<s>
The	O
DRYAD	B-Operating_System
Numeral	O
Cipher/Authentication	O
System	O
(	O
KTC	O
1400	O
D	O
)	O
is	O
a	O
simple	O
,	O
paper	O
cryptographic	O
system	O
employed	O
by	O
the	O
U.S.	O
military	O
for	O
authentication	O
and	O
for	O
encryption	O
of	O
short	O
,	O
numerical	O
messages	O
.	O
</s>
<s>
Each	O
unit	O
with	O
a	O
radio	O
is	O
given	O
a	O
set	O
of	O
matching	O
DRYAD	B-Operating_System
code	O
sheets	O
.	O
</s>
<s>
6	O
hours	O
)	O
,	O
called	O
a	O
cryptoperiod	B-General_Concept
.	O
</s>
<s>
A	O
DRYAD	B-Operating_System
cipher	O
sheet	O
contains	O
25	O
lines	O
or	O
rows	O
of	O
scrambled	O
letters	O
.	O
</s>
<s>
Each	O
row	O
contains	O
a	O
random	O
permutation	B-Algorithm
of	O
the	O
letters	O
A	O
through	O
Y	O
.	O
</s>
<s>
While	O
crude	O
,	O
the	O
DRYAD	B-Operating_System
Numeral	O
Cipher/Authentication	O
System	O
has	O
the	O
advantage	O
of	O
being	O
fast	O
,	O
relatively	O
easy	O
and	O
requires	O
no	O
extra	O
equipment	O
(	O
such	O
as	O
a	O
pencil	O
)	O
.	O
</s>
<s>
But	O
much	O
of	O
the	O
security	O
comes	O
from	O
keeping	O
the	O
cryptoperiod	B-General_Concept
short	O
.	O
</s>
<s>
DRYAD	B-Operating_System
can	O
be	O
used	O
in	O
two	O
modes	O
,	O
authentication	O
or	O
encryption	O
.	O
</s>
<s>
The	O
downside	O
to	O
this	O
method	O
is	O
reduced	O
longevity	O
of	O
the	O
current	O
DRYAD	B-Operating_System
page	O
,	O
since	O
the	O
page	O
is	O
getting	O
twice	O
as	O
much	O
use	O
as	O
a	O
single-authentication	O
scheme	O
.	O
</s>
