<s>
Zfone	B-Application
is	O
software	O
for	O
secure	O
voice	O
communication	O
over	O
the	O
Internet	O
(	O
VoIP	B-Application
)	O
,	O
using	O
the	O
ZRTP	B-Protocol
protocol	O
.	O
</s>
<s>
It	O
is	O
created	O
by	O
Phil	O
Zimmermann	O
,	O
the	O
creator	O
of	O
the	O
PGP	B-Application
encryption	I-Application
software	O
.	O
</s>
<s>
Zfone	B-Application
works	O
on	O
top	O
of	O
existing	O
SIP	B-Application
-	O
and	O
RTP-programs	O
,	O
but	O
should	O
work	O
with	O
any	O
SIP	B-Application
-	O
and	O
RTP-compliant	O
VoIP-program	O
.	O
</s>
<s>
Zfone	B-Application
turns	O
many	O
existing	O
VoIP	B-Application
clients	O
into	O
secure	O
phones	O
.	O
</s>
<s>
It	O
runs	O
in	O
the	O
Internet	O
Protocol	O
stack	O
on	O
any	O
Windows	B-Application
XP	O
,	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
,	O
or	O
Linux	B-Application
PC	O
,	O
and	O
intercepts	O
and	O
filters	O
all	O
the	O
VoIP	B-Application
packets	O
as	O
they	O
go	O
in	O
and	O
out	O
of	O
the	O
machine	O
,	O
and	O
secures	O
the	O
call	O
on	O
the	O
fly	O
.	O
</s>
<s>
A	O
variety	O
of	O
different	O
software	O
VoIP	B-Application
clients	O
can	O
be	O
used	O
to	O
make	O
a	O
VoIP	B-Application
call	I-Application
.	O
</s>
<s>
The	O
Zfone	B-Application
software	O
detects	O
when	O
the	O
call	O
starts	O
,	O
and	O
initiates	O
a	O
cryptographic	O
key	O
agreement	O
between	O
the	O
two	O
parties	O
,	O
and	O
then	O
proceeds	O
to	O
encrypt	O
and	O
decrypt	O
the	O
voice	O
packets	O
on	O
the	O
fly	O
.	O
</s>
<s>
It	O
has	O
its	O
own	O
separate	O
GUI	B-Application
,	O
telling	O
the	O
user	O
if	O
the	O
call	O
is	O
secure	O
.	O
</s>
<s>
Zfone	B-Application
describes	O
itself	O
to	O
end-users	O
as	O
a	O
"	O
bump	O
on	O
the	O
wire	O
"	O
between	O
the	O
VoIP	B-Application
client	O
and	O
the	O
Internet	O
,	O
which	O
acts	O
upon	O
the	O
protocol	O
stack	O
.	O
</s>
<s>
Zfone	B-Application
's	O
libZRTP	O
SDK	B-Application
libraries	O
are	O
released	O
under	O
either	O
the	O
Affero	O
General	O
Public	O
License	O
(	O
AGPL	O
)	O
or	O
a	O
commercial	O
license	O
.	O
</s>
<s>
Note	O
that	O
only	O
the	O
libZRTP	O
SDK	B-Application
libraries	O
are	O
provided	O
under	O
the	O
AGPL	O
.	O
</s>
<s>
The	O
parts	O
of	O
Zfone	B-Application
that	O
are	O
not	O
part	O
of	O
the	O
libZRTP	O
SDK	B-Application
libraries	O
are	O
not	O
licensed	O
under	O
the	O
AGPL	O
or	O
any	O
other	O
open	O
source	O
license	O
.	O
</s>
<s>
Although	O
the	O
source	O
code	O
of	O
those	O
components	O
is	O
published	O
for	O
peer	O
review	O
,	O
they	O
remain	O
proprietary	B-Application
.	O
</s>
<s>
The	O
Zfone	B-Application
proprietary	B-Application
license	I-Application
also	O
contains	O
a	O
time	B-General_Concept
bomb	I-General_Concept
provision	O
.	O
</s>
<s>
It	O
appears	O
that	O
Zfone	B-Application
development	O
has	O
stagnated	O
,	O
however	O
,	O
as	O
the	O
most	O
recent	O
version	O
was	O
released	O
on	O
22	O
Mar	O
2009	O
.	O
</s>
<s>
In	O
addition	O
,	O
since	O
29	O
Jan	O
2011	O
,	O
it	O
has	O
not	O
been	O
possible	O
to	O
download	O
Zfone	B-Application
from	O
the	O
developer	O
's	O
website	O
since	O
the	O
download	O
server	O
has	O
gone	O
offline	O
.	O
</s>
<s>
Availability	O
–	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
,	O
Linux	B-Application
,	O
and	O
Windows	B-Application
as	O
compiled	O
programs	O
as	O
well	O
as	O
an	O
SDK	B-Application
.	O
</s>
<s>
Encryption	O
standards	O
–	O
Based	O
on	O
ZRTP	B-Protocol
,	O
which	O
uses	O
128	O
-	O
or	O
256-bit	O
AES	B-Algorithm
together	O
with	O
a	O
3072-bit	O
key	O
exchange	O
system	O
and	O
voice-based	O
verification	O
to	O
prevent	O
man-in-the-middle	O
attacks	O
.	O
</s>
<s>
VoIP	B-Application
clients	O
–	O
Zfone	B-Application
has	O
been	O
tested	O
with	O
the	O
following	O
VoIP	B-Application
clients	O
:	O
X-Lite	O
,	O
Gizmo5	B-Application
,	O
XMeeting	O
,	O
Google	B-Application
Talk	I-Application
VoIP	B-Application
client	O
,	O
and	O
SJphone	O
.	O
</s>
