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https://archive.devcon.org/archive/watch/6/fast-and-furious-withdrawals-from-optimistic-rollups/
Bridges are very complex and have under-explored security issues. The issue we are focused on with withdrawing from L2, which requires L1 to be absolutely sure of what occurred on L2 (finality was reached) and current proposals like Offchain Lab’s Arbitrum require 1 week for finality. In this talk, we discuss the three designs for “fast” withdraws that allow a user to move Layer 2 to Layer 1 instantly, while a counter-party takes the risk that the withdraw will not finalize.
Speaker(s): Mahsa Moosavi
Skill level: Intermediate
Track: Layer 2s
Keywords: optimistic roll-ups,withdrawal latency,layer 2 scaling solutions,Arbitrum,instant finality
Follow us: https://twitter.com/efdevcon, https://twitter.com/ethereum
Learn more about devcon: https://www.devcon.org/
Learn more about ethereum: https://ethereum.org/
Devcon is the Ethereum conference for developers, researchers, thinkers, and makers.
Devcon 6 was held in Bogotá, Colombia on Oct 11 - 14, 2022.
Devcon is organized and presented by the Ethereum Foundation, with the support of our sponsors. To find out more, please visit https://ethereum.foundation/
A
A
My
talk
today
is
going
to
be
about
Fast
and
Furious
withdrawals
from
optimistic
Roll-Ups.
So
a
little
bit
about
myself.
My
name
is
Masa
and
I'm,
a
PhD
candidate,
I'm,
doing
PhD
at
Concordia
University
with
Jeremy
Clark
and
Montreal
and
I'm
also
integration
engineer
at
the
often
labs
all
right.
So
what
I'm
going
to
talk
about
today
is
research
that
I
was
doing
during
the
PHD
and
it
kind
of
co-located,
with
the
time
that
I
was
doing
the
job
at
the
off
chain.
A
So
it's
kind
of
a
collaboration
between
the
folks
at
FJ
and
Korea.
So
it's
a
research.
We
are
particularly
interested
in
solving
a
research
problem,
but
before
I
dive
into
the
details
of
the
problem,
I
want
to
take
some
time
and
talk
about
how
Roll-Ups
work
in
general
and
how
optimistic
Roll-Ups
work.
So
you
might
have
already
heard
from
other
folks
during
the
conference,
but
it's
always
good
to
keep
refreshing
and
make
sure
everybody
is
on
board.
So,
let's
start
with
the
question
of:
why
is
the
term
slow
and
expensive?
A
So
we
know
ethereum
is
slow,
there's
unscalability
problem
with
ethereum
and
it's
really
a
fundamental
issue
and
why
there
is
a
problem.
Is
that
let's
say
you
have
a
function
that
you
want
to
get
executed
on
an
input
X
and
if
it
was
a
cloud
environment,
the
cloud
service
provider
would
give
you
the
closest
node
in
the
network.
A
And
then
what
happened?
Is
you
send
the
function
to
the
node?
It
would
execute
the
function
for
you
and
it
would
give
back
the
results
and
you
learn
what
the
result
is,
but
on
a
blockchain,
because
we
care
about
trust
and
integrity.
We
can't
really
rely
on
one
node
to
execute
everything
for
us.
So
what
happens
on
a
blockchain?
A
Is
you
send
your
function
to
a
node
and
one
note
what
that
node
does
is
actually
relays
your
function
together
with
the
input
to
all
the
nodes
in
the
network
and
what
happens
is
then
all
the
nodes
in
a
network
execute
your
function.
So
it's
literally
like
you
have
to
pay
for
each
and
every
node
executing
the
function
for
you,
and
then
it
takes
some
time
because
they
have
to
run
a
consensus.
A
Algorithm
among
each
other
so
that
they
could
all
agree
on
the
same
output,
and
it's
only
that
moment
that
you
can
learn
the
result.
So
that's
that's!
