ECID

The Exclusive Chip ID or ECID, is an identifier unique to every unit.

ECID looks like this: 00000XXXXXXXXXXX (hex)

Please be aware that some tools (like TinyUmbrella) display the ECID in decimal format.

The IMG3 Tag

With the iPhone 3GS onward, the ECID tag with your ECID is put in the signature checked section of every firmware file, and sent to Apple's servers for re-signing to get a valid SHSH.

The implication of this is Apple has been disallowing downgrades (even via DFU Mode) on newer devices when a new firmware version is released.

Get your ECID

Mac Instructions

• Put your device in Recovery Mode or DFU Mode.
• Open System Profiler. (in /Applications/Utilities/)
• In the sidebar, go to "USB" (in the "Hardware" section)
• Under "Serial Number", there should be a part called "ECID". There you go.

Windows Instructions

• Put your device in Recovery Mode or DFU Mode.
• Open Device Manager and right click on Apple Mobile Device (Recovery or DFU Mode) for properties
• Click on the details tab
• Click on the dropdown box and select Device Instance Path
• You should find it in the textbox

ECID Caching

Saurik has developed his own server that has been designed to 'cache' every signature that Apple has given you.[1] Rather than being 'live' and talking directly to Apple's servers, it retrieves on file results.

To use this server, open up your hosts file and add the following line

74.208.10.249 gs.apple.com

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. Remember that this does NOT allow you to downgrade if your SHSH blobs for previous versions aren't on file on this server. In Cydia, you need to hit "Make My Life Easier" in order to backup SHSH blobs for that version of iOS.