<s>
The	O
Secure	B-Protocol
Real-time	I-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
SRTP	O
)	O
is	O
a	O
profile	O
for	O
Real-time	B-Protocol
Transport	I-Protocol
Protocol	I-Protocol
(	O
RTP	O
)	O
intended	O
to	O
provide	O
encryption	O
,	O
message	O
authentication	O
and	O
integrity	O
,	O
and	O
replay	O
attack	O
protection	O
to	O
the	O
RTP	O
data	O
in	O
both	O
unicast	B-Protocol
and	O
multicast	O
applications	O
.	O
</s>
<s>
It	O
was	O
developed	O
by	O
a	O
small	O
team	O
of	O
Internet	B-Protocol
Protocol	I-Protocol
and	O
cryptographic	O
experts	O
from	O
Cisco	O
and	O
Ericsson	O
.	O
</s>
<s>
Since	O
RTP	O
is	O
accompanied	O
by	O
the	O
RTP	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
RTCP	B-Protocol
)	O
which	O
is	O
used	O
to	O
control	O
an	O
RTP	O
session	O
,	O
SRTP	O
has	O
a	O
sister	O
protocol	O
,	O
called	O
Secure	B-Protocol
RTCP	I-Protocol
(	O
SRTCP	B-Protocol
)	O
;	O
it	O
securely	O
provides	O
the	O
same	O
functions	O
to	O
SRTP	O
as	O
the	O
ones	O
provided	O
by	O
RTCP	B-Protocol
to	O
RTP	O
.	O
</s>
<s>
Utilization	O
of	O
SRTP	O
or	O
SRTCP	B-Protocol
is	O
optional	O
in	O
RTP	O
or	O
RTCP	B-Protocol
applications	O
;	O
but	O
even	O
if	O
SRTP	O
or	O
SRTCP	B-Protocol
are	O
used	O
,	O
all	O
provided	O
features	O
(	O
such	O
as	O
encryption	O
and	O
authentication	O
)	O
are	O
optional	O
and	O
can	O
be	O
separately	O
enabled	O
or	O
disabled	O
.	O
</s>
<s>
The	O
only	O
exception	O
is	O
the	O
message	O
authentication	O
feature	O
which	O
is	O
indispensable	O
and	O
required	O
when	O
using	O
SRTCP	B-Protocol
.	O
</s>
<s>
SRTP	O
and	O
SRTCP	B-Protocol
use	O
Advanced	B-Algorithm
Encryption	I-Algorithm
Standard	I-Algorithm
(	O
AES	B-Algorithm
)	O
as	O
the	O
default	O
cipher	B-Application
.	O
</s>
<s>
There	O
are	O
two	O
cipher	B-Algorithm
modes	I-Algorithm
defined	O
which	O
allow	O
the	O
AES	B-Algorithm
block	O
cipher	B-Application
to	O
be	O
used	O
as	O
a	O
stream	B-Algorithm
cipher	I-Algorithm
:	O
</s>
<s>
AES	B-Algorithm
running	O
in	O
this	O
mode	O
is	O
the	O
default	O
encryption	O
algorithm	O
,	O
with	O
a	O
default	O
key	O
size	O
of	O
128	O
bits	O
and	O
a	O
default	O
session	O
salt	O
key	O
length	O
of	O
112	O
bits	O
.	O
</s>
<s>
The	O
default	O
values	O
of	O
the	O
encryption	O
key	O
and	O
salt	O
key	O
are	O
the	O
same	O
as	O
for	O
AES	B-Algorithm
in	O
counter	O
mode	O
.	O
</s>
<s>
(	O
AES	B-Algorithm
running	O
in	O
this	O
mode	O
has	O
been	O
chosen	O
to	O
be	O
used	O
in	O
3G	B-Architecture
mobile	I-Architecture
networks	O
.	O
)	O
</s>
<s>
Besides	O
the	O
AES	B-Algorithm
cipher	I-Algorithm
,	O
SRTP	O
allows	O
the	O
ability	O
to	O
disable	O
encryption	O
