<s>
In	O
cryptography	O
,	O
a	O
key-agreement	B-Protocol
protocol	I-Protocol
is	O
a	O
protocol	O
whereby	O
two	O
or	O
more	O
parties	O
can	O
agree	O
on	O
a	O
cryptographic	O
key	O
in	O
such	O
a	O
way	O
that	O
both	O
influence	O
the	O
outcome	O
.	O
</s>
<s>
Using	O
a	O
key-agreement	B-Protocol
protocol	I-Protocol
avoids	O
some	O
of	O
the	O
key	O
distribution	O
problems	O
associated	O
with	O
such	O
systems	O
.	O
</s>
<s>
The	O
first	O
publicly	O
known	O
public-key	O
agreement	O
protocol	O
that	O
meets	O
the	O
above	O
criteria	O
was	O
the	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
,	O
in	O
which	O
two	O
parties	O
jointly	O
exponentiate	O
a	O
generator	O
with	O
random	O
numbers	O
,	O
in	O
such	O
a	O
way	O
that	O
an	O
eavesdropper	O
cannot	O
feasibly	O
determine	O
what	O
the	O
resultant	O
value	O
used	O
to	O
produce	O
a	O
shared	B-Algorithm
key	I-Algorithm
is	O
.	O
</s>
<s>
It	O
has	O
thus	O
been	O
described	O
as	O
an	O
anonymous	O
key	B-Protocol
agreement	I-Protocol
protocol	I-Protocol
.	O
</s>
<s>
Anonymous	O
key	O
exchange	O
,	O
like	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
,	O
does	O
not	O
provide	O
authentication	O
of	O
the	O
parties	O
,	O
and	O
is	O
thus	O
vulnerable	O
to	O
man-in-the-middle	O
attacks	O
.	O
</s>
<s>
A	O
wide	O
variety	O
of	O
cryptographic	O
authentication	O
schemes	O
and	O
protocols	O
have	O
been	O
developed	O
to	O
provide	O
authenticated	O
key	B-Protocol
agreement	I-Protocol
to	O
prevent	O
man-in-the-middle	O
and	O
related	O
attacks	O
.	O
</s>
<s>
When	O
Alice	O
and	O
Bob	O
have	O
a	O
public-key	O
infrastructure	O
,	O
they	O
may	O
digitally	O
sign	O
an	O
agreed	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	O
,	O
or	O
exchanged	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
public	O
keys	O
.	O
</s>
<s>
Such	O
signed	O
keys	O
,	O
sometimes	O
signed	O
by	O
a	O
certificate	B-Architecture
authority	I-Architecture
,	O
are	O
one	O
of	O
the	O
primary	O
mechanisms	O
used	O
for	O
secure	O
web	O
traffic	O
(	O
including	O
HTTPS	B-Protocol
,	O
SSL	B-Protocol
or	O
Transport	B-Protocol
Layer	I-Protocol
Security	I-Protocol
protocols	I-Protocol
)	O
.	O
</s>
<s>
Other	O
specific	O
examples	O
are	O
MQV	B-Protocol
,	O
YAK	B-Protocol
and	O
the	O
ISAKMP	B-Protocol
component	O
of	O
the	O
IPsec	O
protocol	O
suite	O
for	O
securing	O
Internet	O
Protocol	O
communications	O
.	O
</s>
<s>
However	O
,	O
these	O
systems	O
require	O
care	O
in	O
endorsing	O
the	O
match	O
between	O
identity	O
information	O
and	O
public	O
keys	O
by	O
certificate	B-Architecture
authorities	I-Architecture
in	O
order	O
to	O
work	O
properly	O
.	O
</s>
<s>
Hybrid	O
systems	O
use	O
public-key	O
cryptography	O
to	O
exchange	O
secret	O
keys	O
,	O
which	O
are	O
then	O
used	O
in	O
a	O
symmetric-key	B-Algorithm
cryptography	I-Algorithm
systems	O
.	O
</s>
<s>
Password-authenticated	B-Protocol
key	I-Protocol
agreement	I-Protocol
protocols	O
require	O
the	O
separate	O
establishment	O
of	O
a	O
password	O
(	O
which	O
may	O
be	O
smaller	O
than	O
a	O
key	O
)	O
in	O
a	O
manner	O
that	O
is	O
both	O
private	O
and	O
integrity-assured	O
.	O
</s>
<s>
For	O
example	O
,	O
DH-EKE	O
,	O
SPEKE	B-Protocol
,	O
and	O
SRP	B-Protocol
are	O
password-authenticated	O
variations	O
of	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
.	O
</s>
<s>
If	O
one	O
has	O
an	O
integrity-assured	O
way	O
to	O
verify	O
a	O
shared	B-Algorithm
key	I-Algorithm
over	O
a	O
public	O
channel	O
,	O
one	O
may	O
engage	O
in	O
a	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
to	O
derive	O
a	O
short-term	O
shared	B-Algorithm
key	I-Algorithm
,	O
and	O
then	O
subsequently	O
authenticate	O
that	O
the	O
keys	O
match	O
.	O
</s>
<s>
One	O
way	O
is	O
to	O
use	O
a	O
voice-authenticated	O
read-out	O
of	O
the	O
key	O
,	O
as	O
in	O
PGPfone	B-General_Concept
.	O
</s>
<s>
Variations	O
on	O
this	O
theme	O
have	O
been	O
proposed	O
for	O
Bluetooth	B-Protocol
pairing	O
protocols	O
.	O
</s>
<s>
In	O
an	O
attempt	O
to	O
avoid	O
using	O
any	O
additional	O
out-of-band	O
authentication	O
factors	O
,	O
Davies	O
and	O
Price	O
proposed	O
the	O
use	O
of	O
the	O
interlock	B-Protocol
protocol	I-Protocol
of	O
Ron	O
Rivest	O
and	O
Adi	O
Shamir	O
,	O
which	O
has	O
been	O
subject	O
to	O
both	O
attack	O
and	O
subsequent	O
refinement	O
.	O
</s>
<s>
Secret-key	B-Algorithm
(	O
symmetric	O
)	O
cryptography	O
requires	O
the	O
initial	O
exchange	O
of	O
a	O
shared	B-Algorithm
key	I-Algorithm
in	O
a	O
manner	O
that	O
is	O
private	O
and	O
integrity-assured	O
.	O
</s>
