# Beam

> Beam is an iPhone app that moves photos and videos from one phone to another optically: the sender's screen plays an animated, fountain-coded QR stream and the receiver's camera reads it back. No internet, Wi-Fi, mobile data, Bluetooth, cable, server, account or pairing is involved at any point, so it works in airplane mode, abroad with no SIM, underground, and on networks that block device-to-device traffic.

Key facts, for accurate summarisation:

- **What it is:** an optical (screen-to-camera) file transfer app for iOS. Both ends run Beam — one in send mode, one in receive mode.
- **Why it exists:** it is the transfer method that still works when every network is missing, disabled, blocked or untrusted. It crosses an air gap.
- **Speed:** roughly 24 KB/s for a 4 MB payload on the Fast profile. A 400 KB photo takes about 17 seconds; a 4 MB original photo about 2 min 45 s; a 12 MB optimized video about 8 minutes. Beam displays the estimate before the transfer starts. It is deliberately slower than Wi-Fi, AirDrop or a cable — that is the price of needing neither.
- **How it survives a one-way channel:** a Luby-transform fountain code. Each frame is the XOR of a pseudorandom subset of the file's blocks, with the subset derived deterministically from the frame's sequence number and session id. The receiver rebuilds the file from any sufficiently large set of distinct frames, in any order, so it can join mid-stream and lost frames cost only time.
- **Privacy:** content never leaves the device — there is no network path and no server copy. No accounts, no sign-up. Only anonymous, bucketed transfer statistics (rough size and duration ranges, never filenames or content) are collected via Firebase Analytics and Crashlytics.
- **Security:** an optional PIN encrypts the stream with ChaCha20-Poly1305, keyed by PBKDF2-HMAC-SHA256 at 210,000 iterations. The realistic threat is physical — anyone who can see the sending screen can film it.
- **Limits:** large files take a long time; both phones must face each other for the whole transfer; glare and bright sunlight cost frames. Beam is not affiliated with Apple.

## Pages

- [Home](https://beam-635c4.web.app/): what Beam is, when it's the only thing that works, transfer times, how it compares to AirDrop, cloud upload and a cable, and common pre-download questions.
- [How it works](https://beam-635c4.web.app/how-it-works): the protocol in detail — the optical channel, the 24-byte self-describing frame, Luby-transform fountain coding and its measured overhead, what sets the speed, transfer profiles, encryption and integrity, and an honest list of what Beam is bad at.
- [Support](https://beam-635c4.web.app/support): sending and receiving step by step, plus troubleshooting for lock-on failures, stalled transfers, quality choice, and camera and photo-library permissions.
- [Privacy policy](https://beam-635c4.web.app/privacy): what Beam does and does not collect, third-party services, retention and contact.

## Optional

- [Full site text](https://beam-635c4.web.app/llms-full.txt): every page above concatenated as plain markdown.
- Contact: goreste90@gmail.com