<s>
ROT13	B-Algorithm
(	O
"	O
rotate	B-Algorithm
by	I-Algorithm
13	I-Algorithm
places	I-Algorithm
"	O
,	O
sometimes	O
hyphenated	O
ROT-13	B-Algorithm
)	O
is	O
a	O
simple	O
letter	O
substitution	O
cipher	O
that	O
replaces	O
a	O
letter	O
with	O
the	O
13th	O
letter	O
after	O
it	O
in	O
the	O
latin	O
alphabet	B-Application
.	O
</s>
<s>
ROT13	B-Algorithm
is	O
a	O
special	O
case	O
of	O
the	O
Caesar	O
cipher	O
which	O
was	O
developed	O
in	O
ancient	O
Rome	O
.	O
</s>
<s>
Because	O
there	O
are	O
26	O
letters	O
(	O
2×13	O
)	O
in	O
the	O
basic	O
Latin	O
alphabet	B-Application
,	O
ROT13	B-Algorithm
is	O
its	O
own	O
inverse	O
;	O
that	O
is	O
,	O
to	O
undo	O
ROT13	B-Algorithm
,	O
the	O
same	O
algorithm	O
is	O
applied	O
,	O
so	O
the	O
same	O
action	O
can	O
be	O
used	O
for	O
encoding	O
and	O
decoding	O
.	O
</s>
<s>
ROT13	B-Algorithm
is	O
used	O
in	O
online	B-Application
forums	I-Application
as	O
a	O
means	O
of	O
hiding	O
spoilers	O
,	O
punchlines	B-Application
,	O
puzzle	O
solutions	O
,	O
and	O
offensive	O
materials	O
from	O
the	O
casual	O
glance	O
.	O
</s>
<s>
ROT13	B-Algorithm
has	O
inspired	O
a	O
variety	O
of	O
letter	O
and	O
word	O
games	O
online	O
,	O
and	O
is	O
frequently	O
mentioned	O
in	O
newsgroup	B-Application
conversations	O
.	O
</s>
<s>
Applying	O
ROT13	B-Algorithm
to	O
a	O
piece	O
of	O
text	O
merely	O
requires	O
examining	O
its	O
alphabetic	O
characters	O
and	O
replacing	O
each	O
one	O
by	O
the	O
letter	O
13	O
places	O
further	O
along	O
in	O
the	O
alphabet	B-Application
,	O
wrapping	O
back	O
to	O
the	O
beginning	O
if	O
necessary	O
.	O
</s>
<s>
A	O
becomes	O
N	O
,	O
B	O
becomes	O
O	O
,	O
and	O
so	O
on	O
up	O
to	O
M	O
,	O
which	O
becomes	O
Z	O
,	O
then	O
the	O
sequence	O
continues	O
at	O
the	O
beginning	O
of	O
the	O
alphabet	B-Application
:	O
N	O
becomes	O
A	O
,	O
O	O
becomes	O
B	O
,	O
and	O
so	O
on	O
to	O
Z	O
,	O
which	O
becomes	O
M	O
.	O
Only	O
those	O
letters	O
which	O
occur	O
in	O
the	O
English	O
alphabet	B-Application
are	O
affected	O
;	O
numbers	O
,	O
symbols	O
,	O
punctuation	O
,	O
whitespace	O
,	O
and	O
all	O
other	O
characters	O
are	O
left	O
unchanged	O
.	O
</s>
<s>
Because	O
there	O
are	O
26	O
letters	O
in	O
the	O
English	O
alphabet	B-Application
and	O
26	O
=	O
2	O
13	O
,	O
the	O
ROT13	B-Algorithm
function	O
is	O
its	O
own	O
inverse	O
:	O
</s>
<s>
for	O
any	O
basic	O
Latin-alphabet	O
text	O
x	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
two	O
successive	O
applications	O
of	O
ROT13	B-Algorithm
restore	O
the	O
original	O
text	O
(	O
in	O
mathematics	O
,	O
this	O
is	O
sometimes	O
called	O
an	O
involution	B-Algorithm
;	O
in	O
cryptography	O
,	O
a	O
reciprocal	O
cipher	O
)	O
.	O
</s>
<s>
The	O
