<s>
Acrobits	B-Protocol
is	O
a	O
privately	O
owned	O
software	O
development	O
company	O
creating	O
VoIP	B-Application
Clients	O
for	O
mobile	O
platforms	O
,	O
based	O
in	O
Prague	O
,	O
Czech	O
Republic	O
.	O
</s>
<s>
Acrobits	B-Protocol
was	O
founded	O
in	O
November	O
2008	O
,	O
and	O
builds	O
mobile	O
VoIP	B-Application
software	O
with	O
a	O
polished	O
user	O
interface	O
,	O
supporting	O
encrypted	O
calls	O
using	O
SRTP/SDES	O
and	O
ZRTP	B-Protocol
,	O
Google	O
Voice	O
integration	O
,	O
and	O
the	O
G.729	B-Algorithm
Annex	O
A	O
audio	O
codec	O
.	O
</s>
<s>
In	O
2009	O
Acrobits	B-Protocol
Softphone	I-Protocol
was	O
released	O
on	O
the	O
iTunes	B-Application
App	I-Application
Store	I-Application
.	O
</s>
<s>
The	O
following	O
year	O
Acrobits	B-Protocol
released	O
their	O
SIP	B-Application
Client	O
with	O
business	O
features	O
,	O
Groundwire	O
.	O
</s>
<s>
In	O
early	O
2011	O
Acrobits	B-Protocol
Softphone	I-Protocol
was	O
released	O
on	O
the	O
Android	O
Market	O
.	O
</s>
<s>
In	O
2010	O
Acrobits	B-Protocol
also	O
launched	O
a	O
service	O
allowing	O
SIP	B-Application
providers	O
to	O
appear	O
on	O
the	O
list	O
of	O
pre-configured	O
providers	O
in	O
Acrobits	B-Protocol
Softphone	I-Protocol
.	O
</s>
<s>
In	O
2012	O
Acrobits	B-Protocol
added	O
video	O
calls	O
over	O
WiFi	O
support	O
for	O
the	O
iOS	O
version	O
of	O
its	O
softphone	O
.	O
</s>
<s>
Acrobits	B-Protocol
Softphone	I-Protocol
is	O
a	O
VoIP	B-Application
client	O
which	O
uses	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
.	O
</s>
<s>
Acrobits	B-Protocol
Softphone	I-Protocol
is	O
the	O
leading	O
SIP	B-Application
Client	O
on	O
the	O
App	B-Application
Store	I-Application
,	O
featuring	O
push	O
notifications	O
and	O
the	O
G.729	B-Algorithm
Annex	O
A	O
audio	O
codec	O
,	O
backgrounding	O
,	O
Google	O
Voice	O
integration	O
and	O
encrypted	O
calls	O
through	O
ZRTP	B-Protocol
.	O
</s>
<s>
The	O
first	O
version	O
of	O
Acrobits	B-Protocol
Softphone	I-Protocol
was	O
released	O
on	O
the	O
App	B-Application
Store	I-Application
in	O
April	O
2009	O
.	O
</s>
<s>
Version	O
1.0	O
supported	O
only	O
a	O
single	O
SIP	B-Application
account	O
and	O
the	O
G711	O
and	O
GSM	O
codecs	O
.	O
</s>
<s>
During	O
the	O
following	O
months	O
new	O
updates	O
were	O
released	O
rapidly	O
,	O
adding	O
new	O
features	O
,	O
and	O
the	O
app	O
quickly	O
became	O
the	O
most	O
downloaded	O
paid	O
SIP	B-Application
app	O
for	O
iOS	O
worldwide	O
.	O
</s>
<s>
The	O
G729	B-Algorithm
codec	O
was	O
added	O
in	O
Apr	O
2010	O
.	O
</s>
<s>
In	O
August	O
2010	O
,	O
a	O
business-caliber	O
version	O
of	O
Softphone	O
called	O
Groundwire	O
was	O
released	O
on	O
the	O
App	B-Application
Store	I-Application
,	O
adding	O
support	O
for	O
conferencing	O
,	O
voicemail	O
,	O
call	O
transfers	O
,	O
call	O
forwarding	O
and	O
other	O
