File Sender

How are files transferred without a centralized service?
File Sender uses a Kademlia Distributed Hash Table (DHT) to connect sender and recipient. Unlike WebRTC, connections are established through a distributed peer-to-peer signaling system. Direct peer-to-peer connectivity is established between the sender and recipient via Hyperswarm, a stack of networking modules for finding peers and creating reliable connections between them.

How is direct connectivity established?

First, the sender's app generates a new download invitation comprised of a unique Download Code and a PIN Code. The sender's app joins the peer-to-peer network and advertises the Download Code (but not the PIN Code). This allows someone with knowledge of the Download Code to send a 'connect' invitation to the sender.

Next, the recipient's app looks up the sender on the peer-to-peer network via the Download Code. Once connected, the recipient's app provides the PIN code to verify the invitation. If the invitation is verified, the sender's app presents to the recipient a list of files available to download.

Direct connectivity is afforded by an end-to-end encrypted channel based on the Noise Protocol Framework. Once the download is complete, the invitation information is discarded, and no additional downloads are possible.

Would the sender and recipient be able to detect each other's IP addresses?

Yes, a technically inclined sender (recipient) would be able to determine the other party's IP address (but not if the other party uses a VPN). This is no different than the sender (recipient) revealing his or her IP address to Microsoft, Google, or any visited web site, except that there is typically more trust between a sender and recipient who share files as compared to one's trust in an unrelated corporation or web site.

As with any internet activity, the privacy of your IP address is best protected with a VPN service.