<s>
EPOC	O
(	O
Efficient	O
Probabilistic	O
Public	B-Application
Key	I-Application
Encryption	I-Application
)	O
is	O
a	O
probabilistic	O
public-key	B-Application
encryption	I-Application
scheme	O
.	O
</s>
<s>
It	O
is	O
based	O
on	O
the	O
random	B-Application
oracle	I-Application
model	I-Application
,	O
in	O
which	O
a	O
primitive	O
public-key	B-Application
encryption	I-Application
function	O
is	O
converted	O
to	O
a	O
secure	O
encryption	O
scheme	O
by	O
use	O
of	O
a	O
truly	O
random	O
hash	O
function	O
;	O
the	O
resulting	O
scheme	O
is	O
designed	O
to	O
be	O
semantically	O
secure	O
against	O
a	O
chosen	O
ciphertext	O
attack	O
.	O
</s>
<s>
EPOC	O
's	O
primitive	O
encryption	O
function	O
is	O
the	O
OU	O
(	O
Okamoto	O
–	O
Uchiyama	O
)	O
function	O
,	O
in	O
which	O
to	O
invert	O
the	O
OU	O
function	O
is	O
proven	O
to	O
be	O
as	O
hard	O
as	O
factoring	O
a	O
composite	O
integer	O
public	B-Application
key	I-Application
.	O
</s>
