The key that was released is for the XData Ransomware that was targeting the Ukraine around May 19th 2017. Kaspersky confirmed that this was indeed a valid key and updated their RakhniDecryptor to support the decryption of XData encrypted files. XData Heavily Targeted Ukraine on May 19, 2017, was especially the case with Ukraine, which was the victim of a targeted campaign. How to decrypt the encrypted files can be identified by their files being encrypted and renamed to the format of [filename].~xdata~ or [filename]-xdata-.
Source: https://www.bleepingcomputer.com/news/security/xdata-ransomware-master-decryption-keys-released-kaspersky-releases-decryptor-/