Version 2.90 of Transmission has been infected with ransomware. It is believed the Transmission website was compromised. The KeRanger malware imposes a 72 hour lockout window unless the user pays 1 bitcoin (€376.00). Transmission recommend you delete version 2.90 from your computer, or upgrade to version 2.91 which is safe.
The KeRanger malware appears still to be under development, and it is reported to attempt to encrypt Time Machine backup files to prevent users recovering their files.