone model, assuming a static population (i.e., static difficulty) and an adaptive adversary holding less than 50% of the computational power of the network.

Paper

For a full description and formalization, see the Blink paper.

The Blink Client

Config

In the src/config.ini file, enter the endpoints for full nodes (mainnet and/or testnet), the details of the UTXO the entropy transaction must spend, the common prefix security parameter, and your local path to the file holding your secret key.

CLI commands

--mainnet: the client is run on mainnet. If omitted, testnet is default.

--dry-run: the client is run with a default entropy transaction id. If omitted, the client broadcast a new entropy transaction. By using it in combination with --entropy_txid, you can provide a new entropy transaction id to use

Run

The Blink client can be executed either via the Makefile or via the shebang line. If the entropy transaction is already on-chain and its txid is known, you can run the following command:

make ARGS="--dry-run --mainnet --test-txid=your_entropy_txid"

else, to create and broadcast a new entropy transaction run

make ARGS="--mainnet"