<s>
In	O
cryptography	O
the	O
standard	B-General_Concept
model	I-General_Concept
is	O
the	O
model	O
of	O
computation	O
in	O
which	O
the	O
adversary	B-General_Concept
is	O
only	O
limited	O
by	O
the	O
amount	O
of	O
time	O
and	O
computational	O
power	O
available	O
.	O
</s>
<s>
Schemes	O
that	O
can	O
be	O
proven	O
secure	O
using	O
only	O
complexity	O
assumptions	O
are	O
said	O
to	O
be	O
secure	O
in	O
the	O
standard	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
Security	O
proofs	O
are	O
notoriously	O
difficult	O
to	O
achieve	O
in	O
the	O
standard	B-General_Concept
model	I-General_Concept
,	O
so	O
in	O
many	O
proofs	O
,	O
cryptographic	O
primitives	O
are	O
replaced	O
by	O
idealized	O
versions	O
.	O
</s>
<s>
The	O
most	O
common	O
example	O
of	O
this	O
technique	O
,	O
known	O
as	O
the	O
random	B-Application
oracle	I-Application
model	I-Application
,	O
involves	O
replacing	O
a	O
cryptographic	B-Algorithm
hash	I-Algorithm
function	I-Algorithm
with	O
a	O
genuinely	O
random	O
function	O
.	O
</s>
<s>
Another	O
example	O
is	O
the	O
generic	B-General_Concept
group	I-General_Concept
model	I-General_Concept
,	O
where	O
the	O
adversary	B-General_Concept
is	O
given	O
access	O
to	O
a	O
randomly	O
chosen	O
encoding	O
of	O
a	O
group	O
,	O
instead	O
of	O
the	O
finite	O
field	O
or	O
elliptic	O
curve	O
groups	O
used	O
in	O
practice	O
.	O
</s>
<s>
Other	O
models	O
used	O
invoke	O
trusted	O
third	O
parties	O
to	O
perform	O
some	O
task	O
without	O
cheating	O
;	O
for	O
example	O
,	O
the	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
(	O
PKI	O
)	O
model	O
requires	O
a	O
certificate	B-Architecture
authority	I-Architecture
,	O
which	O
if	O
it	O
were	O
dishonest	O
,	O
could	O
produce	O
fake	O
certificates	O
and	O
use	O
them	O
to	O
forge	O
signatures	O
,	O
or	O
mount	O
a	O
man	O
in	O
the	O
middle	O
attack	O
to	O
read	O
encrypted	O
messages	O
.	O
</s>
<s>
Other	O
examples	O
of	O
this	O
type	O
are	O
the	O
common	O
random	O
string	O
model	O
,	O
where	O
it	O
is	O
assumed	O
that	O
all	O
parties	O
have	O
access	O
to	O
some	O
string	O
chosen	O
uniformly	O
at	O
random	O
,	O
and	O
its	O
generalization	O
,	O
the	O
common	B-General_Concept
reference	I-General_Concept
string	I-General_Concept
model	I-General_Concept
,	O
where	O
a	O
string	O
is	O
chosen	O
according	O
to	O
some	O
other	O
probability	O
distribution	O
.	O
</s>
<s>
In	O
some	O
applications	O
,	O
such	O
as	O
the	O
Dolev	O
–	O
Dwork	O
–	O
Naor	O
encryption	O
scheme	O
,	O
it	O
makes	O
sense	O
for	O
a	O
particular	O
party	O
to	O
generate	O
the	O
common	B-General_Concept
reference	I-General_Concept
string	I-General_Concept
,	O
while	O
in	O
other	O
applications	O
,	O
the	O
common	B-General_Concept
reference	I-General_Concept
string	I-General_Concept
must	O
be	O
generated	O
by	O
a	O
trusted	O
third	O
party	O
.	O
</s>