advanced	O
features	O
of	O
business-grade	O
phones	O
.	O
</s>
<s>
With	O
the	O
release	O
of	O
Groundwire	O
,	O
the	O
app	O
reached	O
the	O
level	O
of	O
maturity	O
and	O
completeness	O
and	O
attracted	O
much	O
interest	O
from	O
VoIP	B-Application
providers	O
,	O
who	O
asked	O
for	O
white-label	O
versions	O
of	O
the	O
app	O
,	O
optimized	O
and	O
fine-tuned	O
for	O
their	O
network	O
only	O
.	O
</s>
<s>
Acrobits	B-Protocol
Softphone	I-Protocol
for	O
Android	O
was	O
released	O
in	O
Feb	O
2011	O
,	O
followed	O
by	O
Android	O
Groundwire	O
in	O
April	O
2012	O
.	O
</s>
<s>
Acrobits	B-Protocol
Softphone	I-Protocol
and	O
especially	O
Groundwire	O
support	O
all	O
features	O
and	O
technologies	O
expected	O
of	O
the	O
modern	O
SIP	B-Application
client	O
,	O
plus	O
some	O
unique	O
features	O
described	O
below	O
.	O
</s>
<s>
The	O
challenge	O
with	O
VoIP	B-Application
on	O
mobile	O
devices	O
is	O
to	O
make	O
sure	O
that	O
the	O
device	O
is	O
ready	O
to	O
receive	O
incoming	O
calls	O
while	O
keeping	O
the	O
power	O
consumption	O
as	O
low	O
as	O
possible	O
.	O
</s>
<s>
Due	O
to	O
the	O
inherent	O
mobility	O
of	O
mobile	O
devices	O
,	O
the	O
network	O
conditions	O
change	O
often	O
and	O
frequent	O
SIP	B-Application
re-registrations	O
and	O
keep-alive	O
traffic	O
are	O
needed	O
to	O
make	O
sure	O
the	O
mobile	O
client	O
is	O
properly	O
registered	O
and	O
will	O
receive	O
incoming	O
call	O
at	O
all	O
times	O
.	O
</s>
<s>
Acrobits	B-Protocol
Softphone	I-Protocol
uses	O
a	O
proprietary	O
SIP	B-Application
Instance	O
Server	O
(	O
SIPIS	O
)	O
to	O
register	O
on	O
behalf	O
of	O
user	O
when	O
the	O
mobile	O
app	O
is	O
not	O
running	O
in	O
foreground	O
on	O
the	O
mobile	O
device	O
.	O
</s>
<s>
When	O
a	O
call	O
arrives	O
,	O
the	O
mobile	O
app	O
is	O
woken	O
up	O
using	O
the	O
Apple	B-Operating_System
Push	I-Operating_System
Notification	I-Operating_System
Service	I-Operating_System
(	O
APNS	B-Operating_System
)	O
and	O
the	O
call	O
is	O
handed	O
over	O
to	O
the	O
mobile	O
app	O
.	O
</s>
<s>
Using	O
push	O
notifications	O
does	O
n't	O
require	O
any	O
support	O
on	O
the	O
SIP	B-Application
server	O
side	O
and	O
uses	O
only	O
SIP	B-Application
protocol	O
standard	O
.	O
</s>
<s>
An	O
important	O
point	O
and	O
a	O
potential	O
drawback	O
of	O
this	O
solution	O
is	O
the	O
need	O
to	O
transfer	O
full	O
SIP	B-Application
account	O
credentials	O
to	O
SIPIS	O
server	O
,	O
as	O
it	O
needs	O
them	O
to	O
be	O
able	O
to	O
register	O
,	O
which	O
is	O
an	O
obvious	O
security	O
risk	O
.	O
</s>
<s>
One	O
way	O
to	O
avoid	O
it	O
is	O
to	O
install	O
the	O
SIPIS	O
server	O
on	O
the	O
premises	O
of	O
the	O
VoIP	B-Application
service	O
provider	O
,	O
in	O
which	O
case	O
the	O
security	O
risk	O
is	O
eliminated	O
-	O
the	O
provider	O
already	O
knows	O
the	O
passwords	O
anyway	O
