OSX/KeRanger is OS X ransomware that infected the Transmission BitTorrent client installer and demanded payment of ~$400 in bitcoin in exchange for decryption on encrypted files [1].

References:

1. http://researchcenter.paloaltonetworks.com/2016/03/new-os-x-ransomware-keranger-infected-transmission-bittorrent-client-installer/