<s>
SDES	B-Protocol
(	O
Session	B-Language
Description	I-Language
Protocol	I-Language
Security	O
Descriptions	O
)	O
for	O
Media	O
Streams	O
is	O
a	O
way	O
to	O
negotiate	O
the	O
key	O
for	O
Secure	O
Real-time	O
Transport	O
Protocol	O
.	O
</s>
<s>
The	O
keys	O
are	O
transported	O
in	O
the	O
SDP	B-Language
attachment	O
of	O
a	O
SIP	B-Application
message	O
.	O
</s>
<s>
That	O
means	O
,	O
the	O
SIP	B-Application
transport	O
layer	O
must	O
make	O
sure	O
that	O
no	O
one	O
else	O
can	O
see	O
the	O
attachment	O
.	O
</s>
<s>
Using	O
TLS	O
assumes	O
that	O
the	O
next	O
hop	O
in	O
the	O
SIP	B-Application
proxy	O
chain	O
can	O
be	O
trusted	O
and	O
it	O
will	O
take	O
care	O
about	O
the	O
security	O
requirements	O
of	O
the	O
request	O
.	O
</s>
<s>
The	O
key	O
is	O
base-64	O
encoded	O
in	O
the	O
SDP	B-Language
attachment	O
.	O
</s>
<s>
The	O
SDES	B-Protocol
method	O
does	O
not	O
address	O
the	O
"	O
end-to-end	O
"	O
media	O
encryption	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
user	O
A	O
is	O
talking	O
to	O
user	O
B	O
via	O
a	O
proxy	O
P	O
,	O
SDES	B-Protocol
allows	O
negotiation	O
of	O
keys	O
between	O
A	O
and	O
P	O
or	O
between	O
B	O
and	O
P	O
,	O
but	O
not	O
between	O
A	O
and	O
B	O
.	O
</s>
<s>
Newer	O
,	O
modern	O
protocols	O
,	O
like	O
ZRTP	B-Protocol
,	O
offer	O
end-to-end	O
encryption	O
for	O
SIP/RTP	O
calls	O
.	O
</s>
