<s>
The	O
Secure	B-Protocol
Communications	I-Protocol
Interoperability	I-Protocol
Protocol	I-Protocol
(	O
SCIP	O
)	O
is	O
a	O
US	O
standard	O
for	O
secure	O
voice	O
and	O
data	O
communication	O
,	O
for	O
one-to-one	O
connections	O
,	O
not	O
packet-switched	O
networks	O
.	O
</s>
<s>
SCIP	O
derived	O
from	O
the	O
US	O
Government	O
Future	O
Narrowband	O
Digital	O
Terminal	O
(	O
FNBDT	B-Protocol
)	O
project	O
.	O
</s>
<s>
SCIP	O
has	O
to	O
operate	O
over	O
the	O
wide	O
variety	O
of	O
communications	O
systems	O
,	O
including	O
commercial	O
land	O
line	O
telephone	O
,	O
military	O
radios	O
,	O
communication	O
satellites	O
,	O
Voice	B-Application
over	I-Application
IP	I-Application
and	O
the	O
several	O
different	O
cellular	O
telephone	O
standards	O
.	O
</s>
<s>
Therefore	O
,	O
it	O
was	O
designed	O
to	O
make	O
no	O
assumptions	O
about	O
the	O
underlying	O
channel	O
other	O
than	O
a	O
minimum	O
bandwidth	B-Algorithm
of	O
2400	O
Hz	O
.	O
</s>
<s>
It	O
is	O
similar	O
to	O
a	O
dial-up	B-Application
modem	I-Application
in	O
that	O
once	O
a	O
connection	O
is	O
made	O
,	O
two	O
SCIP	O
phones	O
first	O
negotiate	O
the	O
parameters	O
they	O
need	O
and	O
then	O
communicate	O
in	O
the	O
best	O
way	O
possible	O
.	O
</s>
<s>
US	O
SCIP	O
or	O
FNBDT	B-Protocol
systems	O
were	O
used	O
since	O
2001	O
,	O
beginning	O
with	O
the	O
CONDOR	O
secure	O
cell	O
phone	O
.	O
</s>
<s>
The	O
standard	O
is	O
designed	O
to	O
cover	O
wideband	B-Algorithm
as	O
well	O
as	O
narrowband	O
voice	O
and	O
data	O
security	O
.	O
</s>
<s>
For	O
Type	O
1	O
security	O
(	O
classified	O
calls	O
)	O
,	O
the	O
SCIP	O
signalling	O
plan	O
uses	O
an	O
enhanced	O
FIREFLY	B-Device
messaging	O
system	O
for	O
key	O
exchange	O
.	O
</s>
<s>
FIREFLY	B-Device
is	O
an	O
NSA	O
key	O
management	O
system	O
based	O
on	O
public	B-Application
key	I-Application
cryptography	I-Application
.	O
</s>
<s>
At	O
least	O
one	O
commercial	O
grade	O
implementation	O
uses	O
Diffie-Hellman	B-Protocol
key	I-Protocol
exchange	I-Protocol
.	O
</s>
<s>
SCIP	O
can	O
work	O
with	O
a	O
variety	O
of	O
vocoders	B-Application
.	O
</s>
<s>
The	O
standard	O
requires	O
,	O
as	O
a	O
minimum	O
,	O
support	O
for	O
the	O
mixed-excitation	B-Algorithm
linear	I-Algorithm
prediction	I-Algorithm
(	O
MELP	B-Algorithm
)	O
coder	O
,	O
an	O
enhanced	O
MELP	B-Algorithm
algorithm	O
known	O
as	O
MELPe	B-Algorithm
,	O
with	O
additional	O
preprocessing	O
,	O
analyzer	O
and	O
synthesizer	O
capabilities	O
for	O
improved	O
intelligibility	O
and	O
noise	O
robustness	O
.	O
</s>
<s>
The	O
old	O
MELP	B-Algorithm
and	O
the	O
