<s>
A	O
SHSH	B-Operating_System
blob	I-Operating_System
(	O
based	O
on	O
the	O
acronyms	O
for	O
signed	O
hash	B-Algorithm
and	O
binary	B-General_Concept
large	I-General_Concept
object	I-General_Concept
;	O
also	O
called	O
ECID	O
SHSH	B-Operating_System
,	O
referring	O
to	O
the	O
device	O
's	O
ECID	O
,	O
a	O
unique	O
identification	O
number	O
embedded	O
in	O
its	O
hardware	O
)	O
is	O
an	O
unofficial	O
term	O
referring	O
to	O
the	O
digital	O
signatures	O
that	O
Apple	O
generates	O
and	O
uses	O
to	O
personalize	O
IPSW	B-Operating_System
firmware	O
files	O
for	O
each	O
iOS	B-Operating_System
device	I-Operating_System
.	O
</s>
<s>
Apple	O
's	O
public	O
name	O
for	O
this	O
process	O
is	O
System	O
Software	O
Authorization	O
(	O
formerly	O
called	O
System	O
Software	O
Personalization	O
before	O
iOS	B-Operating_System
7	I-Operating_System
)	O
.	O
</s>
<s>
This	O
process	O
is	O
controlled	O
by	O
the	O
TATSU	O
(	O
"	O
TSS	O
"	O
)	O
Signing	O
Server	O
(	O
gs.apple.com	O
)	O
where	O
updates	O
and	O
restores	O
can	O
only	O
be	O
completed	O
by	O
iTunes	B-Operating_System
if	O
the	O
version	O
of	O
iOS	O
is	O
being	O
signed	O
.	O
</s>
<s>
Developers	O
interested	O
in	O
iOS	B-Application
jailbreaking	I-Application
have	O
made	O
tools	O
for	O
working	O
around	O
this	O
signature	O
system	O
in	O
order	O
to	O
install	O
jailbreakable	O
older	O
iOS	O
versions	O
that	O
are	O
no	O
longer	O
being	O
signed	O
by	O
Apple	O
.	O
</s>
<s>
SHSH	B-Operating_System
blobs	I-Operating_System
are	O
created	O
by	O
a	O
hashing	B-Algorithm
formula	I-Algorithm
that	O
has	O
multiple	O
keys	O
,	O
including	O
the	O
device	O
type	O
,	O
the	O
iOS	O
version	O
being	O
signed	O
,	O
and	O
the	O
device	O
's	O
ECID	O
.	O
</s>
<s>
When	O
Apple	O
wishes	O
to	O
restrict	O
users	O
 '	O
ability	O
to	O
restore	O
their	O
devices	O
to	O
a	O
particular	O
iOS	O
version	O
,	O
Apple	O
can	O
refuse	O
to	O
generate	O
this	O
hash	B-Algorithm
during	O
the	O
restore	O
attempt	O
,	O
and	O
the	O
restore	O
will	O
not	O
be	O
successful	O
(	O
or	O
at	O
least	O
will	O
require	O
bypassing	O
the	O
intended	O
function	O
of	O
the	O
system	O
)	O
.	O
</s>
<s>
When	O
iTunes	B-Operating_System
restores	O
or	O
updates	O
an	O
iOS	O
firmware	O
,	O
Apple	O
has	O
added	O
many	O
checkpoints	O
before	O
the	O
iOS	O
version	O
is	O
installed	O
and	O
on-device	O
consolidation	O
begins	O
.	O
</s>
<s>
At	O
the	O
first	O
"	O
Verifying	O
iPhone	O
software	O
"	O
iTunes	B-Operating_System
communicates	O
with	O
"	O
gs.apple.com	O
 "	O
to	O
verify	O
that	O
the	O
IPSW	B-Operating_System
file	O
provided	O
is	O
still	O
being	O
signed	O
.	O
</s>
<s>
iTunes	B-Operating_System
will	O
communicate	O
with	O
iBoot	O
throughout	O
the	O
process	O
of	O
an	O
update	O
or	O
restore	O
ensuring	O
the	O
firmware	O
has	O
not	O
been	O
modified	O
to	O
a	O
Custom	O
Firmware	O
(	O
"	O
CFW	O
"	O
)	O
.	O
</s>
<s>
iTunes	B-Operating_System
will	O
not	O
update	O
or	O
restore	O
a	O
device	O
when	O
it	O
suspects	O
the	O
file	O
has	O
been	O
modified	O
.	O
</s>
<s>
You	O
cannot	O
install	O
unsigned	O
iOS	O
versions	O
,	O
unless	O
1	O
)	O
you	O
possess	O
SHSH2	B-Operating_System
blobs	I-Operating_System
and	O
have	O
set	O
nonces	O
(	O
requiring	O
exploits	O
)	O
or	O
2	O
)	O
you	O
exploit	O
the	O
chain	O
process	O
.	O
</s>
<s>
The	O
requirement	O
of	O
SHSH	B-Operating_System
Blobs	I-Operating_System
in	O
order	O
to	O
install	O
to	O
unsigned	O
iOS	O
versions	O
can	O
be	O
bypassed	O
using	O
a	O
replay	O
attack	O
,	O
by	O
saving	O
blobs	B-General_Concept
while	O
an	O
iOS	O
firmware	O
is	O
still	O
signed	O
and	O
later	O
using	O
them	O
when	O
installing	O
the	O
firmware	O
.	O
</s>
<s>
Newer	O
iOS	O
versions	O
require	O
more	O
elements	O
,	O
such	O
as	O
a	O
valid	O
nonce	O
,	O
when	O
saving	O
SHSH	B-Operating_System
blobs	I-Operating_System