That's
really
why
it's
so
inexpensive,
okay,
so,
but
why
is
it
that
every
node
has
to
compute
everything?
So
the
answer
is
because
they
have
to
make
sure
that
it's
the
correct
thing.
So,
let's
say
I
have
a
function:
I
execute
it
for
you
and
then
I
come
to
you
and
I'm
like
here's,
the
output.
A
So
what
is
the
most
straightforward
way
to
for
you
to
actually
verify
if
I'm
saying
the
correct
thing,
so
you
have
to
go
and
compute
the
function
yourself
right.
So
the
question
people
have
been
asking
in
terms
of
scalability
of
ethereum.
Is
that
is
there
any
way
I
can
convince
you
that
I've
done
the
execution
correct
without
you
having
to
go
and
execute
everything
yourself.
A
So
there
are
two
main
approaches
to
tackle
this
problem.
The
first
one
is:
you
can
actually
throw
some
math
at
a
problem
and
you
create
kind
of
a
mathematical
proof.
So
you
run
a
function.
You
get
the
output
and
you
then
you
kind
of
create
a
cryptography
proof,
you
hand
it
to
me
and
then
I
can
actually
verify
the
proof
and
see
if
it's
a
correct
thing
or
not.
So
very
if,
like
verifying
the
proof,
is
less
work
than
actually
having
to
execute
everything,
then
I
actually
save
some
money
and
time
right.
A
A
So
it's
really
a
cost
effective
solution,
especially
if
you're
one
of
those
nodes
that
have
to
verify
those
proof
and
the
second
solution
is
to
use
kind
of
like
economics,
so
I'm
like
I,
execute
this
function
and
here's
the
output
and
here's
a
million
dollar
worth
of
eth
packing
my
assertion
and
it's
open
for
you
to
dispute
anybody
can
come
along
and
they
can
prove
that
it's
the
wrong
output,
but
they
have
to
stake
money
as
well.
So
the
way
it
works
is
let's
say:
Alice
is
saying
what
apple
is
right.
A
Bob
is
saying
the
other
output
is
right
and
then
there's
a
dispute
and
disagreement
right,
and
it's
only
in
that
point
that
ethereum
goes
back
to
a
normal
blockchain
mode,
where
every
node
has
to
execute
everything.
So
this
is
how
optimistic
rollups
work
and
we
call
it
optimistic
because,
because
we're
really
optimistic
that
not
every
node
has
to
compute
everything,
because
it
really
notes
always
assert
the
right
output,
and
you
see
why
so
the
way
an
optimistic
rollups
work
is
that
I
I
execute
a
function
and
I
put
an
assertion.
A
I,
stick
on
that
and
let's
say
Bobby's
come
coming
and
he
says
this
is
the
wrong
output,
so
he
finds
a
dispute.
So
what
happens
is
ethereum
would
be
like
hey
I
know
you
guys
are
disagreeing
on
this
output.
I
want
you
to
pinpoint
me
to
the
exact
step
inside
the
computation
where
you
started
to
disagree.
So
maybe
you
were
fine
for
the
first
30
of
the
computation
and
it's
only
this
little
step
where
you
started
to
disagree.
A
So
it's
actually
going
to
verify
that
little
step,
so
it
ends
up
being
very
cost
effective,
even
if
ethereum
has
to
step
in
and
that's
why?
Actually,
it's
near
really
not
easy
to
get
away
with
the
wrong
output,
because
everybody
knows
that
at
the
end
of
the
day
a
term
can
come
along
and
it
can
settle
things
and
you
can
lose
money
and
then
yeah
and
then
that's
why?
Actually,
the
only
way
you
can
get
away
with
the
wrong
output
is
that
nobody
is
watching
the
protocol.
A
So
at
least
what
honest
party
watches
the
protocol,
then
you
you
can
never
get
out
of
it
yeah
and
yeah.
This
is
how
optimistic
rollups
work
and
like
in
in
real
world.
It
never
happens
and
they're
dispute
like
an
arbitrum
there's,
never
been
a
dispute
before
so.
