From Diving into a Silverlight Exploit and Shellcode – Analysis and Techniques:
... We decided to translate this Assembly code into a Python script and write a generic tool decrypting payloads from Infinity (using the encrypted payload and the key as parameters).
This is the Python script.
If you have the encrypted payload, let’s say from a PCAP file, it isn’t a problem to find the key also since it is hardcoded in the shellcode a few packets before.
Usage:
InfinityPayloadDecrypter.py <encoded_file> <output_file> <key>