<s>
HMAC-based	B-Algorithm
one-time	I-Algorithm
password	I-Algorithm
(	O
HOTP	B-Algorithm
)	O
is	O
a	O
one-time	O
password	O
(	O
OTP	O
)	O
algorithm	O
based	O
on	O
HMAC	B-Algorithm
.	O
</s>
<s>
It	O
is	O
a	O
cornerstone	O
of	O
the	O
Initiative	B-General_Concept
for	I-General_Concept
Open	I-General_Concept
Authentication	I-General_Concept
(	O
OATH	O
)	O
.	O
</s>
<s>
HOTP	B-Algorithm
was	O
published	O
as	O
an	O
informational	O
IETF	O
RFC	O
4226	O
in	O
December	O
2005	O
,	O
documenting	O
the	O
algorithm	O
along	O
with	O
a	O
Java	O
implementation	O
.	O
</s>
<s>
The	O
HOTP	B-Algorithm
algorithm	O
is	O
a	O
freely	O
available	O
open	O
standard	O
.	O
</s>
<s>
The	O
HOTP	B-Algorithm
algorithm	O
provides	O
a	O
method	O
of	O
authentication	O
by	O
symmetric	O
generation	O
of	O
human-readable	O
passwords	O
,	O
or	O
values	O
,	O
each	O
used	O
for	O
only	O
one	O
authentication	O
attempt	O
.	O
</s>
<s>
Parties	O
intending	O
to	O
use	O
HOTP	B-Algorithm
must	O
establish	O
some	O
;	O
typically	O
these	O
are	O
specified	O
by	O
the	O
authenticator	O
,	O
and	O
either	O
accepted	O
or	O
not	O
by	O
the	O
authenticated	O
:	O
</s>
<s>
Both	O
parties	O
compute	O
the	O
HOTP	B-Algorithm
value	O
derived	O
from	O
the	O
secret	O
key	O
K	O
and	O
the	O
counter	O
C	O
.	O
Then	O
the	O
authenticator	O
checks	O
its	O
locally	O
generated	O
value	O
against	O
the	O
value	O
supplied	O
by	O
the	O
authenticated	O
.	O
</s>
<s>
The	O
recommendation	O
is	O
made	O
that	O
persistent	O
throttling	O
of	O
HOTP	B-Algorithm
value	O
verification	O
take	O
place	O
,	O
to	O
address	O
their	O
relatively	O
small	O
size	O
and	O
thus	O
vulnerability	O
to	O
brute-force	O
attacks	O
.	O
</s>
<s>
After	O
verification	O
,	O
the	O
authenticator	O
can	O
authenticate	O
itself	O
simply	O
by	O
generating	O
the	O
next	O
HOTP	B-Algorithm
value	O
,	O
returning	O
it	O
,	O
and	O
then	O
the	O
authenticated	O
can	O
generate	O
their	O
own	O
HOTP	B-Algorithm
value	O
to	O
verify	O
it	O
.	O
</s>
<s>
The	O
HOTP	B-Algorithm
value	O
is	O
the	O
human-readable	O
design	O
output	O
,	O
a	O
d-digit	O
decimal	O
number	O
(	O
without	O
omission	O
of	O
leading	O
0s	O
)	O
:	O
</s>
<s>
HOTP	B-Algorithm
value	O
=	O
HOTP(K, C )	O
mod	O
10d	O
.	O
</s>
<s>
That	O
is	O
,	O
the	O
value	O
is	O
the	O
d	O
least	O
significant	O
base-10	O
digits	O
of	O
HOTP	B-Algorithm
.	O
</s>
<s>
HOTP	B-Algorithm
is	O
a	O
truncation	O
of	O
the	O
HMAC	B-Algorithm
of	O
the	O
counter	O
C	O
(	O
under	O
the	O
key	O
K	O
and	O
hash	B-Algorithm
function	I-Algorithm
H	O
)	O
:	O
</s>
<s>
HOTP(K, C )	O
=	O
truncate(HMAC(K, C )	O
)	O
,	O
</s>
<s>
The	O
use	O
of	O
"	O
19	O
"	O
in	O
the	O
above	O
formula	O
relates	O
to	O
the	O
size	O
of	O
the	O
output	O
from	O
the	O
hash	B-Algorithm
function	I-Algorithm
.	O
</s>
<s>
With	O
the	O
default	O
of	O
SHA-1	B-Algorithm
,	O
the	O
output	O
is	O
,	O
and	O
so	O
the	O
last	O
byte	O
is	O
byte	O
19	O
(	O
0-origin	O
)	O
.	O
</s>
<s>
Hardware	O
tokens	O
implementing	O
OATH	O
HOTP	B-Algorithm
tend	O
to	O
be	O
significantly	O
cheaper	O
than	O
their	O
competitors	O
based	O
on	O
proprietary	O
algorithms	O
.	O
</s>
<s>
As	O
of	O
2010	O
,	O
OATH	O
HOTP	B-Algorithm
hardware	O
tokens	O
can	O
be	O
purchased	O
for	O
a	O
marginal	O
price	O
.	O
</s>
<s>
Some	O
products	O
can	O
be	O
used	O
for	O
strong	O
passwords	O
as	O
well	O
as	O
OATH	O
HOTP	B-Algorithm
.	O
</s>
<s>
Software	O
tokens	O
are	O
available	O
for	O
(	O
nearly	O
)	O
all	O
major	O
mobile/smartphone	O
platforms	O
(	O
J2ME	B-Language
,	O
Android	B-Application
,	O
iPhone	B-Device
,	O
BlackBerry	B-Operating_System
,	O
Maemo	B-Operating_System
,	O
macOS	B-Application
,	O
and	O
Windows	B-Operating_System
Mobile	I-Operating_System
)	O
.	O
</s>
<s>
Although	O
the	O
reception	O
from	O
some	O
of	O
the	O
computer	O
press	O
has	O
been	O
negative	O
during	O
2004	O
and	O
2005	O
,	O
after	O
IETF	O
adopted	O
HOTP	B-Algorithm
as	O
RFC	O
4226	O
in	O
December	O
2005	O
,	O
various	O
vendors	O
started	O
to	O
produce	O
HOTP-compatible	O
tokens	O
and/or	O
whole	O
authentication	O
solutions	O
.	O
</s>
<s>
According	O
to	O
the	O
article	O
"	O
Road	O
Map	O
:	O
Replacing	O
Passwords	O
with	O
OTP	O
Authentication	O
"	O
on	O
strong	O
authentication	O
,	O
published	O
by	O
Burton	O
Group	O
(	O
a	O
division	O
of	O
Gartner	O
,	O
Inc	O
.	O
)	O
in	O
2010	O
,	O
"	O
Gartner	O
's	O
expectation	O
is	O
that	O
the	O
hardware	O
OTP	O
form	O
factor	O
will	O
continue	O
to	O
enjoy	O
modest	O
growth	O
while	O
smartphone	B-Application
OTPs	O
will	O
grow	O
and	O
become	O
the	O
default	O
hardware	O
platform	O
over	O
time.	O
"	O
</s>
