Ephemera documentation

How the encryption works

The two devices in a room negotiate an AES-256-GCM session key themselves, using ephemeral ECDH on curve P-256. Each device generates a fresh key pair, commits to a hash of its public key and a random nonce, then reveals the actual key and nonce — the commit-then-reveal order stops a malicious relay from grinding a keypair that would reproduce an expected result after seeing the other side's key first. Both sides then run the shared ECDH output through HKDF-SHA256 (salted with the room ID, bound to both public keys) to derive the actual session key. The server relays public keys and commitments — both meaningless to an eavesdropper — and never sees the shared secret or the resulting key.

Every message and image is sealed with that key using AES-256-GCM before it leaves your device, with a fresh random IV per message and the room ID bound in as additional authenticated data (so a ciphertext can't be replayed into a different room). The server's WebSocket handler only accepts handshake frames and encrypted envelopes — a plaintext text or image message is rejected outright, which is what makes end-to-end encryption a property of the protocol rather than a client-side convention.

Both devices display a short safety code derived from the two agreed public keys. Comparing it (read one aloud, or look at both screens) confirms the handshake wasn't intercepted — if a relay substituted its own keys toward either side, the resulting safety codes would differ. Keys are ephemeral per room and never stored, so a key recovered later cannot decrypt an earlier conversation.

How rooms work

Creating a room requests a short room code from the server. Anyone with that code — or a direct /r/<room-id> link, which auto-joins — can enter the same room. Everyone connected to a room communicates over a shared WebSocket connection, so encrypted text and images sent by one participant appear instantly for everyone else in the room.

What you can send

• Text messages, delivered instantly to everyone in the room.
• Images up to 2 MB. Click a shared image to open a full-view preview, then copy or download it from there.

Only images are supported — there is currently no support for other file types.

Room lifecycle

Because the system currently allows only 3 active rooms at once, you may occasionally see a capacity message when trying to create a new room. This is a deliberate constraint of the current MVP, not a bug.

What happens if you lose a room code

There is no recovery flow. Ephemera has no accounts, no room directory, and no admin access — by design, since adding any of those would work against the goal of a zero-sign-in, fully ephemeral tool. If you forget a code:

1. Ask another participant still in the room to share the code or room link again.
2. If nobody has it, create a new room.
3. If the system is at capacity, wait a few minutes for an abandoned room to expire.

Known limitations

• No forced verification — the safety code detects a tampered handshake, but nothing requires participants to actually compare it.
• No authentication or identity — anyone with a room's code can join it, and the safety code confirms a shared key, not who's holding the other device.
• No persistent message history.
• No room recovery if a code is lost.
• Images only — no other file types.
• Image "copy to clipboard" depends on browser support and secure-context rules, so it may not work in every browser.

Source

Ephemera's source is not currently public. If that changes, this page will link to it.