Let's
say,
let's
see
how
it
works
on
arbitrom,
on
a
smart
contract
level.
So
what
happens
with
Arbitron
puts
two
smart
contracts
on
the
L1
inbox
and
outbox,
and
what
Alice's
Alice
does
is
she
wants
to
execute
some
function
right?
A
A
So
what
happens?
Is
somebody
outside
L1
comes
along?
They
dump
the
contents
of
the
inbox
smart
contract
on
L2
and
then
they're
going
to
execute
everything
on
the
arbos,
and
it's
really
based
on,
like
the
the
how
they
were
sorted
inside
inbox,
smart
contract
and
then
they're
gonna
update
everybody's
account
on
the
arbos,
and
then
they
put
the
like
the
output
of
this
execution
inside
the
outbox
more
contract
in
the
form
of
an
assertion.
So
an
assertion
is
basically
a
fingerprint
of
the
latest
state
of
the
arbos.
A
So
a
term
doesn't
care
what
is
our
Os
or
how
our
OS
works?
It
actually
works
a
lot
like
ethereum.
It
runs
a
blockchain,
it
has
blocks,
it
has
miners,
we
call
them
validators,
it
has
block
explorers,
but
a
term
doesn't
care
about
anything
about
that.
It
only
cares
about
like
here's.
The
list
of
function
that
I'm
seeing
that
are
going
to
be
executed
on
Layer
Two
I
want
to
see
the
the
output
of
this
execution.
That's
all
it
cares
about.
A
So
we
have
to
wait
for
seven
days
because
we
want
to
make
sure
that
everybody
have
time
to
come
along
and
they
react
to
it.
If
there's
any
dispute
and
if
there's
not
no
dispute,
it's
going
to
be
finalized,
so
let's
say
Alice
wants
to
do
something
slightly
different.
This
time
she
wants
to
withdraw
100
eth
from
L2
and
to
make
that
happen.
Arbitrom
has
to
introduce
a
smart
contract
which
is
called
Bridge,
so
Bridge's
job
is
basically
to
Escrow
whatever
tokens
or
ethers,
people
want
to
move
to
the
L2.
A
So
if
Alice
wants
to
withdraw
100
ether
from
L2,
it
means
that
she
has
previously
moved
100
ether
to
the
rbos.
So
what
bridge
does
is
hey?
Alice
I
know
you're
moving
100
East
to
the
L2.
That's
fine
just
give
me
100
each
I'm
gonna
score
it
for
you
and
then
I'm
gonna,
mint
100
is
on
L2
out
of
thin
air
you'll,
be
the
owner,
do
whatever
you
want,
go
ahead,
transfer
it
to
Bob
and
the
end
of
the
day.
A
What
I
care
about
is
somebody
comes
along
and
they
want
their
ether
back
from
me
and
that's
called
a
withdraw
transaction,
so
yeah,
so
like
any
other
transaction
I
just
described.
Alice
puts
a
message
for
the
withdrawal
transaction
inside
the
Inbox
and
arbitrum.
Does
it
things
so
yeah
somebody
dumps
the
contents
of
the
Inbox
and
the
arbos
it
gets
executed,
and
then
the
assertion
would
be
placed
inside
the
outbox.
A
Now
we
have
to
wait
for
seven
days
so
that
we
make
sure
that
the
assertion
is
finalized
and
our
withdraw
transaction
can
go
through
and
only
in
that
case,
when
it
is
finalized.
Alice
knows
that
her
transaction
is
in
assertion
number
20..
She
can
go
to
the
outbox
and
she'll
be
like
hey
upbox.
This
is
the
assertion.
It's
just
finalized.
It's
sitting
there
for
you
to
go
and
see,
go
check
it
please,
and
then,
if
you're
happy
with
that,
I
won
my
100
East
back
from
the
bridge,
so
outbox
is
Happy.
