Can anyone at My Signal Vault read my files?

The honest answer

Your files are stored encrypted, and the keys that open them are themselves encrypted. But decryption happens on our server when you download a file or when a vault delivers one, which means the running application is capable of decrypting your data. It is not zero knowledge encryption and we will not claim otherwise.

What that gives you is protection against every realistic attack on stored data: a stolen bucket, a leaked backup, a misconfigured server, the storage provider's own staff. In all of those cases what exists is ciphertext with no usable keys.

What it does not give you is a mathematical guarantee against the operator. If you need that, no service that emails your attachment to someone with no account can provide it, because something has to hold the key.

What we do about it in practice

• Nothing decrypts unless a request requires it. There is no browsing interface over your files for anyone but you.
• Every read of a file is written to your audit log, with the time and the address. You can see it.
• Decrypted working copies are written with owner only permissions and removed as soon as the transfer finishes, including when a download is aborted part way.
• Administrators can see account level facts — plan, storage used, vault count, activity — and cannot browse the contents of a vault or a file through any interface.

If you need more

Encrypt the file yourself before uploading it, using a tool you control, and give the passphrase to your recipient another way. We then hold something we could not read under any circumstances, and the vault still delivers it exactly as before.

For a written message rather than a file, the same idea applies: if it is sensitive enough that server side decryption bothers you, put it in an encrypted file and attach that instead of typing it into the message body.