outright	O
,	O
using	O
the	O
so-called	O
NULL	O
cipher	B-Application
,	O
which	O
can	O
be	O
assumed	O
as	O
an	O
alternate	O
supported	O
cipher	B-Application
.	O
</s>
<s>
In	O
fact	O
,	O
the	O
NULL	O
cipher	B-Application
does	O
not	O
perform	O
any	O
encryption	O
;	O
The	O
encryption	O
algorithm	O
functions	O
as	O
the	O
identity	O
function	O
,	O
and	O
copies	O
the	O
input	O
stream	O
to	O
the	O
output	O
stream	O
without	O
any	O
changes	O
.	O
</s>
<s>
It	O
is	O
mandatory	O
for	O
this	O
cipher	B-Algorithm
mode	I-Algorithm
to	O
be	O
implemented	O
in	O
any	O
SRTP-compatible	O
system	O
.	O
</s>
<s>
To	O
authenticate	O
the	O
message	O
and	O
protect	O
its	O
integrity	O
,	O
the	O
HMAC-SHA1	B-Algorithm
algorithm	O
is	O
used	O
.	O
</s>
<s>
The	O
HMAC	B-Algorithm
is	O
calculated	O
over	O
the	O
packet	O
payload	O
and	O
material	O
from	O
the	O
packet	O
header	O
,	O
including	O
the	O
packet	O
sequence	O
number	O
.	O
</s>
<s>
A	O
key	B-Algorithm
derivation	I-Algorithm
function	I-Algorithm
is	O
used	O
to	O
derive	O
the	O
different	O
keys	O
used	O
in	O
a	O
crypto	O
context	O
(	O
SRTP	O
and	O
SRTCP	B-Protocol
encryption	O
keys	O
and	O
salts	O
,	O
SRTP	O
and	O
SRTCP	B-Protocol
authentication	O
keys	O
)	O
from	O
one	O
single	O
master	O
key	O
in	O
a	O
cryptographically	O
secure	O
way	O
.	O
</s>
<s>
Thus	O
,	O
the	O
key	O
management	O
protocol	O
needs	O
to	O
exchange	O
only	O
one	O
master	O
key	O
,	O
all	O
the	O
necessary	O
session	O
keys	O
are	O
generated	O
by	O
applying	O
the	O
key	B-Algorithm
derivation	I-Algorithm
function	I-Algorithm
.	O
</s>
<s>
Periodic	O
application	O
of	O
the	O
key	B-Algorithm
derivation	I-Algorithm
function	I-Algorithm
prevents	O
an	O
attacker	O
from	O
collecting	O
large	O
amounts	O
of	O
ciphertext	O
encrypted	O
with	O
one	O
single	O
session	O
key	O
.	O
</s>
<s>
Furthermore	O
,	O
multiple	O
applications	O
of	O
the	O
key	B-Algorithm
derivation	I-Algorithm
function	I-Algorithm
provides	O
backwards	O
and	O
forward	O
security	O
in	O
the	O
sense	O
that	O
a	O
compromised	O
session	O
key	O
does	O
not	O
compromise	O
other	O
session	O
keys	O
derived	O
from	O
the	O
same	O
master	O
key	O
.	O
</s>
<s>
Two	O
protocols	O
specifically	O
designed	O
to	O
be	O
used	O
with	O
SRTP	O
are	O
ZRTP	B-Protocol
and	O
MIKEY	B-Protocol
.	O
</s>
<s>
There	O
are	O
several	O
vendors	O
which	O
offer	O
products	O
that	O
use	O
the	O
SDES	B-Protocol
key	O
exchange	O
method	O
.	O
</s>
<s>
Although	O
SRTP	O
could	O
be	O
used	O
to	O
operate	O
in	O
a	O
VPN	B-Application
,	O
in	O
conjunction	O
with	O
web	O
browsers	O
,	O
no	O
VPN	B-Application
networks	O
are	O
known	O
to	O
be	O
using	O
it	O
.	O
</s>