A
So
here's
the
problem,
I'm
sure
you've
noticed
withdrawals
take
seven
days
to
be
to
be
finalized
and
it
could
be
even
longer
if
there's
any
dispute
and
although
there's
never
been
a
dispute
before,
but
there
could
be,
and
people
really
don't
want
to
wait
for
seven
days.
Sometimes
they
have
a
pressing
reason.
A
If
you
run
an
validator
on
arbitrol,
what
happens
is
what
is
validator
is
just
a
software
that
goes
through
the
inbox
transactions,
one
by
one.
It's
going
to
execute
those,
and,
at
the
end
of
the
day,
it's
just
gonna
compare
the
output
of
this
execution
with
the
state
that
is
being
asserted.
So
if
you
run
an
orbits
from
validator,
you
can
be
100
sure
that
this
transaction
is
really
going
to
be
finalized.
The
assertion
is
going
to
be
finalized,
so
we
found
that
this.
A
This
is
really
interesting
that
we
can
leverage
to
get
a
relief
of
this
pain.
Point
of
having
to
wait
for
seven
days,
so
the
question
we
were
asking
in
this
research
is:
can
Alice
withdraw
100
eth
from
L2
back
to
L1,
without
having
to
wait
for
seven
days,
so
we
we
have
come
up
with
four
different
solutions
in
his
work
and
I'm
gonna
go
through
them
one
by
one
and
they
actually
get
better
as
we
go
through.
Okay,
so
the
first
solution
is
Alice
can
actually
use
a
centralized
exchange
like
hop
right.
A
So
what
you
can
do
is
you
can
go
to
The
Exchange.
She
can
trade
her
hundred
each
on
a
two
with
the
one
on
L1
and
it's
really
easy.
So
these
centralized
exchanges
they
have
piles
of
eth
on
both
L2
and
L1,
and
you
can
actually
trade
with
them.
So
this
solution
works.
A
lot
of
people
have
been
using
it,
but
the
problem
with
that
is
it's
centralized.
A
So
if
it
happens
to
be
some
somebody
somewhere
that
that
has
the
exact
same
amount
of
if
on
L1
and
is
willing
to
trade
that
with
Alice,
then
they
can
do
something
called
Atomic
swap
so
I'm
not
gonna,
go
through
the
details
of
what
Atomic
swap
is,
but
it's
basically
like
a
little
of
crypto,
combined
with
smart
contract
that
allows
you
to
basically
do
transactions
on
two
different
chains
at
the
same
time.
So
it's
like
arbits
from
here
is
a
different
chain
than
a
term
right.
So
it's
like
both
Alice
and
Bob.
A
They
agree
on
Atomic
swap
so
they
have
to
lock
up
some
money
and
then
there
is
a
secret
somewhere
that
if
it's,
if
it's
revealed
on
one
chain,
then
you
can
use
it
to
send
a
transaction
on
another
chain.
So
it's
either
like
either.
Both
transaction
goes
through
at
the
same
time
or
they
don't
go
through
at
all.
So
this
is
the
beauty
of
atomic
swap.
So
it's
perfect.
A
It
works,
but
the
problem
with
that
that
you
might
have
noticed
is
that
Bob
can
agree
to
do
the
atomic
swap,
but
then
he
can
just
disappear
in
the
middle
of
the
protocol,
so
there's
some
time
window
for
Alice
to
go
back
and
get
whatever
she
has
locked
up
before,
but
it's
actually
very
awkward
because
she's
not
getting
what
she
wants
and
she's
actually
losing
some
time
here.
So
we
go
to
the
third
solution
so
we're
like
hey
Alice.
A
We
know
that
you
initiated
your
withdraw
transaction
on
L2
and
from
L2
and
then
now
you're
waiting
for
your
assertion
to
be
finalized.
What
we
can
do
is
we
can
give
you
a
ticket.
Actually
that
says
you
are
actually
the
owner
of
this
hundred
each
and
then
you
can
do
whatever
you
want
with
the
ticket.
You
can
go
ahead.
