<s>
The	O
Horton	B-General_Concept
principle	I-General_Concept
is	O
a	O
design	O
rule	O
for	O
cryptographic	B-General_Concept
systems	I-General_Concept
and	O
can	O
be	O
expressed	O
as	O
"	O
Authenticate	O
what	O
is	O
being	O
meant	O
,	O
not	O
what	O
is	O
being	O
said	O
"	O
or	O
"	O
mean	O
what	O
you	O
sign	O
and	O
sign	O
what	O
you	O
mean	O
"	O
not	O
merely	O
the	O
encrypted	O
version	O
of	O
what	O
was	O
meant	O
.	O
</s>
<s>
The	O
Horton	B-General_Concept
principle	I-General_Concept
becomes	O
important	O
when	O
using	O
message	B-Algorithm
authentication	I-Algorithm
codes	I-Algorithm
(	O
or	O
MACs	O
)	O
in	O
a	O
cryptographic	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
When	O
applied	O
in	O
this	O
case	O
,	O
the	O
Horton	B-General_Concept
principle	I-General_Concept
works	O
by	O
authenticating	O
the	O
meaning	O
instead	O
of	O
the	O
message	O
.	O
</s>
<s>
Systems	O
can	O
manage	O
this	O
problem	O
by	O
adding	O
metadata	O
such	O
as	O
a	O
protocol	O
number	O
or	O
by	O
formatting	O
messages	O
with	O
an	O
explicit	O
structure	O
,	O
such	O
as	O
XML	B-Protocol
.	O
</s>