new	O
MELPe	B-Algorithm
are	O
interoperable	O
and	O
both	O
operate	O
at	O
2400	O
bit/s	O
,	O
sending	O
a	O
54	O
bit	O
data	O
frame	O
every	O
22.5	O
milliseconds	O
but	O
the	O
MELPe	B-Algorithm
has	O
optional	O
additional	O
rates	O
of	O
1200	O
bit/s	O
and	O
600	O
bit/s	O
.	O
</s>
<s>
2400	O
bit/s	O
MELPe	B-Algorithm
is	O
the	O
only	O
mandatory	O
voice	B-Application
coder	I-Application
required	O
for	O
SCIP	O
.	O
</s>
<s>
Other	O
voice	B-Application
coders	I-Application
can	O
be	O
supported	O
in	O
terminals	O
.	O
</s>
<s>
G.729D	O
is	O
the	O
most	O
widely	O
supported	O
non-mandatory	O
voice	B-Application
coder	I-Application
in	O
SCIP	O
terminals	O
as	O
it	O
offers	O
a	O
good	O
compromise	O
between	O
higher	O
voice	O
quality	O
without	O
dramatically	O
increasing	O
the	O
required	O
throughput	O
.	O
</s>
<s>
SCIP	O
231	O
defines	O
AES	B-Algorithm
based	O
cryptography	O
which	O
can	O
be	O
used	O
multinationally	O
.	O
</s>
<s>
The	O
output	O
from	O
the	O
block	O
cipher	O
is	O
xored	O
with	O
the	O
MELP	B-Algorithm
data	O
frames	O
to	O
create	O
the	O
cipher	O
text	O
that	O
is	O
then	O
transmitted	O
.	O
</s>
<s>
Note	O
that	O
since	O
overall	O
SCIP	O
encryption	O
is	O
effectively	O
a	O
stream	B-Algorithm
cipher	I-Algorithm
,	O
it	O
is	O
essential	O
that	O
the	O
same	O
state	O
vector	O
value	O
never	O
be	O
used	O
twice	O
for	O
a	O
given	O
TEK	O
.	O
</s>
<s>
At	O
MELP	B-Algorithm
data	O
rates	O
,	O
a	O
42-bit	O
counter	O
allows	O
a	O
call	O
over	O
three	O
thousand	O
years	O
long	O
before	O
the	O
encryption	O
repeats	O
.	O
</s>
<s>
For	O
Type	O
1	O
security	O
,	O
SCIP	O
uses	O
BATON	B-Algorithm
,	O
a	O
128-bit	O
block	O
design	O
.	O
</s>
<s>
With	O
this	O
or	O
other	O
128-bit	O
ciphers	O
,	O
such	O
as	O
AES	B-Algorithm
,	O
SCIP	O
specifies	O
that	O
two	O
data	O
frames	O
are	O
encrypted	O
with	O
each	O
cipher	O
output	O
bloc	O
,	O
the	O
first	O
beginning	O
at	O
bit	O
1	O
,	O
the	O
second	O
at	O
bit	O
57	O
(	O
i.e.	O
</s>
<s>
At	O
least	O
one	O
commercial	O
grade	O
implementation	O
uses	O
the	O
Triple	B-Algorithm
DES	I-Algorithm
cipher	O
.	O
</s>
<s>
The	O
mandatory	O
data	O
service	O
uses	O
an	O
ARQ	B-Error_Name
protocol	I-Error_Name
with	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
(	O
FEC	O
)	O
to	O
ensure	O
reliable	O
transmission	O
.	O
</s>
<s>
For	O
voice	O
,	O
SCIP	O
simply	O
sends	O
a	O
stream	O
of	O
voice	O
data	O
frames	O
(	O
typically	O
MELPe	B-Algorithm
frames	O
,	O
but	O
possibly	O
G.729D	O
or	O
another	O
codec	O
if	O
that	O
has	O
been	O
negotiated	O
between	O
the	O
terminals	O
)	O
.	O
</s>
