<s>
In	O
public-key	B-Application
cryptography	I-Application
,	O
the	O
Station-to-Station	O
(	O
STS	O
)	O
protocol	O
is	O
a	O
cryptographic	O
key	B-Protocol
agreement	I-Protocol
scheme	O
.	O
</s>
<s>
The	O
protocol	O
is	O
based	O
on	O
classic	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
,	O
and	O
provides	O
mutual	O
key	O
and	O
entity	O
authentication	O
.	O
</s>
<s>
Unlike	O
the	O
classic	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
,	O
which	O
is	O
not	O
secure	O
against	O
a	O
man-in-the-middle	O
attack	O
,	O
this	O
protocol	O
assumes	O
that	O
the	O
parties	O
have	O
signature	O
keys	O
,	O
which	O
are	O
used	O
to	O
sign	O
messages	O
,	O
thereby	O
providing	O
security	O
against	O
man-in-the-middle	O
attacks	O
.	O
</s>
<s>
It	O
also	O
entails	O
two-way	O
explicit	O
key	O
confirmation	O
,	O
making	O
it	O
an	O
authenticated	O
key	B-Protocol
agreement	I-Protocol
with	O
key	O
confirmation	O
(	O
AKC	O
)	O
protocol	O
.	O
</s>
<s>
In	O
the	O
following	O
explanations	O
,	O
exponentiation	O
(	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
)	O
operations	O
provide	O
the	O
basis	O
for	O
key	B-Protocol
agreement	I-Protocol
,	O
though	O
this	O
is	O
not	O
a	O
requirement	O
.	O
</s>
<s>
An	O
asymmetric	O
signature	O
keypair	B-Application
for	O
each	O
party	O
Required	O
for	O
authentication	O
.	O
</s>
<s>
The	O
public	O
portion	O
of	O
this	O
keypair	B-Application
may	O
be	O
shared	O
prior	O
to	O
session	O
establishment	O
.	O
</s>
<s>
Alice	O
decrypts	O
and	O
verifies	O
Bob	O
's	O
signature	O
using	O
his	O
asymmetric	O
public	B-Application
key	I-Application
.	O
</s>
<s>
Bob	O
decrypts	O
and	O
verifies	O
Alice	O
's	O
signature	O
using	O
her	O
asymmetric	O
public	B-Application
key	I-Application
.	O
</s>
<s>
Public	B-Application
key	I-Application
certificates	O
may	O
be	O
sent	O
in	O
steps	O
2	O
and	O
3	O
if	O
the	O
keys	O
are	O
not	O
known	O
in	O
advance	O
.	O
</s>
<s>
It	O
uses	O
random	O
number	O
challenges	O
instead	O
of	O
the	O
above	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
technique	O
.	O
</s>
<s>
Bob	O
concatenates	O
the	O
random	O
numbers	O
(	O
y	O
,	O
x	O
)	O
(	O
order	O
is	O
important	O
)	O
and	O
signs	O
them	O
using	O
his	O
asymmetric	B-Application
key	I-Application
B	O
.	O
</s>
<s>
Alice	O
concatenates	O
the	O
random	O
numbers	O
(	O
x	O
,	O
y	O
)	O
(	O
order	O
is	O
important	O
)	O
and	O
signs	O
them	O
using	O
her	O
asymmetric	B-Application
key	I-Application
A	O
.	O
</s>
<s>
In	O
cases	O
where	O
encryption	O
is	O
a	O
not	O
viable	O
choice	O
in	O
session	O
establishment	O
,	O
K	O
can	O
instead	O
be	O
used	O
to	O
create	O
a	O
MAC	B-Algorithm
.	O
</s>
<s>
The	O
paper	O
goes	O
on	O
to	O
counsel	O
,	O
however	O
,	O
that	O
using	O
K	O
for	O
both	O
a	O
MAC	B-Algorithm
and	O
as	O
the	O
session	O
key	O
violates	O
the	O
principle	O
that	O
keys	O
should	O
not	O
be	O
used	O
for	O
more	O
than	O
one	O
purpose	O
,	O
and	O
presents	O
various	O
workarounds	O
.	O
</s>
<s>
STS-MAC	O
is	O
vulnerable	O
to	O
some	O
unknown	O
key-share	O
attacks	O
,	O
whereby	O
an	O
active	O
attacker	O
can	O
inject	O
his	O
own	O
identity	O
into	O
the	O
session	O
establishment	O
procedure	O
in	O
place	O
of	O
either	O
initiator	O
or	O
responder	O
.	O
</s>