transformation	O
can	O
be	O
done	O
using	O
a	O
lookup	B-Data_Structure
table	I-Data_Structure
,	O
such	O
as	O
the	O
following	O
:	O
</s>
<s>
For	O
example	O
,	O
in	O
the	O
following	O
joke	O
,	O
the	O
punchline	O
has	O
been	O
obscured	O
by	O
ROT13	B-Algorithm
:	O
</s>
<s>
Transforming	O
the	O
entire	O
text	O
via	O
ROT13	B-Algorithm
form	O
,	O
the	O
answer	O
to	O
the	O
joke	O
is	O
revealed	O
:	O
</s>
<s>
A	O
second	O
application	O
of	O
ROT13	B-Algorithm
would	O
restore	O
the	O
original	O
.	O
</s>
<s>
ROT13	B-Algorithm
is	O
a	O
special	O
case	O
of	O
the	O
encryption	O
algorithm	O
known	O
as	O
a	O
Caesar	O
cipher	O
,	O
used	O
by	O
Julius	O
Caesar	O
in	O
the	O
1st	O
century	O
BC	O
.	O
</s>
<s>
Johann	O
Ernst	O
Elias	O
Bessler	O
,	O
an	O
18th	O
century	O
clockmaker	O
and	O
constructor	O
of	O
perpetual	O
motion	O
machines	O
,	O
pointed	O
out	O
that	O
ROT13	B-Algorithm
encodes	O
his	O
surname	O
as	O
Orffyre	O
.	O
</s>
<s>
ROT13	B-Algorithm
was	O
in	O
use	O
in	O
the	O
net.jokes	O
newsgroup	B-Application
by	O
the	O
early	O
1980s	O
.	O
</s>
<s>
ROT13	B-Algorithm
is	O
typically	O
supported	O
as	O
a	O
built-in	O
feature	O
to	O
newsreading	O
software	O
.	O
</s>
<s>
Email	O
addresses	O
are	O
also	O
sometimes	O
encoded	O
with	O
ROT13	B-Algorithm
to	O
hide	O
them	O
from	O
less	O
sophisticated	O
spam	B-Protocol
bots	I-Protocol
.	O
</s>
<s>
In	O
encrypted	O
,	O
normal	O
,	O
English-language	O
text	O
of	O
any	O
significant	O
size	O
,	O
ROT13	B-Algorithm
is	O
recognizable	O
from	O
some	O
letter/word	O
patterns	O
.	O
</s>
<s>
The	O
words	O
"	O
n	O
"	O
,	O
"	O
V	O
"	O
(	O
capitalized	O
only	O
)	O
,	O
and	O
"	O
gur	O
"	O
(	O
ROT13	B-Algorithm
for	O
"	O
a	O
"	O
,	O
"	O
I	O
"	O
,	O
and	O
"	O
the	O
"	O
)	O
,	O
and	O
words	O
ending	O
in	O
"	O
yl	O
"	O
(	O
"	O
ly	O
"	O
)	O
are	O
examples	O
.	O
</s>
<s>
ROT13	B-Algorithm
is	O
not	O
intended	O
to	O
be	O
used	O
where	O
secrecy	O
is	O
of	O
any	O
concern	O
—	O
the	O
use	O
of	O
a	O
constant	O
shift	O
means	O
that	O
the	O
encryption	O
effectively	O
has	O
no	O
key	O
,	O
and	O
decryption	O
requires	O
no	O
more	O
knowledge	O
than	O
the	O
fact	O
that	O
ROT13	B-Algorithm
is	O
in	O
use	O
.	O
</s>
<s>
Because	O
of	O
its	O
utter	O
unsuitability	O
for	O
real	O
secrecy	O
,	O
ROT13	B-Algorithm
has	O
become	O
a	O
catchphrase	O
to	O
refer	O
to	O
any	O
conspicuously	O
weak	O
encryption	O
scheme	O
;	O
a	O
critic	O
might	O
claim	O
that	O
"	O
56-bit	O
DES	B-Algorithm
is	O
little	O
better	O
than	O
ROT13	B-Algorithm
these	O
days	O
"	O
.	O
</s>
<s>
Also	O
,	O
in	O
a	O
play	O
on	O
real	O
terms	O
like	O
"	O
double	O
DES	B-Algorithm
"	O
,	O
the	O
terms	O
"	O
double	B-Algorithm
ROT13	I-Algorithm
"	O
,	O
"	O
ROT26	O
