If I've got this right, the file is encrypted using a secret key which is generated on the client and appended to the anchor in the link, like: http://send.fir

If I've got this right, the file is encrypted using a secret key which is generated on the client and appended to the anchor in the link, like: http://send.firefox.com/download/<fileid>/#<secret> http://send.firefox.com/download/<fileid>/#<secret> Anyone who obtains the link (e.g. via email interception) gains access to the file. Since browsers don't transmit the anchor when requesting a resource [1], Firefox servers never see a copy of the key. Provided you trust their JavaScript. [1] https://stackoverflow.com/questions/3067491/is-the-anchor-part-of-a-url-being-sent-to-a-web-server https://stackoverflow.com/questions/3067491/is-the-anchor-pa...