ECID

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

ECID looks like this: 00000XXXXXXXXXXX

The Img3 Tag

With the iPhone 3GS, 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) on newer devices when a new firmware version is released.

Get your ECID

• Windows: use [ECID Grabber] Tool
• Read USB-Device infos when Apple Device in connected in Recovery Mode or DFU
  • Recovery Mode:

    iProduct: "Apple Mobile Device (Recovery Mode)"

    iSerialNumber: "CPID:8920 CPRV:15 CPFM:03 SCEP:03 BDID:00 ECID:XXXXXXXXXXXXXXXX IBFL:01 SRNM:[XXXXXXXXXXX] IMEI:[XXXXXXXXXXXXXXX]"

  • DFU:

    iProduct: "Apple Mobile Device (DFU Mode)"

    iSerialNumber: "CPID:8920 CPRV:15 CPFM:03 SCEP:03 BDID:00 ECID:XXXXXXXXXXXXXXXX SRTG:[iBoot-XXX.X.X]"

ECID Caching

Saurik has developed his own server that has been designed to 'cache' every signature that Apple has given you. 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

74.208.105.171 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.

Caching Apple's Signature Server