<s>
ZRTP	B-Protocol
(	O
composed	O
of	O
Z	O
and	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
)	O
is	O
a	O
cryptographic	O
key-agreement	B-Protocol
protocol	I-Protocol
to	O
negotiate	O
the	O
keys	O
for	O
encryption	O
between	O
two	O
end	O
points	O
in	O
a	O
Voice	B-Application
over	I-Application
IP	I-Application
(	O
VoIP	B-Application
)	O
phone	O
telephony	O
call	O
based	O
on	O
the	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
It	O
uses	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
and	O
the	O
Secure	B-Protocol
Real-time	I-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
SRTP	O
)	O
for	O
encryption	O
.	O
</s>
<s>
ZRTP	B-Protocol
was	O
developed	O
by	O
Phil	O
Zimmermann	O
,	O
with	O
help	O
from	O
Bryce	O
Wilcox-O	O
'	O
Hearn	O
,	O
Colin	O
Plumb	O
,	O
Jon	O
Callas	O
and	O
Alan	O
Johnston	O
and	O
was	O
submitted	O
to	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
(	O
IETF	O
)	O
by	O
Zimmermann	O
,	O
Callas	O
and	O
Johnston	O
on	O
March	O
5	O
,	O
2006	O
and	O
published	O
on	O
April	O
11	O
,	O
2011	O
as	O
.	O
</s>
<s>
ZRTP	B-Protocol
(	O
"	O
Z	O
"	O
is	O
a	O
reference	O
to	O
its	O
inventor	O
,	O
Zimmermann	O
;	O
"	O
RTP	O
"	O
stands	O
for	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
)	O
is	O
described	O
in	O
the	O
Internet	O
Draft	O
as	O
a	O
"	O
key	B-Protocol
agreement	I-Protocol
protocol	I-Protocol
which	O
performs	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
during	O
call	O
setup	O
in-band	O
in	O
the	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
media	O
stream	O
which	O
has	O
been	O
established	O
using	O
some	O
other	O
signaling	O
protocol	O
such	O
as	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
.	O
</s>
<s>
This	O
generates	O
a	O
shared	O
secret	O
which	O
is	O
then	O
used	O
to	O
generate	O
keys	O
and	O
salt	O
for	O
a	O
Secure	B-Protocol
RTP	I-Protocol
(	O
SRTP	O
)	O
session.	O
"	O
</s>
<s>
One	O
of	O
ZRTP	B-Protocol
's	O
features	O
is	O
that	O
it	O
does	O
not	O
rely	O
on	O
SIP	O
signaling	O
for	O
the	O
key	O
management	O
,	O
or	O
on	O
any	O
servers	O
at	O
all	O
.	O
</s>
<s>
It	O
supports	O
opportunistic	B-Protocol
encryption	I-Protocol
by	O
auto-sensing	O
if	O
the	O
other	O
VoIP	B-Application
client	O
supports	O
ZRTP	B-Protocol
.	O
</s>
<s>
This	O
protocol	O
does	O
not	O
require	O
prior	O
shared	O
secrets	O
or	O
rely	O
on	O
a	O
Public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
(	O
PKI	O
)	O
or	O
on	O
certification	O
authorities	O
,	O
in	O
fact	O
ephemeral	B-Protocol
Diffie	I-Protocol
–	I-Protocol
Hellman	I-Protocol
keys	O
are	O
generated	O
on	O
each	O
session	O
establishment	O
:	O
this	O
allows	O
the	O
complexity	O
of	O
creating	O
and	O
maintaining	O
a	O
trusted	O
third-party	O
to	O
be	O
bypassed	O
.	O
</s>
<s>
ZRTP	B-Protocol
can	O
be	O
used	O
with	O
any	O
signaling	O
protocol	O
,	O
including	O
SIP	O
,	O
H.323	B-Protocol
,	O
Jingle	B-Protocol
,	O
and	O
distributed	B-Operating_System
hash	I-Operating_System
table	I-Operating_System
systems	O
.	O
</s>
<s>
ZRTP	B-Protocol
is	O
independent	O
of	O
the	O
signaling	O
layer	O
,	O
because	O
all	O
its	O
key	O
negotiations	O
occur	O
via	O
the	O
RTP	O
media	O
stream	O
.	O
</s>
<s>
ZRTP/S	O
,	O
a	O
ZRTP	B-Protocol
protocol	O
extension	O
,	O
can	O
run	O
on	O
any	O
kind	O
of	O
legacy	O
telephony	O
networks	O
including	O
GSM	O
,	O
UMTS	B-Protocol
,	O
ISDN	B-Protocol
,	O
PSTN	O
,	O
SATCOM	O
,	O
