Every note submitted to the server will eventually be destroyed. All unread notes are destroyed after 30 days. All notes that have been viewed are destroyed immediately after viewing, and the browser will be redirected away from the note after 3 minutes. When the note is destroyed, it is overwritten with random data. This application does not make any assumptions about the underlying filesystem. Better safe than sorry, we say.
The note can be viewed only once. If someone attempts to view the note after the initial viewing, the note has been destroyed, so the client is redirected to a standard 404 error.
AES-256 Encrypted with HMAC-SHA512
Before storing the note on disk, every note is compressed with ZLIB, then encrypted with 256-bit AES in CTR mode using HMAC-SHA512 encryption. The AES-256 and HMAC-SHA512 keys are never stored on disk and are part of the URL.
The application uses a per note random key that is stored in the URL and is part of the encryption and decryption of the notes. The application supports using a custom passphrase to encrypt and decrypt the note. This custom passphrase is created using a password-based key derivation function (PBKDF2). The private passphrase is not stored to disk in the application, but this does not prevent the system administrator from modifying the code to do so. Therefore, it is best that you host this application yourself, on your own server.
Part of Hashcash tokens is the ability to create a uniquely identifiable resource string. Typically, this is an email address. However, with this application, your browser fingerprint is used as the resource string. The browser fingerprint is a 32-bit MurmurHash number. By itself, it cannot identify who it belongs to. However, the system administrator could tie that fingerprint to an IP address and user agent string by looking in his web server logs.