<s>
The	O
Time-Stamp	B-Protocol
Protocol	I-Protocol
,	O
or	O
TSP	O
is	O
a	O
cryptographic	O
protocol	O
for	O
certifying	O
timestamps	O
using	O
X.509	O
certificates	O
and	O
public	B-Architecture
key	I-Architecture
infrastructure	I-Architecture
.	O
</s>
<s>
It	O
has	O
been	O
extended	O
to	O
create	O
the	O
ANSI	B-Protocol
ASC	I-Protocol
X9.95	I-Protocol
Standard	I-Protocol
.	O
</s>
<s>
In	O
the	O
protocol	O
a	O
Time	O
Stamp	O
Authority	O
(	O
TSA	O
)	O
is	O
a	O
trusted	O
third	O
party	O
that	O
can	O
provide	O
a	O
timestamp	O
to	O
be	O
associated	O
with	O
a	O
hashed	B-Algorithm
version	O
of	O
some	O
data	O
.	O
</s>
<s>
It	O
is	O
a	O
request-response	O
protocol	O
,	O
where	O
the	O
request	O
contains	O
a	O
hash	B-Algorithm
of	O
the	O
data	O
to	O
be	O
signed	O
.	O
</s>
<s>
This	O
is	O
sent	O
to	O
the	O
TSA	O
and	O
the	O
response	O
contains	O
a	O
Time	O
Stamp	O
Token	O
(	O
TST	O
)	O
which	O
itself	O
includes	O
the	O
hash	B-Algorithm
of	O
the	O
data	O
,	O
a	O
unique	O
serial	O
number	O
,	O
a	O
timestamp	O
and	O
a	O
digital	O
signature	O
.	O
</s>
<s>
The	O
protocol	O
can	O
operate	O
over	O
a	O
number	O
of	O
different	O
transports	O
,	O
including	O
email	O
,	O
TCP	B-Protocol
sockets	I-Protocol
or	O
HTTP	B-Protocol
.	I-Protocol
</s>
<s>
When	O
presented	O
with	O
a	O
TST	O
,	O
someone	O
may	O
verify	O
that	O
the	O
data	O
existed	O
at	O
the	O
timestamp	O
in	O
the	O
TST	O
by	O
verifying	O
the	O
signature	O
using	O
the	O
public	O
key	O
of	O
the	O
TSA	O
and	O
that	O
the	O
hash	B-Algorithm
of	O
the	O
data	O
matches	O
that	O
included	O
in	O
the	O
TST	O
.	O
</s>