"	O
,	O
or	O
"	O
2ROT13	B-Algorithm
"	O
crop	O
up	O
with	O
humorous	O
intent	O
(	O
due	O
to	O
the	O
fact	O
that	O
,	O
since	O
applying	O
ROT13	B-Algorithm
to	O
an	O
already	O
ROT13-encrypted	O
text	O
restores	O
the	O
original	O
plaintext	O
,	O
ROT26	O
is	O
equivalent	O
to	O
no	O
encryption	O
at	O
all	O
)	O
,	O
including	O
a	O
spoof	O
academic	O
paper	O
entitled	O
"	O
On	O
the	O
2ROT13	B-Algorithm
Encryption	O
Algorithm	O
"	O
.	O
</s>
<s>
By	O
extension	O
,	O
triple-ROT13	O
(	O
used	O
in	O
joking	O
analogy	O
with	O
3DES	O
)	O
is	O
equivalent	O
to	O
regular	O
ROT13	B-Algorithm
.	I-Algorithm
</s>
<s>
In	O
December	O
1999	O
,	O
it	O
was	O
found	O
that	O
Netscape	B-Protocol
Communicator	I-Protocol
used	O
ROT13	B-Algorithm
as	O
part	O
of	O
an	O
insecure	O
scheme	O
to	O
store	O
email	O
passwords	O
.	O
</s>
<s>
In	O
2001	O
,	O
Russian	O
programmer	O
Dimitry	O
Sklyarov	O
demonstrated	O
that	O
an	O
eBook	O
vendor	O
,	O
New	O
Paradigm	O
Research	O
Group	O
(	O
NPRG	O
)	O
,	O
used	O
ROT13	B-Algorithm
to	O
encrypt	O
their	O
documents	O
;	O
it	O
has	O
been	O
speculated	O
that	O
NPRG	O
may	O
have	O
mistaken	O
the	O
ROT13	B-Algorithm
toy	O
example	O
—	O
provided	O
with	O
the	O
Adobe	O
eBook	O
software	B-Application
development	I-Application
kit	I-Application
—	O
for	O
a	O
serious	O
encryption	O
scheme	O
.	O
</s>
<s>
Windows	O
XP	O
uses	O
ROT13	B-Algorithm
on	O
some	O
of	O
its	O
registry	O
keys	O
.	O
</s>
<s>
ROT13	B-Algorithm
is	O
also	O
used	O
in	O
the	O
Unix	B-Application
fortune	I-Application
program	I-Application
to	O
conceal	O
potentially	O
offensive	O
dicta	O
.	O
</s>
<s>
ROT13	B-Algorithm
provides	O
an	O
opportunity	O
for	O
letter	O
games	O
.	O
</s>
<s>
Some	O
words	O
will	O
,	O
when	O
transformed	O
with	O
ROT13	B-Algorithm
,	O
produce	O
another	O
word	O
.	O
</s>
<s>
The	O
pair	O
gnat	B-Language
and	O
tang	O
is	O
an	O
example	O
of	O
words	O
that	O
are	O
both	O
ROT13	B-Algorithm
reciprocals	O
and	O
reversals	O
.	O
</s>
<s>
Westley	O
's	O
computer	B-Application
program	I-Application
can	O
be	O
encoded	O
in	O
ROT13	B-Algorithm
or	O
reversed	O
and	O
still	O
compiles	B-Language
correctly	O
.	O
</s>
<s>
Its	O
operation	O
,	O
when	O
executed	O
,	O
is	O
either	O
to	O
perform	O
ROT13	B-Algorithm
encoding	O
on	O
,	O
or	O
to	O
reverse	O
its	O
input	O
.	O
</s>
<s>
The	O
newsgroup	B-Application
alt.folklore.urban	O
coined	O
a	O
word	O
—	O
furrfu	O
—	O
that	O
was	O
the	O
ROT13	B-Algorithm
encoding	O
of	O
the	O
frequently	O
encoded	O
utterance	O
"	O
sheesh	O
"	O
.	O
</s>
<s>
ROT5	O
is	O
a	O
practice	O
similar	O
to	O
ROT13	B-Algorithm
that	O
applies	O
to	O
numeric	O
digits	O
(	O
0	O
to	O
9	O
)	O
.	O
</s>
<s>
ROT13	B-Algorithm
