Tim Cook has picked up a new and unexpected supporter in Apple's legal struggle with the FBI: WhatsApp CEO Jan Koum. Sharing Cook's letter in a Facebook post, Koum gave the Apple CEO his full support. "We must not allow this dangerous precedent to be set," Koum wrote. "Today our freedom and our liberty is at stake."

WhatsApp drew praise when it began rolling out strong encryption of WhatsApp messages in 2014, built using the TextSecure protocol. Still, Whatsapp's implementation of the protocol means the central service can switch off that encryption at any time, potentially without notifying users that their messages are no longer encrypted. That leaves Koum and his company vulnerable to the same legal compulsion that the FBI is currently using against Apple.

WhatsApp has faced down similar court orders before. In December, the program was banned from Brazil for a full day after the company refused to comply with two local court orders.