Contributing to Lightning Network: Building a Secure Preimage RPC for LND

Contributing to core infrastructure like lnd (Lightning Network Daemon) is a great way to understand the protocol deeply. Recently, I implemented a new RPC method called GeneratePreimage to solve a common developer friction point.

The Problem: To create secure Hold Invoices, developers need a 32-byte preimage and its SHA-256 hash. LND didn't have a native way to generate these, forcing developers to roll their own crypto logic on the client side—which can lead to security risks if weak entropy is used.

The Solution: I implemented a stateless RPC that uses Go's crypto/rand to generate secure preimages. It returns both the preimage and the hash without modifying the database.

Implementation Details: I defined the API contract in Protobuf and implemented the handler in Go.

Here is the core logic I added to rpcserver.go:

func (r *rpcServer) GeneratePreimage(ctx context.Context,
    req *lnrpc.GeneratePreimageRequest) (*lnrpc.GeneratePreimageResponse, error) {

    var preimage lntypes.Preimage

    // 1. Validate user input or generate new secure random bytes
    if len(req.Preimage) > 0 {
        var err error
        preimage, err = lntypes.MakePreimage(req.Preimage)
        if err != nil {
            return nil, err
        }
    } else {
        if _, err := io.ReadFull(rand.Reader, preimage[:]); err != nil {
            return nil, fmt.Errorf("unable to generate preimage: %w", err)
        }
    }

    // 2. Return Preimage and Hash
    hash := preimage.Hash()
    return &lnrpc.GeneratePreimageResponse{
        Preimage: preimage[:],
        Hash:     hash[:],
    }, nil
}

Learnings
This contribution taught me the importance of stateless utility design in distributed systems. By avoiding unnecessary database writes, we keep the node performant while significantly improving the Developer Experience (DX) for anyone building applications on top of LND.

Check out the Pull Request on GitHub to see the full code!