<s>
A	O
cryptosystem	B-General_Concept
is	O
considered	O
to	O
have	O
information-theoretic	B-Algorithm
security	I-Algorithm
(	O
also	O
called	O
unconditional	B-Algorithm
security	I-Algorithm
)	O
if	O
the	O
system	O
is	O
secure	O
against	O
adversaries	B-General_Concept
with	O
unlimited	O
computing	O
resources	O
and	O
time	O
.	O
</s>
<s>
An	O
encryption	O
protocol	O
with	O
information-theoretic	B-Algorithm
security	I-Algorithm
is	O
impossible	O
to	O
break	O
even	O
with	O
infinite	O
computational	O
power	O
.	O
</s>
<s>
Protocols	O
proven	O
to	O
be	O
information-theoretically	B-Algorithm
secure	I-Algorithm
are	O
resistant	O
to	O
future	O
developments	O
in	O
computing	O
.	O
</s>
<s>
The	O
concept	O
of	O
information-theoretically	B-Algorithm
secure	I-Algorithm
communication	O
was	O
introduced	O
in	O
1949	O
by	O
American	O
mathematician	O
Claude	O
Shannon	O
,	O
one	O
of	O
the	O
founders	O
of	O
classical	O
information	O
theory	O
,	O
who	O
used	O
it	O
to	O
prove	O
the	O
one-time	B-Algorithm
pad	I-Algorithm
system	O
was	O
secure	O
.	O
</s>
<s>
Information-theoretically	B-Algorithm
secure	I-Algorithm
cryptosystems	B-General_Concept
have	O
been	O
used	O
for	O
the	O
most	O
sensitive	O
governmental	O
communications	O
,	O
such	O
as	O
diplomatic	O
cables	O
and	O
high-level	O
military	O
communications	O
.	O
</s>
<s>
There	O
are	O
a	O
variety	O
of	O
cryptographic	O
tasks	O
for	O
which	O
information-theoretic	B-Algorithm
security	I-Algorithm
is	O
a	O
meaningful	O
and	O
useful	O
requirement	O
.	O
</s>
<s>
Secret	O
sharing	O
schemes	O
such	O
as	O
Shamir	B-Architecture
's	I-Architecture
are	O
information-theoretically	B-Algorithm
secure	I-Algorithm
(	O
and	O
also	O
perfectly	O
secure	O
)	O
in	O
that	O
having	O
less	O
than	O
the	O
requisite	O
number	O
of	O
shares	O
of	O
the	O
secret	O
provides	O
no	O
information	O
about	O
the	O
secret	O
.	O
</s>
<s>
More	O
generally	O
,	O
secure	B-Protocol
multiparty	I-Protocol
computation	I-Protocol
protocols	O
often	O
have	O
information-theoretic	B-Algorithm
security	I-Algorithm
.	O
</s>
<s>
Reductions	B-Algorithm
between	O
cryptographic	O
primitives	O
or	O
tasks	O
can	O
often	O
be	O
achieved	O
information-theoretically	O
.	O
</s>
<s>
Such	O
reductions	B-Algorithm
are	O
important	O
from	O
a	O
theoretical	O
perspective	O
because	O
they	O
establish	O
that	O
primitive	O
can	O
be	O
realized	O
if	O
primitive	O
can	O
be	O
realized	O
.	O
</s>
<s>
Symmetric	B-Algorithm
encryption	I-Algorithm
can	O
be	O
constructed	O
under	O
an	O
information-theoretic	O
notion	O
of	O
security	O
called	O
entropic	O
security	O
,	O
which	O
assumes	O
that	O
the	O
adversary	B-General_Concept
knows	O
almost	O
nothing	O
about	O
the	O
message	O
being	O
sent	O
.	O
</s>
<s>
For	O
example	O
,	O
RSA	B-Architecture
relies	O
on	O
the	O
assertion	O
that	O
factoring	O
large	O
numbers	O
is	O
hard	O
.	O
</s>
<s>
Another	O
scheme	O
using	O
pattern-reconfigurable	O
transmit	O
antennas	O
for	O
Alice	O
called	O
reconfigurable	O
multiplicative	B-Algorithm
noise	I-Algorithm
(	O
RMN	O
)	O
complements	O
additive	O
artificial	O
noise	O
.	O
</s>
<s>
The	O
different	O
works	O
mentioned	O
in	O
the	O
previous	O
part	O
employ	O
,	O
in	O
one	O
way	O
or	O
another	O
,	O
the	O
randomness	O
present	O
in	O
the	O
wireless	O
channel	O
to	O
transmit	O
information-theoretically	B-Algorithm
secure	I-Algorithm
messages	O
.	O
</s>
