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 could disallow 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]"