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Nonce is a signing method that randomizes Apple's cryptographic signature hash blobs (SHSH blobs) and is used with the BBTicket (baseband signing ticket), the APTicket (firmware signing ticket), and SEP (Secure Enclave).


The device bootloaders (baseband, firmware, and SEP) generate a random number at the restore, then send them to iTunes which sends them to Apple. Then Apple sends the APTicket / BBTicket SHSH certificate with the number generated. This level is the most critical: the number within the certificate is matched to the number generated on device, and if they match iTunes will prepare the matched certificate and finally will stitch the randomized blobs to the firmware. If the blobs don't match the restore will fail and the bootloaders will reject the certificate.




Jailbreak difficulties

With nonce signatures, signatures can't be cached, because signatures will not match if they have been reused. In other words, the widely used replay attack is no longer possible unless the nonce(s) (SepNonce + ApNonce on 64 bit, ApNonce only on 32 bit) the device generates matches the one on the APTicket.

See also