<s>
In	O
cryptography	O
,	O
an	O
adversary	B-General_Concept
(	O
rarely	O
opponent	O
,	O
enemy	O
)	O
is	O
a	O
malicious	O
entity	O
whose	O
aim	O
is	O
to	O
prevent	O
the	O
users	O
of	O
the	O
cryptosystem	B-General_Concept
from	O
achieving	O
their	O
goal	O
(	O
primarily	O
privacy	O
,	O
integrity	O
,	O
and	O
availability	O
of	O
data	O
)	O
.	O
</s>
<s>
An	O
adversary	B-General_Concept
's	O
efforts	O
might	O
take	O
the	O
form	O
of	O
attempting	O
to	O
discover	O
secret	O
data	O
,	O
corrupting	O
some	O
of	O
the	O
data	O
in	O
the	O
system	O
,	O
spoofing	O
the	O
identity	O
of	O
a	O
message	O
sender	O
or	O
receiver	O
,	O
or	O
forcing	O
system	O
downtime	O
.	O
</s>
<s>
Actual	O
adversaries	B-General_Concept
,	O
as	O
opposed	O
to	O
idealized	O
ones	O
,	O
are	O
referred	O
to	O
as	O
attackers	O
.	O
</s>
<s>
This	O
notion	O
of	O
an	O
adversary	B-General_Concept
helps	O
both	O
intuitive	O
and	O
formal	O
reasoning	O
about	O
cryptosystems	B-General_Concept
by	O
casting	O
security	O
analysis	O
of	O
cryptosystems	B-General_Concept
as	O
a	O
'	O
game	O
 '	O
between	O
the	O
users	O
and	O
a	O
centrally	O
co-ordinated	O
enemy	O
.	O
</s>
<s>
The	O
notion	O
of	O
security	O
of	O
a	O
cryptosystem	B-General_Concept
is	O
meaningful	O
only	O
with	O
respect	O
to	O
particular	O
attacks	O
(	O
usually	O
presumed	O
to	O
be	O
carried	O
out	O
by	O
particular	O
sorts	O
of	O
adversaries	B-General_Concept
)	O
.	O
</s>
<s>
There	O
are	O
several	O
types	O
of	O
adversaries	B-General_Concept
depending	O
on	O
what	O
capabilities	O
or	O
intentions	O
they	O
are	O
presumed	O
to	O
have	O
.	O
</s>
<s>
In	O
actual	O
security	O
practice	O
,	O
the	O
attacks	O
assigned	O
to	O
such	O
adversaries	B-General_Concept
are	O
often	O
seen	O
,	O
so	O
such	O
notional	O
analysis	O
is	O
not	O
merely	O
theoretical	O
.	O
</s>
<s>
How	O
successful	O
an	O
adversary	B-General_Concept
is	O
at	O
breaking	O
a	O
system	O
is	O
measured	O
by	O
its	O
advantage	O
.	O
</s>
<s>
An	O
adversary	B-General_Concept
's	O
advantage	O
is	O
the	O
difference	O
between	O
the	O
adversary	B-General_Concept
's	O
probability	O
of	O
breaking	O
the	O
system	O
and	O
the	O
probability	O
that	O
the	O
system	O
can	O
be	O
broken	O
by	O
simply	O
guessing	O
.	O
</s>
