Update: tihmstar has stated that the tool is working perfectly, and that he has used it successfully multiple times today. However, some users have been encountering a TSS server error when using the tool nonetheless.
Hot on the heels of Prometheus' release at the 33c3 convention two days ago, tihmstar's tool for upgrading and downgrading to unsigned iOS firmwares seems already to have hit a snag. Following some changes to Apple's TSS servers today, the nature of which are yet to be investigated, the tool has been rendered non-functional at the present time.