The author of the original Petya malware released his master key. The master key is destroyed after use and cannot be recovered. WARNING: This tool is an experiment in unlocking a particular kind of Ransomware. It works for Stage 1 of encryption if the system was not rebooted after the infection. Research about a possibility to decrypt Stage 2 is in progress. The latest version of the decoder is available here:https://github.com/hasherezade/petya_recovery (64 bit ELF)”]
Source: https://hshrzd.wordpress.com/2016/03/31/petya-key-decoder/