To this end, I have constructed a server that duplicates the functionality exposed by Apple's signature server, except using "on file" results rather than live requests.
All we need, then, is to make iTunes use it. Luckily, most operating systems also have the ability to locally define bypasses on specific hostnames through a file calledhosts. Using this, we can redirect requests to Apple's signature server to Cydia.
So, open the file C:\Windows\System32\drivers\etc\hosts (Windows) or/etc/hosts (Mac OS X) and add the following entry to the bottom of the file.
Now, when iTunes thinks it is talking to Apple, it is talking to Cydia instead. Doing this will allow iTunes to access signatures already stored by Cydia's "on file" feature.
This server will also act as a cache for any SHSH blobs it hasn't seen, acting as an intermediary to Apple's server. This effectively registers your device with the "on file" mechanism, which means you can now enjoy the protections of being able to downgrade your firmware in the future even if you aren't jailbroken.
This point should be stressed: even if you don't jailbreak, and even if you never intend to jailbreak, you should consider using the new "on file" service.
Let's say that Apple releases an OS upgrade in the future, you take it, and they break something important. Maybe they break your e-mail account, or your todo list. Your business is now crippled.
If only you could downgrade, right? Alas, Apple won't let you anymore. That's where the new signature cache server comes in: by doing your restores through this server you secure your ability to not accept upgrades from Apple if the need is dire.