<s>
In	O
cryptography	O
,	O
the	O
interlock	B-Protocol
protocol	I-Protocol
,	O
as	O
described	O
by	O
Ron	O
Rivest	O
and	O
Adi	O
Shamir	O
,	O
is	O
a	O
protocol	B-Protocol
designed	O
to	O
frustrate	O
eavesdropper	O
attack	O
against	O
two	O
parties	O
that	O
use	O
an	O
anonymous	O
key	O
exchange	O
protocol	B-Protocol
to	O
secure	O
their	O
conversation	O
.	O
</s>
<s>
A	O
further	O
paper	O
proposed	O
using	O
it	O
as	O
an	O
authentication	O
protocol	B-Protocol
,	O
which	O
was	O
subsequently	O
broken	O
.	O
</s>
<s>
Most	O
cryptographic	B-Protocol
protocols	I-Protocol
rely	O
on	O
the	O
prior	O
establishment	O
of	O
secret	O
or	O
public	O
keys	O
or	O
passwords	O
.	O
</s>
<s>
However	O
,	O
the	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
protocol	B-Protocol
introduced	O
the	O
concept	O
of	O
two	O
parties	O
establishing	O
a	O
secure	O
channel	O
(	O
that	O
is	O
,	O
with	O
at	O
least	O
some	O
desirable	O
security	O
properties	O
)	O
without	O
any	O
such	O
prior	O
agreement	O
.	O
</s>
<s>
Unauthenticated	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
,	O
as	O
an	O
anonymous	O
key	O
agreement	O
protocol	B-Protocol
,	O
has	O
long	O
been	O
known	O
to	O
be	O
subject	O
to	O
man	O
in	O
the	O
middle	O
attack	O
.	O
</s>
<s>
The	O
Interlock	B-Protocol
Protocol	I-Protocol
was	O
described	O
as	O
a	O
method	O
to	O
expose	O
a	O
middle-man	O
who	O
might	O
try	O
to	O
compromise	O
two	O
parties	O
that	O
use	O
anonymous	O
key	O
agreement	O
to	O
secure	O
their	O
conversation	O
.	O
</s>
<s>
The	O
Interlock	B-Protocol
protocol	I-Protocol
works	O
roughly	O
as	O
follows	O
:	O
</s>
<s>
The	O
strength	O
of	O
the	O
protocol	B-Protocol
lies	O
in	O
the	O
fact	O
that	O
half	O
of	O
an	O
encrypted	O
message	O
cannot	O
be	O
decrypted	O
.	O
</s>
<s>
Davies	O
and	O
Price	O
proposed	O
the	O
use	O
of	O
the	O
Interlock	B-Protocol
Protocol	I-Protocol
for	O
authentication	O
in	O
a	O
book	O
titled	O
Security	O
for	O
Computer	O
Networks	O
.	O
</s>
<s>
Passwords	O
may	O
be	O
sent	O
using	O
the	O
Interlock	B-Protocol
Protocol	I-Protocol
between	O
A	O
and	O
B	O
as	O
follows	O
:	O
</s>
<s>
where	O
Ea	O
,	O
b(M )	O
is	O
message	O
M	O
encrypted	O
with	O
the	O
key	O
derived	O
from	O
the	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
exchange	O
between	O
A	O
and	O
B	O
,	O
<1>/<2>	O
denote	O
first	O
and	O
second	O
halves	O
,	O
and	O
Pa/Pb	O
are	O
the	O
passwords	O
of	O
A	O
and	O
B	O
.	O
</s>
<s>
A	O
modified	O
Interlock	B-Protocol
Protocol	I-Protocol
can	O
require	O
B	O
(	O
the	O
server	O
)	O
to	O
delay	O
all	O
responses	O
for	O
a	O
known	O
duration	O
:	O
</s>
<s>
Where	O
"	O
data	O
"	O
is	O
the	O
encrypted	O
data	O
that	O
immediately	O
follows	O
the	O
Interlock	B-Protocol
Protocol	I-Protocol
exchange	O
(	O
it	O
could	O
be	O
anything	O
)	O
,	O
encoded	O
using	O
an	O
all-or-nothing	O
transform	O
to	O
prevent	O
in-transit	O
modification	O
of	O
the	O
message	O
.	O
</s>
<s>
Of	O
course	O
,	O
Z	O
could	O
choose	O
to	O
not	O
perform	O
the	O
Interlock	B-Protocol
Protocol	I-Protocol
with	O
B	O
(	O
opting	O
to	O
instead	O
send	O
his	O
own	O
Mb	O
)	O
but	O
then	O
the	O
session	O
would	O
be	O
between	O
A	O
and	O
Z	O
,	O
not	O
A	O
,	O
Z	O
,	O
and	O
B	O
:	O
Z	O
would	O
n't	O
be	O
in	O
the	O
middle	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
the	O
interlock	B-Protocol
protocol	I-Protocol
cannot	O
be	O
effectively	O
used	O
to	O
provide	O
authentication	O
,	O
although	O
it	O
can	O
ensure	O
that	O
no	O
third	O
party	O
can	O
modify	O
the	O
messages	O
in	O
transit	O
without	O
detection	O
.	O
</s>