You
can
transfer
it
to
Bob.
A
That's
fine,
even
though
the
seven
day
has
not
passed
so
so
it's
basically
like
we're
giving
a
tradable
ticket
or
a
claim
of
whatever
is
going
to
go
out
of
the
bridge
after
the
seven
days
and
as
I
said,
these
are
tradable
they
can
transfer
it.
So
if
Bob
happens
to
be
an
Orbitz
from
validator,
it
means
that
he
can
be
sure
if
the
assertion
is
going
to
go
finalized
or
not.
A
So
what
she
does
is
what
Alice
does
is
that
she
can
actually
go
to
Bob
and
she'll,
be
like
hey,
Bob,
I
know
that
you're
you're
actually
validating
everything
and
you
might
not
even
care
about
my
withdrawal,
but
what
I
can
offer
you
is
that
I
have
this
ticket
that
worth
100
ether
in
seven
days.
So
what
you
can
do
is
I
give
you
a
discount
or
a
bonus.
You
can
buy
my
ticket
because
you
know
that
this
assertion
is
going
to
be
finalized
and
then
you
get
the
ticket.
A
You
get
the
ticket,
you
get
the
bonus
and
then,
at
the
end
of
the
day
you
have
profit.
So
Bob
can
do
that.
It's
great
but,
as
you
may
have
noticed,
Bob
here
is
having
two
roles
so
first
off
he
has
to
have
the
liquidity,
meaning
that
he
has
to
be
sitting
on
a
pile
of
East
and
his.
He
has
to
be
willing
to
trade
it
with
Alice.
The
other
thing
is:
Bob
needs
to
run
an
evalidator
on
arbitrom,
so
he
needs
some
Assurance.
A
So
that's
the
problem.
Finding
these
bulbs
that
work
for
the
both
side
of
equations
is
not
really
easy.
Okay,
so
we
are
at
a
last
solution.
We
so
we
were
like.
Is
there
any
way
we
can
split
Bob?
So
is
there
any
way
we
can
have
Alice?
She
wants
to
withdraw
100
East
from
L2.
Then
we
have
David,
who
is
the
arbits
from
validator,
but
he
doesn't
have
the
liquidity
and
we
have
Carol
who
has
the
liquidity
but
she's,
not
a
validator.
A
So
is
there
any
way
these
three
can
arrange
something
that
can
give
Alice
what
she
wants
at
the
end
of
the
day,
and
it
turns
out
that,
yes,
there
is
a
solution
for
that.
We
have
this
solution
and
our
solution
is
not
necessarily
the
only
one
but
but
we
think
it
works
great.
So
the
solution
is
to
set
up
something
called
a
prediction
market.
So
what
is
a
prediction
market?
A
A
You
cannot
get
out
of
your
position,
you
have
to
sit
on
it
and
then
you
have
to
wait
for
the
event
to
happen,
but
on
the
prediction
Market
on
the
other
side,
what
happens
is,
let's
say
you
think
the
odds
of
Joe
Biden
being
the
next
president
of
the
United
States
is
so
high.
So
what
you
do
is
You're
Gonna
Go
place
a
bet
against
it
in
three
months.
The
odds
are
actually
going
down.
So
what
you
do
is
you
can
actually
sell
your
position
and
can
get
out
of
your
position.
A
So
this
is
the
difference
with
a
betting
market,
and
then
it's
really
like
a
betting
Market,
but
it
designed
more
kind
of
like
stocks,
I'd
say
so.
Okay,
so
we
have
this
prediction
Market.
The
way
we
set
up
it
is
we
put
the
market,
it's
really
like
a
vending
machine,
it's
there's
no
bookkeeping
or
anything.
It's
autonomous,
smart
contract,
we
put
it
on
L1,
and
so
a
user
would
go
to
it.
I
can
go
to
it.
A
I
deposit
one
ether
inside
the
market-
and
it
would
give
me
two
shares
one
yester
and
one
no
share
so,
and
these
Shares
are
betting
against.