and	O
ROT5	O
can	O
be	O
used	O
together	O
in	O
the	O
same	O
message	O
,	O
sometimes	O
called	O
ROT18	O
(	O
18	O
=	O
13	O
+	O
5	O
)	O
or	O
ROT13.5	O
.	O
</s>
<s>
ROT47	O
is	O
a	O
derivative	O
of	O
ROT13	B-Algorithm
which	O
,	O
in	O
addition	O
to	O
scrambling	O
the	O
basic	O
letters	O
,	O
treats	O
numbers	O
and	O
common	O
symbols	O
.	O
</s>
<s>
Instead	O
of	O
using	O
the	O
sequence	O
A	O
–	O
Z	O
as	O
the	O
alphabet	B-Application
,	O
ROT47	O
uses	O
a	O
larger	O
set	O
of	O
characters	O
from	O
the	O
common	O
character	B-General_Concept
encoding	I-General_Concept
known	O
as	O
ASCII	B-Protocol
.	O
</s>
<s>
through	O
126	O
'	O
~	O
'	O
,	O
94	O
in	O
total	O
,	O
taken	O
in	O
the	O
order	O
of	O
the	O
numerical	O
values	O
of	O
their	O
ASCII	B-Protocol
codes	I-Protocol
,	O
are	O
rotated	O
by	O
47	O
positions	O
,	O
without	O
special	O
consideration	O
of	O
case	O
.	O
</s>
<s>
The	O
use	O
of	O
a	O
larger	O
alphabet	B-Application
produces	O
a	O
more	O
thorough	O
obfuscation	O
than	O
that	O
of	O
ROT13	B-Algorithm
;	O
for	O
example	O
,	O
a	O
telephone	O
number	O
such	O
as	O
+	O
1-415-839-6885	O
is	O
not	O
obvious	O
at	O
first	O
sight	O
from	O
the	O
scrambled	O
result	O
Z'\c`d\gbh\eggd	O
.	O
</s>
<s>
The	O
GNU	B-Language
C	I-Language
library	I-Language
,	O
a	O
set	O
of	O
standard	O
routines	O
available	O
for	O
use	O
in	O
computer	B-General_Concept
programming	I-General_Concept
,	O
contains	O
a	O
function	O
—	O
memfrob( )	O
—	O
which	O
has	O
a	O
similar	O
purpose	O
to	O
ROT13	B-Algorithm
,	O
although	O
it	O
is	O
intended	O
for	O
use	O
with	O
arbitrary	O
binary	O
data	O
.	O
</s>
<s>
The	O
function	O
operates	O
by	O
combining	O
each	O
byte	B-Application
with	O
the	O
binary	O
pattern	O
00101010	O
(	O
42	O
)	O
using	O
the	O
exclusive	O
or	O
(	O
XOR	O
)	O
operation	O
.	O
</s>
<s>
This	O
effects	O
a	O
simple	B-Algorithm
XOR	I-Algorithm
cipher	I-Algorithm
.	O
</s>
<s>
Like	O
ROT13	B-Algorithm
,	O
XOR	O
(	O
and	O
therefore	O
memfrob( )	O
)	O
is	O
self-reciprocal	O
,	O
and	O
provides	O
a	O
similar	O
,	O
virtually	O
absent	O
,	O
level	O
of	O
security	O
.	O
</s>
<s>
The	O
ROT13	B-Algorithm
and	O
ROT47	O
are	O
fairly	O
easy	O
to	O
implement	O
using	O
the	O
Unix	O
terminal	O
application	O
tr	B-Application
;	O
to	O
encrypt	O
the	O
string	O
"	O
The	O
Quick	O
Brown	O
Fox	O
Jumps	O
Over	O
The	O
Lazy	O
Dog	O
"	O
in	O
ROT13	B-Algorithm
:	O
</s>
<s>
In	O
Emacs	B-Application
,	O
one	O
can	O
ROT13	B-Algorithm
the	O
buffer	O
or	O
a	O
selection	O
with	O
the	O
following	O
commands	O
:	O
</s>
<s>
and	O
in	O
the	O
Vim	B-Application
text	I-Application
editor	I-Application
,	O
one	O
can	O
ROT13	B-Algorithm
a	O
buffer	O
with	O
the	O
command	O
:	O
</s>