.	O
</s>
<s>
Acrobits	B-Protocol
Softphone	I-Protocol
supports	O
encrypted	O
voice	O
and	O
video	O
calls	O
using	O
the	O
standard	O
SRTP	B-Protocol
protocol	O
.	O
</s>
<s>
It	O
is	O
able	O
to	O
encrypt	O
media	O
packets	O
with	O
the	O
AES-128	B-Algorithm
,	O
AES-192	B-Algorithm
or	O
AES-256	B-Algorithm
ciphers	O
and	O
authenticate	O
them	O
using	O
either	O
32-bit	O
or	O
80-bit	O
HMAC-SHA1	B-Algorithm
algorithm	O
.	O
</s>
<s>
For	O
key	O
exchange	O
,	O
Acrobits	B-Protocol
Softphone	I-Protocol
offers	O
support	O
for	O
SDES	B-Protocol
and	O
ZRTP	B-Protocol
protocols	O
.	O
</s>
<s>
The	O
SDES	B-Protocol
protocol	O
transmits	O
the	O
encryption	O
keys	O
in	O
plain	O
text	O
inside	O
SIP+SDP	O
messages	O
.	O
</s>
<s>
This	O
key	O
exchange	O
protocol	O
is	O
therefore	O
pretty	O
much	O
useless	O
for	O
most	O
users	O
,	O
unless	O
they	O
have	O
a	O
complete	O
control	O
over	O
the	O
SIP	B-Application
signalling	O
system	O
to	O
ensure	O
that	O
the	O
TLS	B-Protocol
transport	O
protocol	O
is	O
used	O
all	O
the	O
way	O
from	O
the	O
originating	O
to	O
the	O
receiving	O
device	O
.	O
</s>
<s>
Even	O
if	O
a	O
SIP	B-Application
provider	O
guarantees	O
usage	O
of	O
TLS	B-Protocol
everywhere	O
in	O
his	O
infrastructure	O
,	O
the	O
provider	O
itself	O
is	O
still	O
able	O
to	O
see	O
the	O
encryption	O
keys	O
in	O
plain	O
text	O
,	O
because	O
its	O
SIP	B-Application
proxies	O
must	O
decrypt	O
the	O
SIP+SDP	O
messages	O
in	O
order	O
to	O
route	O
them	O
forward	O
.	O
</s>
<s>
To	O
address	O
the	O
above	O
shortcomings	O
of	O
the	O
SDES	B-Protocol
protocol	O
,	O
Phil	O
Zimmermann	O
devised	O
a	O
military	O
grade	O
key	O
exchange	O
protocol	O
,	O
ZRTP	B-Protocol
,	O
which	O
is	O
built	O
on	O
ideas	O
from	O
public-key	B-Application
cryptography	I-Application
.	O
</s>
<s>
Using	O
ZRTP	B-Protocol
,	O
two	O
devices	O
can	O
securely	O
exchange	O
encryption	O
keys	O
even	O
over	O
an	O
inherently	O
insecure	O
communication	O
channel	O
.	O
</s>
<s>
Moreover	O
,	O
by	O
employing	O
human	O
brains	O
to	O
compare	O
short	O
authentication	O
strings	O
(	O
SAS	O
)	O
spoken	O
by	O
the	O
other	O
party	O
,	O
ZRTP	B-Protocol
severely	O
reduces	O
the	O
probability	O
of	O
a	O
successful	O
man-in-the-middle	O
attack	O
,	O
which	O
requires	O
a	O
single	O
shot	O
guess	O
of	O
the	O
correct	O
SAS	O
out	O
of	O
65536	O
possibilities	O
.	O
</s>
<s>
Acrobits	B-Protocol
Softphone	I-Protocol
supports	O
the	O
following	O
algorithms	O
employed	O
by	O
ZRTP	B-Protocol
:	O
</s>
<s>
SRTP	B-Protocol
Cipher	O
:	O
</s>
<s>
SRTP	B-Protocol
Authentication	O
:	O
</s>
<s>
ZRTP	B-Protocol
Hash	O
:	O
</s>
<s>
In	O
addition	O
to	O
their	O
flagship	O
products	O
Acrobits	B-Protocol
creates	O
white	O
label	O
SIP	B-Application
Solutions	O
for	O
VoIP	B-Application
providers	O
around	O
the	O
world	O
.	O
</s>
