<s>
Proof	O
of	O
work	O
was	O
later	O
popularized	O
by	O
Bitcoin	B-Protocol
as	O
a	O
foundation	O
for	O
consensus	B-Operating_System
in	O
a	O
permissionless	O
decentralized	O
network	O
,	O
in	O
which	O
miners	O
compete	O
to	O
append	O
blocks	O
and	O
mine	O
new	O
currency	O
,	O
each	O
miner	O
experiencing	O
a	O
success	O
probability	O
proportional	O
to	O
the	O
computational	O
effort	O
expended	O
.	O
</s>
<s>
PoW	O
and	O
PoS	O
(	O
proof	B-Protocol
of	I-Protocol
stake	I-Protocol
)	O
remain	O
the	O
two	O
best	O
known	O
Sybil	O
deterrence	O
mechanisms	O
.	O
</s>
<s>
It	O
is	O
verified	O
with	O
a	O
single	O
computation	O
by	O
checking	O
that	O
the	O
SHA-1	B-Algorithm
hash	I-Algorithm
of	O
the	O
stamp	O
(	O
omit	O
the	O
header	O
name	O
X-Hashcash	O
:	O
including	O
the	O
colon	O
and	O
any	O
amount	O
of	O
whitespace	O
following	O
it	O
up	O
to	O
the	O
digit	O
'	O
1	O
 '	O
)	O
begins	O
with	O
52	O
binary	O
zeros	O
,	O
that	O
is	O
13	O
hexadecimal	O
zeros	O
:	O
</s>
<s>
Proof-of-work	O
systems	O
are	O
being	O
used	O
by	O
other	O
,	O
more	O
complex	O
cryptographic	O
systems	O
such	O
as	O
bitcoin	B-Protocol
,	O
which	O
uses	O
a	O
system	O
similar	O
to	O
Hashcash	O
.	O
</s>
<s>
At	O
the	O
IACR	O
conference	O
Crypto	O
2022	O
researchers	O
presented	O
a	O
paper	O
describing	O
Ofelimos	O
,	O
a	O
blockchain	O
protocol	O
with	O
a	O
consensus	B-Operating_System
mechanism	I-Operating_System
based	O
on	O
"	O
proof	O
of	O
useful	O
work	O
"	O
(	O
PoUW	O
)	O
.	O
</s>
<s>
Rather	O
than	O
miners	O
consuming	O
energy	O
in	O
solving	O
complex	O
,	O
but	O
essentially	O
useless	O
,	O
puzzles	O
to	O
validate	O
transactions	O
,	O
Ofelimos	O
achieves	O
consensus	B-Operating_System
while	O
simultaneously	O
providing	O
a	O
decentralized	O
optimization	O
problem	O
solver	O
.	O
</s>
<s>
The	O
paper	O
gives	O
an	O
example	O
that	O
implements	O
a	O
variant	O
of	O
WalkSAT	B-Application
,	O
a	O
local	O
search	O
algorithm	O
to	O
solve	O
Boolean	O
problems	O
.	O
</s>
<s>
In	O
2009	O
,	O
the	O
Bitcoin	B-Protocol
network	O
went	O
online	O
.	O
</s>
<s>
Bitcoin	B-Protocol
is	O
a	O
proof-of-work	O
digital	O
currency	O
that	O
,	O
like	O
Finney	O
's	O
RPoW	O
,	O
is	O
also	O
based	O
on	O
the	O
Hashcash	O
PoW	O
.	O
</s>
<s>
But	O
in	O
Bitcoin	B-Protocol
,	O
double-spend	O
protection	O
is	O
provided	O
by	O
a	O
decentralized	O
P2P	O
protocol	O
for	O
tracking	O
transfers	O
of	O
coins	O
,	O
rather	O
than	O
the	O
hardware	O
trusted	O
computing	O
function	O
used	O
by	O
RPoW	O
.	O
</s>
<s>
Bitcoin	B-Protocol
has	O
better	O
trustworthiness	O
because	O
it	O
is	O
protected	O
by	O
computation	O
.	O
</s>
<s>
Bitcoins	B-Protocol
are	O
"	O
mined	O
"	O
using	O
the	O
Hashcash	O
proof-of-work	O
function	O
by	O
individual	O
miners	O
and	O
verified	O
by	O
the	O
decentralized	O
nodes	O
in	O
the	O
P2P	O
bitcoin	B-Protocol
network.The	O
difficulty	O
is	O
periodically	O
adjusted	O
to	O
keep	O
the	O
block	O
time	O
around	O
a	O
target	O
time	O
.	O
</s>
<s>
Since	O
the	O
creation	O
of	O
Bitcoin	B-Protocol
,	O
proof-of-work	O
has	O
been	O
the	O
predominant	O
design	O
of	O
Peer-to-peer	O
cryptocurrency	O
.	O
</s>
<s>
2018	O
estimates	O
from	O
the	O
University	O
of	O
Cambridge	O
equate	O
Bitcoin	B-Protocol
’s	O
energy	O
consumption	O
to	O
that	O
of	O
Switzerland	O
.	O
</s>
<s>
Within	O
the	O
Bitcoin	B-Protocol
community	O
there	O
are	O
groups	O
working	O
together	O
in	O
mining	B-Protocol
pools	I-Protocol
.	O
</s>
<s>
This	O
trend	O
toward	O
mining	B-Protocol
pools	I-Protocol
and	O
specialized	O
ASICs	O
has	O
made	O
mining	O
some	O
cryptocurrencies	O
economically	O
infeasible	O
for	O
most	O
players	O
without	O
access	O
to	O
the	O
latest	O
ASICs	O
,	O
nearby	O
sources	O
of	O
inexpensive	O
energy	O
,	O
or	O
other	O
special	O
advantages	O
.	O
</s>
<s>
These	O
miners	O
compete	O
to	O
solve	O
crypto	O
challenges	O
on	O
the	O
Bitcoin	B-Protocol
blockchain	I-Protocol
,	O
and	O
their	O
solutions	O
must	O
be	O
agreed	O
upon	O
by	O
all	O
nodes	O
and	O
reach	O
consensus	B-Operating_System
.	O
</s>
<s>
The	O
solutions	O
are	O
then	O
used	O
to	O
validate	O
transactions	O
,	O
add	O
blocks	O
and	O
generate	O
new	O
bitcoins	B-Protocol
.	O
</s>
<s>
However	O
,	O
the	O
Bitcoin-style	O
mining	O
process	O
is	O
very	O
energy	O
intensive	O
because	O
the	O
proof	O
of	O
work	O
shaped	O
like	O
a	O
lottery	O
mechanism	O
.	O
</s>
<s>
In	O
January	O
2022	O
Vice-Chair	O
of	O
the	O
European	O
Securities	O
and	O
Markets	O
Authority	O
Erik	O
Thedéen	O
called	O
on	O
the	O
EU	O
to	O
ban	O
the	O
proof	O
of	O
work	O
model	O
in	O
favor	O
of	O
the	O
proof	B-Protocol
of	I-Protocol
stake	I-Protocol
model	O
due	O
its	O
lower	O
energy	O
emissions	O
.	O
</s>
