<s>
PGPfone	B-General_Concept
was	O
a	O
secure	O
voice	O
telephony	O
system	O
developed	O
by	O
Philip	O
Zimmermann	O
in	O
1995	O
.	O
</s>
<s>
The	O
PGPfone	B-General_Concept
protocol	O
had	O
little	O
in	O
common	O
with	O
Zimmermann	O
's	O
popular	O
PGP	B-Application
email	O
encryption	O
package	O
,	O
except	O
for	O
the	O
use	O
of	O
the	O
name	O
.	O
</s>
<s>
It	O
used	O
ephemeral	B-Protocol
Diffie-Hellman	I-Protocol
protocol	O
to	O
establish	O
a	O
session	O
key	O
,	O
which	O
was	O
then	O
used	O
to	O
encrypt	O
the	O
stream	O
of	O
voice	O
packets	O
.	O
</s>
<s>
PGPfone	B-General_Concept
could	O
be	O
used	O
point-to-point	O
(	O
with	O
two	O
modems	B-Application
)	O
over	O
the	O
public	O
switched	O
telephone	O
network	O
,	O
or	O
over	O
the	O
Internet	O
as	O
an	O
early	O
Voice	B-Application
over	I-Application
IP	I-Application
system	O
.	O
</s>
<s>
In	O
1996	O
,	O
there	O
were	O
no	O
protocol	O
standards	O
for	O
Voice	B-Application
over	I-Application
IP	I-Application
.	O
</s>
<s>
Ten	O
years	O
later	O
,	O
Zimmermann	O
released	O
the	O
successor	O
to	O
PGPfone	B-General_Concept
,	O
Zfone	B-Application
and	O
ZRTP	B-Protocol
,	O
a	O
newer	O
and	O
secure	O
VoIP	B-Application
protocol	O
based	O
on	O
modern	O
VoIP	B-Application
standards	O
.	O
</s>
<s>
Zfone	B-Application
builds	O
on	O
the	O
ideas	O
of	O
PGPfone	B-General_Concept
.	O
</s>
<s>
According	O
to	O
the	O
MIT	O
PGPfone	B-General_Concept
web	O
page	O
,	O
"	O
MIT	O
is	O
no	O
longer	O
distributing	O
PGPfone	B-General_Concept
.	O
</s>
