Security — Cynth

Identity & authentication

Every device proves itself. Both ways.

Each device generates an Ed25519 identity key when you set it up. That key never leaves the device. A TLS certificate is derived from it, and every connection — device to device, or device to relay — runs mutual TLS over QUIC: both ends cryptographically prove their identity before a single byte of your data moves.

There is no shared password, no session token that can be replayed from a server, and no account credential that grants access to your files. Pairing devices and adding contacts is an exchange of public keys; content is end-to-end encrypted between the paired endpoints.

What that means concretely

A stolen server credential cannot decrypt or impersonate your transfers — the private keys aren't on servers.
A man-in-the-middle cannot splice into a transfer — mutual TLS fails closed on both ends.
Losing a device means revoking one device key — not resetting your identity.

Transfer paths

Direct first. The fallback stays blind.

Cynth connects your devices in strict priority order and never silently downgrades privacy — every path carries the same end-to-end encryption:

01 · LAN direct

Same network

Devices find each other via mDNS and transfer at local speed. Traffic never leaves your network. Our servers aren't involved in the data path at all.

02 · WAN direct

Across the internet

Devices behind NATs punch a direct UDP path, coordinated by the signaling server. The signaling server sees connection metadata — never file data.

03 · Encrypted relay

Last resort

When no direct path exists, encrypted frames are forwarded through a DERP-style relay. The relay moves ciphertext between two authenticated endpoints. It cannot decrypt what it carries.

Visibility

What our servers can and cannot see.

Three services exist: an API server (accounts, device registry, contacts), a signaling server (presence, connection offers, hole-punch coordination), and the relay. Here is the honest inventory:

Data	Visible to Cynth servers?
File contents	Never. End-to-end encrypted; keys exist only on your devices. The relay forwards ciphertext.
File and folder names	Never. Carried inside the encrypted transfer stream.
Stored copies of files	None. There is no upload step and no server-side storage of transfer data — nothing to breach, subpoena, or lose.
Transfer size & timing	Relay path only. Like any pipe, the relay can observe how many encrypted bytes pass and when. Direct paths expose nothing to us.
IP addresses	Yes, transiently. Signaling and relay need endpoint addresses to coordinate connections — inherent to making any connection work.
Account & device registry	Yes. The API server stores account data, registered device public keys, and your contact list — metadata needed for pairing, never content.

identity Ed25519 key, per device — never leaves it auth mutual TLS over QUIC, both ends verified paths LAN direct → WAN direct → blind relay content E2E encrypted on every path, no exceptions storage no server-side copies of transferred data

Threat model

What Cynth protects against — and what it can't.

Protected against

Server breach exposing your files — impossible by construction; servers never hold file data or decryption keys.
Interception in transit — mutual TLS over QUIC on every path; a network observer sees ciphertext.
Relay operator snooping — including us. The relay is architecturally blind to content.
Impersonation — connections require the private key of a paired device; there is nothing server-side to phish that unlocks your transfers.
Compelled disclosure of content — we cannot hand over what we never possess.

Not protected against — honestly

A compromised endpoint. If malware controls your device, it sees what you see. End-to-end encryption protects the path, not the ends.
Traffic analysis. Your ISP, or we on the relay path, can observe that encrypted traffic flowed between two addresses, its volume, and its timing — not its content. If you need to hide that a transfer happened, use Cynth over a VPN or Tor.
A malicious recipient. Cynth delivers to the device you chose. It cannot control what a trusted contact does with a file after it arrives.
Metadata on our API. Account, device-registry, and contact records exist server-side and are subject to lawful process like any provider's. Content is not — see above.

Cynth is proprietary software. We don't ask you to trust claims you can't check: run a transfer with a packet capture open. On LAN you'll see traffic that never leaves your network; on the relay path you'll see only TLS-protected QUIC between your device and d.cynth.app. The claims on this page are observable behavior, not marketing.

Reporting a vulnerability

Found something? Email security@cynth.app. Plain-text reports welcome; we respond to every substantive report and credit finders who want credit. No bug-bounty theater — just a direct line to the people who wrote the code.

Early access

Send with nothing in the middle.

Cynth is in active development. Join the list and you'll get one email when it's ready for your platform.

Get early access

Private because of how it's built — not because we promise.