If
this
session
is
going
to
be
finalized
or
not
so
at
the
very
beginning,
I'm
actually
covered
for
both
cases.
So
if
the
assertion
is
finalized,
my
yester
is
going
to
be
worth
one
ether
and
my
no
shares
actually
worth
nothing
and
on
the
other
side,
if
the
assertion
fails,
there's
a
dispute
and
then
my
no
share
is
going
to
be
worth
one
ether
and
the
gestures
are
actually
nothing.
A
So
in
any
case,
I
can
go
and
redeem
my
one
ether
back
okay.
So
how
does
it
work?
So?
Let's
say
Alice
wants.
Alice
is
the
one
who
wants
to
do
this
withdrawal
right,
so
she
initiates
the
withdrawal
and
she's
like
okay.
I
want
my
there
now
I,
don't
want
to
wait
for
seven
days.
So
what
I
do
is
I
come
along
I,
go
to
this
market
and
I
I
and
pay
attention
to
this
that
she
wants
to
withdraw
100
ether.
A
So
what
she
does
is
she
deposits
100
if
inside
the
smart
contract
the
market
and
what
Market
does
it
actually
gives
Alice
hundred
shares
of
yes
and
100
shares
of
no?
So
if
Alice
actually
believes
that
this
assertion
is
going
to
be
finalized
and
at
the
end
of
the
day,
her
ether
is
going
to
get
out
of
the
bridge,
but
she
would
do
so.
She
would
keep
the
gestures
for
herself
because
it's
a
bet
against
the
assertions
being
finalized
and
she
would
get
away.
A
There
are
no
shares
right
out
of
the
no
shares
but
she's
not
going
to
do
that
and
I
tell
you
why
what
she
does
is
she
actually
keeps
the
gestures
for
herself
at
the
Notions
for
herself
and
she's
gonna.
Give
the
yes
shares
to
David
who's
running
the
orbits
from
validator,
so
she
goes
to
David
and
she's,
like
hey.
I
know
that
you're
already
validating
the
orbits
from
chain
and
David
doesn't
care
about
this
withdrawal
at
all.
A
A
So
at
the
end
of
the
day
you
pay
99
ether
and
if
it's
going
to
be
finalized,
you
can
always
go
back
to
the
market
and
you
can
redeem
it
400
East,
so
Bob's,
gonna,
profit
right,
so
yeah
sorry
Dave
is
going
to
pray.
I
was
keeping
saying
Bob
right,
sorry
about
that,
so
I'm
in
debut
yeah,
so
so
Dave
is
going
to
profit
right.
So
he's
willing
to
do
that.
That's
fine!
So
what
she
does
with
the
no
shares
that
she
kept
for
herself
now
she
can
actually
go
to
Cairo.
A
Who
has
the
liquidity,
but
she
doesn't
know
what's
going
on
on
arbitrome
chain
and
Alice
would
be
like
hi
I'm
willing
to
trade
my
ticket
with
you.
This
is
a
ticket,
it's
worth
100!
If
it
belongs
to
me
now,
but
I
can
sell
it
to
you
and
I
know
that
you
don't
have
any
idea
about
this
assertion.
Number
10!
You
don't
know
if
it's
going
to
be
finalized
or
not.
That's
fine,
you
don't
have
to
do
anything,
I
provide
insurance,
so
what
I
do
is
I'm
gonna
give
hundred
no
shares
to
you
for
free.
A
So
in
any
case,
Alice
is
ensuring
Carol,
so
Carol
would
be
able
to
do
the
trade
with
her
there's
no
risk
for
her.
So
this
is
great.
This
is
the
best
solution
that
I
I
guess
we
have
come
up
with
and
for
the
implementation.
This
is
a
work
under
progress.
So
what
we
did
so
far
is
we
modified
arbits
from
Nitro
to
support
our
Solutions
and
we
provide
measurements.
We
fully
implemented
solution.
Three.
