computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...

, a SHSH blob, short for Signature HaSH blob, is a digital signature that

Apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...

generates and uses to control the iOS versions that users can install on their iOS devices generally only allowing the newest iOS version to be installable. Apple's public name for this process is ''System Software Authorization'' (before

iOS 7 iOS 7 is the seventh major release of the iOS mobile operating system developed by Apple Inc., being the successor to iOS 6. It was announced at the company's Worldwide Developers Conference on June 10, 2013, and was released on September 18 ...

, ''System Software Personalization''). The term “SHSH blob” is unofficial and based on abbreviations for signed hash and

binary large object Object storage (also known as object-based storage or blob storage) is a computer data storage approach that manages data as "blobs" or "objects", as opposed to other storage architectures like file systems, which manage data as a file hierarchy, ...

. An alternative term, ''ECID SHSH'', refers to the device's ECID, a unique identification number embedded in its hardware. This process is controlled by the TATSU ("TSS") Signing Server (gs.apple.com) where updates and restores can only be completed by

iTunes iTunes is a media player, media library, and mobile device management (MDM) utility developed by Apple. It is used to purchase, play, download and organize digital multimedia on personal computers running the macOS and Windows operating s ...

if the version of iOS is being signed. Developers interested in

iOS jailbreaking iOS jailbreaking is the use of a privilege escalation exploit to remove software restrictions imposed by Apple on devices running iOS and iOS-based operating systems. It is typically done through a series of kernel patches. A jailbroken devic ...

have made tools for working around this signature system in order to install jailbreakable older iOS versions that are no longer being signed by Apple.

Technical details

SHSH blobs are created by a hashing formula that has multiple keys, including the device type, the iOS version being signed, and the device's ECID. When Apple wishes to restrict users' ability to restore their devices to a particular iOS version, Apple can refuse to generate this hash during the restore attempt, and the restore will not be successful (or at least will require bypassing the intended function of the system). This protocol is part of iPhone 3GS and later devices.

TATSU Signing Server

When iTunes restores or updates an iOS firmware, Apple has added many checkpoints before the iOS version is installed and on-device consolidation begins. At the first "Verifying iPhone software" iTunes communicates with "gs.apple.com" to verify that the IPSW file provided is still being signed. The TATSU server will give back a list of versions being signed. If the version is not being signed, then iBEC and iBoot will decline the image, giving an error of "error 3194" or "declined to authorize the image". iTunes will communicate with iBoot throughout the process of an update or restore ensuring the firmware has not been modified to a Custom Firmware ("CFW"). iTunes will not update or restore a device when it suspects the file has been modified. This is a chain process, before installing the firmware, the installed iBoot has to verify the to-be-installed iBoot, and so on. You cannot install unsigned iOS versions, unless 1) you possess SHSH2 blobs and have set nonces (requiring exploits) or 2) you exploit the chain process.

Exploits and countermeasures

The requirement of SHSH Blobs in order to install to unsigned iOS versions can be bypassed using a

replay attack A replay attack (also known as a repeat attack or playback attack) is a form of network attack in which valid data transmission is maliciously or fraudulently repeated or delayed. This is carried out either by the originator or by an adversary w ...

, by saving blobs while an iOS firmware is still signed and later using them when installing the firmware. Newer iOS versions require more elements, such as a valid nonce, when saving SHSH blobs. Saving blobs for devices using the A12 SoC or newer also requires getting a matching nonce for a generator from a device to save valid blobs that can be used later in a restore. Even with SHSH blobs saved correctly, it is still sometimes not possible to jump to certain iOS versions due to incompatibility of the SEP (Secure Enclave) between versions. Tools to save SHSH blobs for newer iOS versions include the applicatio
blobsaver
and the command line too
tsschecker
To use SHSH blobs to install an unsigned iOS version on a device, tools lik
futurerestore
(based on idevicerestore) or it
GUIs
can be used, which allows specification of iOS firmware files and SHSH blobs to be used in the restore.

Previous bypass methods

For iOS 3 and 4, SHSH blobs were made of static keys (such as the device type, iOS version, and ECID), which meant that the SHSH blobs for a specific iOS version and device would be the same upon every restore. To subvert that system using a

man-in-the-middle attack In cryptography and computer security, a man-in-the-middle (MITM) attack, or on-path attack, is a cyberattack where the attacker secretly relays and possibly alters the communications between two parties who believe that they are directly communi ...

, server responses for the unique SHSH blobs from Apple for the jailbroken device are cached, so that if a user changes the

hosts file The computer file hosts is an operating system file that maps hostnames to IP addresses. It is a plain text file. Originally a file named HOSTS.TXT was manually maintained and made available via file sharing by Stanford Research Institute for th ...

on a computer to redirect the SHSH blobs check to cache instead of Apple's servers,

would be tricked into checking those cached SHSH blobs and allowing the device to be restored to that version. iOS 5 and later versions of iOS implement an addition to this system, a random number (a

cryptographic nonce In cryptography, a nonce is an arbitrary number that can be used just once in a cryptographic communication. It is often a random or pseudo-random number issued in an authentication protocol to ensure that each communication session is unique, ...

) in the "APTicket", making that simple

no longer effective. First released in 2009, TinyUmbrella is a tool for finding information about SHSH blobs saved on third party servers, saving SHSH blobs locally, and running a local server to replay SHSH blobs to trick iTunes into restoring older devices to iOS 3 and 4. In June 2011, iH8sn0w released iFaith, a tool that can grab partial SHSH blobs from a device for its currently-installed iOS version (limited to iPhone 4 and older devices). In late 2011, the iPhone Dev Team added features to

redsn0w iOS jailbreaking is the use of a privilege escalation exploit to remove software restrictions imposed by Apple on devices running iOS and iOS-based operating systems. It is typically done through a series of kernel patches. A jailbroken device ...

that include the ability to save SHSH blobs with APTickets and stitch them into custom firmware in order to restore a device to iOS 5 or later. Replaying SHSH blobs for newer devices (

Apple A12 The Apple A12 Bionic is a 64-bit ARM-based system on a chip (SoC) designed by Apple Inc., part of the Apple silicon series, It first appeared in the iPhone XS and XS Max, iPhone XR, iPad Air (3rd generation), iPad Mini (5th generation), iPad ...

and later) is not always possible, because there are no

boot ROM Boot ROM is a piece of read-only memory (ROM) that is used for booting a computer system. It contains instructions that are run after the CPU is reset to the reset vector, and it typically loads a bootloader. There are two types of boot ROM: ...

(hardware level) exploits available for these devices. As of October 2012, redsn0w includes features for restoring newer devices between different versions of iOS 5, but it cannot downgrade newer devices from iOS 6 to iOS 5.

References

{{Reflist, 2 Apple Inc. software IOS jailbreaking Hardware restrictions

Technical details

TATSU Signing Server

Exploits and countermeasures

Previous bypass methods

See also

References