What is a Bitcoin SPV client?

A Bitcoin SPV (Simplified Payment Verification) client is a type of lightweight Bitcoin wallet that operates on the principle of simplified verification, allowing users to interact with the Bitcoin network without downloading and storing the entire blockchain. SPV clients are designed to provide a more efficient and lightweight alternative to full node clients, making them particularly suitable for mobile wallets, desktop wallets, and other resource-constrained environments.

Here's how an SPV client typically works:

1. **Header Chain Validation**: Instead of downloading and storing the entire blockchain, an SPV client only downloads and verifies block headers, which contain key metadata about each block, such as its timestamp, nonce, and the hash of the previous block. By validating the chain of block headers, SPV clients can ensure the integrity of the blockchain without needing to store the full transaction history.

2. **Merkle Path Verification**: When a user initiates a transaction, the SPV client requests a Merkle path proof from full nodes on the network. This Merkle path allows the SPV client to efficiently verify that the transaction is included in a specific block without needing to download the entire block. By checking the Merkle path against the Merkle root provided in the block header, the SPV client can confirm the inclusion of the transaction in the blockchain.

3. **Simplified Payment Verification**: Once the SPV client has verified the chain of block headers and confirmed the inclusion of a transaction in the blockchain, it can trustlessly accept incoming payments without needing to download and validate the entire transaction history. This simplified verification process allows SPV clients to provide users with a lightweight and efficient way to send and receive Bitcoin transactions.

While SPV clients offer convenience and efficiency, they rely on certain assumptions about the security of the Bitcoin network. Specifically, SPV clients assume that the majority of the network's hash power is honest and that the blockchain is secure against attacks such as double spending. As a result, SPV clients may not provide the same level of security as full node clients, particularly in scenarios where users are concerned about potential network attacks or malicious behavior.

Overall, Bitcoin SPV clients provide users with a lightweight and convenient way to interact with the Bitcoin network, offering a balance between efficiency and security for everyday transactions and use cases.