.	O
</s>
<s>
Saving	O
blobs	B-General_Concept
for	O
devices	O
using	O
the	O
A12	B-Device
SoC	I-Device
or	O
newer	O
also	O
requires	O
getting	O
a	O
matching	O
nonce	O
for	O
a	O
generator	O
from	O
a	O
device	O
to	O
save	O
valid	O
blobs	B-General_Concept
that	O
can	O
be	O
used	O
later	O
in	O
a	O
restore	O
.	O
</s>
<s>
Even	O
with	O
SHSH	B-Operating_System
blobs	I-Operating_System
saved	O
correctly	O
,	O
it	O
is	O
still	O
sometimes	O
not	O
possible	O
to	O
jump	O
to	O
certain	O
iOS	O
versions	O
due	O
to	O
incompatibility	O
of	O
the	O
SEP(Secure Enclave )	O
between	O
versions	O
.	O
</s>
<s>
Tools	O
to	O
save	O
SHSH	B-Operating_System
blobs	I-Operating_System
for	O
newer	O
iOS	O
versions	O
include	O
the	O
application	O
and	O
the	O
command	O
line	O
tool	O
.	O
</s>
<s>
To	O
use	O
SHSH	B-Operating_System
blobs	I-Operating_System
to	O
install	O
an	O
unsigned	O
iOS	O
version	O
on	O
a	O
device	O
,	O
tools	O
like	O
(	O
based	O
on	O
idevicerestore	O
)	O
or	O
its	O
can	O
be	O
used	O
,	O
which	O
allows	O
specification	O
of	O
iOS	O
firmware	O
files	O
and	O
SHSH	B-Operating_System
blobs	I-Operating_System
to	O
be	O
used	O
in	O
the	O
restore	O
.	O
</s>
<s>
For	O
iOS	O
3	O
and	O
4	O
,	O
SHSH	B-Operating_System
blobs	I-Operating_System
were	O
made	O
of	O
static	O
keys	O
(	O
such	O
as	O
the	O
device	O
type	O
,	O
iOS	O
version	O
,	O
and	O
ECID	O
)	O
,	O
which	O
meant	O
that	O
the	O
SHSH	B-Operating_System
blobs	I-Operating_System
for	O
a	O
specific	O
iOS	O
version	O
and	O
device	O
would	O
be	O
the	O
same	O
upon	O
every	O
restore	O
.	O
</s>
<s>
To	O
subvert	O
that	O
system	O
using	O
a	O
man-in-the-middle	O
attack	O
,	O
server	O
requests	O
the	O
unique	O
SHSH	B-Operating_System
blobs	I-Operating_System
from	O
Apple	O
for	O
the	O
jailbroken	B-Application
device	O
and	O
caches	O
those	O
SHSH	B-Operating_System
blobs	I-Operating_System
on	O
servers	O
,	O
so	O
that	O
if	O
a	O
user	O
changes	O
the	O
hosts	B-Protocol
file	I-Protocol
on	O
a	O
computer	O
to	O
redirect	O
the	O
SHSH	B-Operating_System
blobs	I-Operating_System
check	O
to	O
cache	O
instead	O
of	O
Apple	O
's	O
servers	O
,	O
iTunes	B-Operating_System
would	O
be	O
tricked	O
into	O
checking	O
those	O
cached	O
SHSH	B-Operating_System
blobs	I-Operating_System
and	O
allowing	O
the	O
device	O
to	O
be	O
restored	O
to	O
that	O
version	O
.	O
</s>
<s>
First	O
released	O
in	O
2009	O
,	O
TinyUmbrella	O
is	O
a	O
tool	O
for	O
finding	O
information	O
about	O
SHSH	B-Operating_System
blobs	I-Operating_System
saved	O
on	O
third	O
party	O
servers	O
,	O
saving	O
SHSH	B-Operating_System
blobs	I-Operating_System
locally	O
,	O
and	O
running	O
a	O
local	O
server	O
to	O
replay	O
SHSH	B-Operating_System
blobs	I-Operating_System
to	O
trick	O
iTunes	B-Operating_System
into	O
restoring	O
older	O
devices	O
to	O
iOS	O
3	O
and	O
4	O
.	O
</s>
<s>
In	O
June	O
2011	O
,	O
iH8sn0w	O
released	O
iFaith	O
,	O
a	O
tool	O
that	O
can	O
grab	O
partial	O
SHSH	B-Operating_System
blobs	I-Operating_System
from	O
a	O
device	O
for	O
its	O
currently-installed	O
iOS	O
version	O
(	O
limited	O
to	O
iPhone	O
4	O
and	O
older	O
devices	O
)	O
.	O
</s>
<s>
In	O
late	O
2011	O
,	O
the	B-Application
iPhone	I-Application
Dev	I-Application
Team	I-Application
added	O
features	O
to	O
redsn0w	O
that	O
include	O
the	O
ability	O
to	O
save	O
SHSH	B-Operating_System
blobs	I-Operating_System
with	O
APTickets	O
and	O
stitch	O
them	O
into	O
custom	O
firmware	O
in	O
order	O
to	O
restore	O
a	O
device	O
to	O
iOS	O
5	O
or	O
later	O
.	O
</s>
<s>
Replaying	O
SHSH	B-Operating_System
blobs	I-Operating_System
for	O
newer	O
devices	O
(	O
iPad	O
2	O
and	O
later	O
)	O
is	O
not	O
always	O
possible	O
,	O
because	O
there	O
are	O
no	O
boot	B-General_Concept
ROM	I-General_Concept
(	O
hardware	O
level	O
)	O
exploits	O
available	O
for	O
these	O
devices	O
.	O
</s>
