With Python's boto, how can I get the contents of an encrypted file?

With an unencrypted file, I can do the following:

key = s3_bucket.get_key(path)
value_as_string = key.get_contents_as_string()

But if the file's encrypted, I need to change something about that. I can't figure out what from reading the docs. What do I change?

I know the master symmetric key, which is a string like 30 chars or so long.

via Chebli Mohamed