So
far,
it's
a
proof
of
concept.
So
what
I
did
was
I
implemented
an
L1
Market.
A
It's
really
like
a
vending
machine,
as
I
said,
so,
there's
no
bookkeeping
or
anything
and
we
modified
the
outbox
so
that
it
would
support
our
Market
and
for
the
solution
for
the
prediction.
Market
one
it's
still
under
progress,
but
we
modified
the
roll
up
and
outbox
more
contracts.
So
there
could
be
prediction:
Market,
friendly,
okay,
so
I'm
gonna
walk
you
through
the
the
protocol
of
solution,
three,
how
it
works
and
how
what
what
are
the
functions
that
we
added
to
the
product,
the
Arbitron
protocols?
A
So
let's
say
Alice
wants
to
do
a
withdraw,
so
she
puts
her
a
withdrawal
transaction
inside
the
Inbox
and
then
what
she
does.
This
is
the
function
that
we
added
to
the
outbox.
She
wants
to
participate
inside
the
market,
so
Market
needs
to
know
that
she
actually
locked
up
her
her
ether
inside
the
market.
So
what
Alice
needs
to
do
first
is
she
has
to
go
to
the
outbox
and
cause
the
transfer
spender
function
with?
What?
Basically
it
does?
Is
it
it
transfer
her
exit,
her
withdrawal
to
the
market
smart
contract?
A
So
it's
like
it's
collateralizing
her
exit
and
then
once
that's
done,
she
can
go
to
the
market
and
she
can
actually
open
a
market
on
her
exit
and
Market
would
check
if
Alice
has
really
deposited
her
exit
inside
the
market
or
not,
and
if
yes,
that's
fine,
she
can
open
a
market.
Then
the
market
is
open
for
some
time.
Until
there's
a
trade
Bob
comes
along
he's
a
Trader.
He
is
willing
to
trade
it
with
Alice.
A
What
he
does
is
he
submits
a
bid
that
actually
is
greater
than
what
is
Alice
as
Alice
is
asking
for
and
that
that's
when
the
trade
occurs,
so
the
trade
happens,
everything
gets
settled
down,
but
Market
does
it
first.
Gonna
first
is
gonna,
send
the
East
back
to
Alice,
so
Alice
is
happy,
she
got
her
ether
and
then
what
it
does
is
because
it's
owning
the
exit.
A
Now,
what
it
can
do
is
it
it
can
actually
go
inside
the
outbox
and
change
the
owner
of
this
exit,
so
it
reached
inside
the
outbox
and
it
changes
the
owner
to
the
ball.
So
now
Bob
owns
the
exit.
We
wait
for
seven
days
and
then
what
happens
is
he
can
now
go
to
the
outbox
smart
contract
execute
his
withdrawal
transaction
and
then
because
he's
the
owner,
the
bridge
would
send
100
East
back
to
him.
A
So
easy
peasy
everyone's
happy,
and
then
we
run
some
measurements
too
to
find
out
how
much
at
one
gas
it
would
cost
us
to
run
this
function.
So
it
turns
out
it's
really
cheap.
So
for
transferring
the
spender
inside
the
output,
smart
contract,
it's
around
80
six
K,
and
it's
really
like
the
most
expensive
one,
because
it's
the
like
the
it's
the
first
transferring
of
spender.
So
it's
actually
you're
paying
for
creation
of
a
mapping.
So
the
because
there's
a
mapping
that
keeps
track
of
all
the
owners
of
an
exit.
A
So
this
is
really
expensive.
The
first
one
is
really
expensive,
but
the
LA,
the
second
one
and
the
third
one
I
guess
they
were
around
like
40K
away,
which
was
really
great
and
for
opening
the
market
we
had
to
pay
around
300K
and
for
for
submitting
the
bid
I
had
to
pay
around
100K
and
then
for
executing
the
transaction
it
cost
90k,
and
that
sums
up
the
talk,
I
I,
hope
you
enjoyed
it.