When investigating an application I found a cryptographic key. Based on the documentation related to how the key is used I know it's some form of symmetric key. However I don't know what cipher it is used in and I can't find out from the application as that would require reverse engineering compiled c++, A skill I don't possess. The key does not appear to be user chosen and is represented as 64 hexadecimal characters (256 bits). My question is, is there any program or test that could be easily preformed on the key to determine if it is valid for common ciphers? Such as a check to see if it's a valid aes-256 key or similar. Thanks
As mentioned, there are no constraints on valid keys. This would limit the keyspace, making the algorithm less secure. You say you don't have reverse engineering skills to throw at this problem, and you don't mention the app used. This leaves us at a dead end because we can't help as far as i can see. You could however try detecting the magic constants with an IDA plugin like FindCrypt (http://www.hexblog.com/?p=27), that would require only basic use of RE tools.
Can you open it with a certificate tool like Cleopatra? If you are on Windows give it a crt or pem file extension and Windows will usually open it in their Certificate viewer.