How the encryption works
What happens to your files and credentials between your browser and our team, and what we can and cannot see.
Customers with an account sign in with their work email. Everyone else uses the single-use link their SC Signal contact sends, which opens the upload page directly.
Sealed before it leaves your device
The upload page carries SC Signal's public key. When you press Seal and send, your browser creates a one-time key pair, combines it with our public key, and derives a fresh encryption key for each file and one for the manifest. The manifest holds your file names, your note and any credentials you entered.
Files are encrypted in 8 MiB pieces. Each piece is numbered and the last one is marked as last, so any piece that is changed, reordered or dropped fails to decrypt.
Only the sealed pieces travel to us. They are stored in a private bucket that has no public address and are deleted after 30 days.
Who can open it
Only someone holding SC Signal's private key and its passphrase. That key never touches our web servers. Staff unlock it in their own browser, or on an offline machine, to open an upload. Every download and every decryption is recorded in an audit trail.
Our hosting provider, our storage, and anyone who intercepts the traffic see only ciphertext.
What we can see without the key
- Who started the upload: the signed-in email address, or the label on the single-use link we sent you.
- When it arrived, how many files it held and their encrypted size.
- Your IP address and country, as recorded by our network edge.
File names, contents, your note and credentials are all inside the encryption.
- Key agreement
- ECDH on NIST P-384
- Key derivation
- HKDF with SHA-384
- Encryption
- AES-256-GCM
- Receipt digest
- SHA-384 of each original file
- Implementation
- Your browser's built-in Web Crypto. No third-party code.
These are the algorithms in the NSA's CNSA 1.0 suite for protecting national security systems.
Secure upload is not authorized for Controlled Unclassified Information (CUI), ITAR- or EAR-controlled technical data, or classified information. Strong encryption does not make it an approved system for that data. If your work with us involves it, ask your SC Signal contact for an approved transfer method.
The upload page shows the fingerprint of the key it encrypts to. If you want to be certain, ask your SC Signal contact to read you the fingerprint over the phone and compare.
Not available
- Send a dedicated or temporary account rather than a personal one.
- Send the link and any context that names the account by different routes.
- Change the credentials when our work is done. We will tell you when.