I ran into the same issue today, and it seems to be happening due to the recent deprecation of dispatch_get_current_queue() in iOS6.
By changing [RNCryptor initWithHandler:] to create a new queue the decryption works correctly.
NSString *responseQueueName = [@"net.robnapier.response." stringByAppendingString:NSStringFromClass([self class])];
_responseQueue = dispatch_queue_create([responseQueueName UTF8String], NULL);
You can find the fix and an associated unit test (based on your code) on the async_decrypt branch of my fork on github.