On November 18, 2014, Open Whisper Systems announced a partnership with WhatsApp to provide end-to-end encryption by incorporating the encryption protocol used in Signal into each WhatsApp client platform.  Open Whisper Systems said that they had already incorporated the protocol into the latest WhatsApp client for Android, and that support for other clients, group/media messages, and key verification would be coming soon after.  WhatsApp confirmed the partnership to reporters, but there was no announcement or documentation about the encryption feature on the official website, and further requests for comment were declined.  In April 2015, German magazine Heise Security used ARP spoofing to confirm that the protocol had been implemented for Android-to-Android messages, and that WhatsApp messages from or to iPhones running iOS were still not end-to-end encrypted.  They expressed the concern that regular WhatsApp users still could not tell the difference between end-to-end encrypted messages and regular messages.  On April 5, 2016, WhatsApp and Open Whisper Systems announced that they had finished adding end-to-end encryption to "every form of communication" on WhatsApp, and that users could now verify each other's keys.  Users were also given the option to enable a trust on first use mechanism in order to be notified if a correspondent's key changes.  According to a white paper that was released along with the announcement, WhatsApp messages are encrypted with the Signal Protocol.  WhatsApp calls are encrypted with SRTP, and all client-server communications are "layered within a separate encrypted channel".  The Signal Protocol library used by WhatsApp is open-source and published under the GPLv3 license.