Philadelphia is a ransomware kit offered within various hacking communities. Written in AutoIt, it encrypts files using AES-256 encryption, file names using RC4 encryption and uses the *.locked file extension. It is based on a similar ransomware kit called 'Stampado' that is written by the same author.
To use the decrypter you will require a file pair containing both an encrypted file and its non-encrypted original version. Due to the file name encryption this can be a bit tricky. The best way is to simply compare file sizes. Encrypted files will have the size of the original file rounded up to the next 16 byte boundary.
So if a the original file was 1020 bytes large, the encrypted file will be 1024. Select both the encrypted and non-encrypted file and drag and drop both of them onto the decrypter file in your download directory.