UHF/VHF	O
radio	O
,	O
because	O
it	O
is	O
a	O
narrow-band	O
bitstream-oriented	O
protocol	O
and	O
performs	O
all	O
key	O
negotiations	O
inside	O
the	O
bitstream	O
between	O
two	O
endpoints	O
.	O
</s>
<s>
Alan	O
Johnston	O
named	O
the	O
protocol	O
ZRTP	B-Protocol
because	O
in	O
its	O
earliest	O
Internet	O
drafts	O
it	O
was	O
based	O
on	O
adding	O
header	O
extensions	O
to	O
RTP	O
packets	O
,	O
which	O
made	O
ZRTP	B-Protocol
a	O
variant	O
of	O
RTP	O
.	O
</s>
<s>
In	O
view	O
of	O
that	O
change	O
,	O
ZRTP	B-Protocol
is	O
now	O
a	O
pseudo-acronym	O
.	O
</s>
<s>
The	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
key	I-Protocol
exchange	I-Protocol
by	O
itself	O
does	O
not	O
provide	O
protection	O
against	O
a	O
man-in-the-middle	O
attack	O
.	O
</s>
<s>
A	O
specific	O
attack	O
theorized	O
against	O
the	O
ZRTP	B-Protocol
protocol	O
involves	O
creating	O
a	O
synthetic	O
voice	O
of	O
both	O
parties	O
to	O
read	O
a	O
bogus	O
SAS	O
which	O
is	O
known	O
as	O
a	O
"	O
Rich	O
little	O
attack	O
"	O
,	O
but	O
this	O
class	O
of	O
attack	O
is	O
not	O
believed	O
to	O
be	O
a	O
serious	O
risk	O
to	O
the	O
protocol	O
's	O
security	O
.	O
</s>
<s>
The	O
SAS	O
is	O
used	O
to	O
authenticate	O
the	O
key	O
exchange	O
,	O
which	O
is	O
essentially	O
a	O
cryptographic	B-Algorithm
hash	I-Algorithm
of	O
the	O
two	O
Diffie	B-Protocol
–	I-Protocol
Hellman	I-Protocol
values	O
.	O
</s>
<s>
The	O
SAS	O
value	O
is	O
rendered	O
to	O
both	O
ZRTP	B-Protocol
endpoints	O
.	O
</s>
<s>
The	O
use	O
of	O
hash	B-Algorithm
commitment	O
in	O
the	O
DH	O
exchange	O
constrains	O
the	O
attacker	O
to	O
only	O
one	O
guess	O
to	O
generate	O
the	O
correct	O
SAS	O
in	O
the	O
attack	O
,	O
which	O
means	O
the	O
SAS	O
may	O
be	O
quite	O
short	O
.	O
</s>
<s>
ZRTP	B-Protocol
provides	O
a	O
second	O
layer	O
of	O
authentication	O
against	O
a	O
MitM	O
attack	O
,	O
based	O
on	O
a	O
form	O
of	O
key	O
continuity	O
.	O
</s>
<s>
It	O
does	O
this	O
by	O
caching	O
some	O
hashed	O
key	O
information	O
for	O
use	O
in	O
the	O
next	O
call	O
,	O
to	O
be	O
mixed	O
in	O
with	O
the	O
next	O
call	O
's	O
DH	O
shared	O
secret	O
,	O
giving	O
it	O
key	O
continuity	O
properties	O
analogous	O
to	O
SSH	B-Protocol
.	O
</s>
<s>
GNU	O
ZRTP4J	O
which	O
is	O
used	O
in	O
Jitsi	B-Protocol
(	O
formerly	O
SIP	B-Protocol
Communicator	I-Protocol
)	O
.	O
</s>
<s>
ortp	O
for	O
use	O
in	O
Linphone	B-Protocol
.	O
</s>
<s>
libzrtp	O
which	O
can	O
be	O
used	O
in	O
FreeSWITCH	B-Language
.	O
</s>
<s>
Signal	B-Application
and	O
its	O
predecessor	O
,	O
RedPhone	O
,	O
used	O
ZRTP	B-Protocol
for	O
encrypted	O
calls	O
on	O
Android	B-Application
and	O
iOS	O
.	O
</s>
<s>
As	O
of	O
March	O
2017	O
,	O
Signal	B-Application
's	O
voice	O
and	O
video	O
calling	O
functionality	O
uses	O
the	O
app	O
's	O
Signal	B-Protocol
Protocol	I-Protocol
channel	O
for	O
authentication	O
instead	O
of	O
ZRTP	B-Protocol
.	O
</s>
<s>
Commercial	O
implementations	O
of	O
ZRTP	B-Protocol
are	O
available	O
in	O
RokaCom	O
from	O
RokaCom	O
,	O
and	O
PrivateWave	O
Professional	O
from	O
PrivateWave	O
and	O
more	O
recently	O
in	O
Silent	O
Phone	O
from	O
Silent	O
Circle	O
,	O
a	O
company	O
founded	O
by	O
Zimmermann	O
.	O
</s>
<s>
Draytek	O
support	O
ZRTP	B-Protocol
in	O
some	O
of	O
their	O
VoIP	B-Application
hardware	O
and	O
software	O
.	O
</s>
<s>
A	O
list	O
of	O
free	O
SIP	O
Providers	O
with	O
ZRTP	B-Protocol
support	O
has	O
been	O
published	O
.	O
</s>
