<s>
In	O
cryptocurrencies	O
,	O
an	O
unspent	B-Protocol
transaction	I-Protocol
output	I-Protocol
(	O
UTXO	B-Protocol
)	O
represents	O
some	O
amount	O
of	O
digital	O
currency	O
which	O
has	O
been	O
authorized	O
by	O
one	O
account	O
to	O
be	O
spent	O
by	O
another	O
.	O
</s>
<s>
UTXOs	O
are	O
formatted	O
with	O
the	O
recipient	O
's	O
public	O
key	O
,	O
thus	O
restricting	O
the	O
ability	O
to	O
spend	O
that	O
UTXO	B-Protocol
to	O
the	O
account	O
that	O
can	O
prove	O
ownership	O
of	O
the	O
associated	O
private	O
key	O
.	O
</s>
<s>
The	O
UTXO	B-Protocol
can	O
only	O
be	O
spent	O
if	O
it	O
includes	O
the	O
digital	O
signature	O
associated	O
with	O
the	O
public	O
key	O
attached	O
the	O
last	O
time	O
it	O
was	O
sent	O
.	O
</s>
<s>
Each	O
UTXO	B-Protocol
represents	O
a	O
chain	O
of	O
ownership	O
implemented	O
as	O
a	O
chain	O
of	O
digital	O
signatures	O
going	O
back	O
to	O
the	O
origin	O
of	O
the	O
coin	O
.	O
</s>
<s>
Bitcoin	B-Protocol
is	O
an	O
example	O
of	O
a	O
cryptocurrency	O
that	O
uses	O
the	O
UTXO	B-Protocol
model	O
.	O
</s>
<s>
An	O
extended	O
version	O
of	O
the	O
UTXO	B-Protocol
model	O
(	O
EUTXO	O
)	O
is	O
used	O
for	O
the	O
Cardano	B-Protocol
blockchain	B-Application
.	O
</s>
<s>
The	O
many	O
cryptocurrencies	O
that	O
use	O
the	O
UTXO	B-Protocol
model	O
do	O
not	O
use	O
accounts	O
or	O
balances	O
.	O
</s>
<s>
The	O
UTXO	B-Protocol
model	O
treats	O
currency	O
as	O
objects	O
.	O
</s>
<s>
The	O
history	O
of	O
a	O
UTXO	B-Protocol
is	O
stored	O
only	O
in	O
the	O
blocks	O
when	O
it	O
is	O
transferred	O
,	O
and	O
to	O
find	O
the	O
total	O
balance	O
of	O
an	O
account	O
one	O
must	O
scan	O
each	O
block	O
to	O
find	O
the	O
latest	O
UTXOs	O
which	O
point	O
to	O
that	O
account	O
.	O
</s>
<s>
Checking	O
account	O
balances	O
is	O
quicker	O
,	O
but	O
like	O
the	O
UTXO	B-Protocol
model	O
,	O
fully	O
verifying	O
the	O
origin	O
of	O
coins	O
still	O
requires	O
auditing	O
past	O
blocks	O
to	O
the	O
coin	O
's	O
origin	O
.	O
</s>
<s>
The	O
total	O
UTXOs	O
present	O
in	O
a	O
blockchain	B-Application
represents	O
a	O
set	O
,	O
every	O
transaction	O
consumes	O
elements	O
from	O
this	O
set	O
and	O
creates	O
new	O
ones	O
that	O
are	O
added	O
to	O
the	O
set	O
.	O
</s>
<s>
The	O
complete	O
UTXO	B-Protocol
set	O
can	O
be	O
summed	O
to	O
calculate	O
the	O
total	O
supply	O
of	O
a	O
cryptocurrency	O
at	O
a	O
given	O
point	O
in	O
time	O
.	O
</s>
<s>
Bitcoin	B-Protocol
UTXO	B-Protocol
lifespans	O
have	O
been	O
studied	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
a	O
valid	O
blockchain	B-Application
transaction	O
,	O
unspent	O
outputs	O
(	O
and	O
only	O
unspent	O
outputs	O
)	O
may	O
be	O
used	O
to	O
fund	O
further	O
transactions	O
.	O
</s>
<s>
The	O
requirement	O
that	O
only	O
unspent	O
outputs	O
may	O
be	O
used	O
in	O
further	O
transactions	O
is	O
necessary	O
to	O
prevent	O
double-spending	B-Operating_System
and	O
fraud	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
inputs	O
on	O
a	O
blockchain	B-Application
are	O
removed	O
from	O
the	O
UTXO	B-Protocol
set	O
when	O
a	O
transaction	O
occurs	O
,	O
whilst	O
at	O
the	O
same	O
time	O
,	O
outputs	O
create	O
new	O
UTXOs	O
,	O
which	O
are	O
added	O
to	O
the	O
UTXO	B-Protocol
set	O
.	O
</s>
<s>
These	O
unspent	B-Protocol
transaction	I-Protocol
outputs	I-Protocol
may	O
be	O
used	O
(	O
by	O
the	O
holders	O
of	O
private	O
keys	O
;	O
for	O
example	O
,	O
persons	O
with	O
cryptocurrency	B-Protocol
wallets	I-Protocol
)	O
for	O
the	O
purpose	O
of	O
future	O
transactions	O
.	O
</s>
<s>
The	O
UTXO	B-Protocol
model	O
can	O
be	O
attributed	O
to	O
Hal	O
Finney	O
's	O
Reusable	O
Proofs	O
of	O
Work	O
proposal	O
,	O
which	O
is	O
based	O
on	O
Adam	O
Back	O
's	O
1997	O
Hashcash	O
proposal	O
.	O
</s>
