Use Case: Atomic Cross-Chain Transfer
-------------------------------------
An AT that enables an atomic cross-chain transfer to be executed by creating two separate instances of itself
on two seperate blockchains. When constructing the first instance a 32 byte secret needs to be "hashed" using
SHA256 and stored in the first 32 bytes of "initial data" followed by another 32 bytes for the address of the
other party, and then the number of minutes to wait until refunding the AT's balance back to its creator.
The initial creator would then wait for the other party to create a matching AT (with a significantly smaller
number of minutes for refund). The hash values would need to be identical. Once the initiator has seen enough
confirmations of the other AT then they will send the secret to the other AT as a message. This AT will first
copy this message into its data storage and then check if when hashed it matches the initial data values that
were provided when it was created. If they match then the funds are sent to the address stored in the initial
data.
Finally the creator of the second AT can now work out from checking their AT's state what the secret is. They
would then send a message to the originally created AT instance with the same secret and it would pay them so
the atomic cross-chain transfer will be completed.
Note that if the second party never actually creates their AT then the first party just needs to wait for the
refund to occur. Likewise once the second party has created their AT then if the first party fails to send it
the secret on time then a refund will occur (and the first party will also be refunded). The first party will
need to be careful not to send the secret too close to the deadline otherwise there is the possibility that a
refund will occur in the second AT but the secret will have already been seen in the message tx.
Another concern is possible attacks by "clogging the AT" up with tiny false message txs to try and prevent it
from paying out. This is perhaps best prevented by making sure that the minimum fee for the ATs is well above
anything that would permit such an attack to be economically beneficial.
The algorithm described at https://en.bitcoin.it/wiki/Atomic_cross-chain_trading (which the Bitcointalk forum
member TierNolan first designed) is what this AT performs (although it doesn't need multisig addresses as the
ATs take over the role of being escrows).
Variables
---------
@00 ==> @hash_part1
@01 ==> @hash_part2
@02 ==> @hash_part3
@03 ==> @hash_part4
@04 ==> @address_part1
@05 ==> @address_part2
@06 ==> @address_part3
@07 ==> @address_part4
@08 ==> @refund_minutes
@09 ==> @refund_timestamp
@0a ==> @last_timestamp
@0b ==> @secret_part1
@0c ==> @secret_part2
@0d ==> @secret_part3
@0e ==> @secret_part4
@0f ==> @tx_type
@10 ==> @comparator
@11 ==> @secret_temp1
@12 ==> @secret_temp2
@13 ==> @secret_temp3
@14 ==> @secret_temp4
Script Assembly
---------------
if @refund_timestamp not zero goto loop 1e0a00000025
set @refund_timestamp to AT creation time 35010309000000
set @last_timestamp to @refund_timestamp 020a00000009000000
add @refund_minutes to @refund_timestamp 370604090000000900000008000000
:loop (0x25)
if @last_timestamp < @refund_timestamp goto :txloop 210a000000090000000f
goto refund 1af7000000
:txloop (0x34)
store tx after @last_timestamp in A 3304030a000000
check if A is zero and store result to @comparator 35250110000000
if @comparator is zero finish 2610000000
get type for tx in A and store in @tx_type 3505030f000000
get timestamp for tx in A and store in @last_timestamp 3507030a000000
if @tx_type is not 0 goto :check_message 1e0f0000000b
goto txloop 1a34000000
:check_message (0x60)
store message of tx in A to B 320903
swap A and B 322801
sha256 of A and store in B 320402
set @secret_temp1 to A1 35000111000000
set @secret_temp2 to A2 35010112000000
set @secret_temp3 to A3 35020113000000
set @secret_temp4 to A4 35030114000000
set A1 to @hash_part1 33100100000000
set A2 to @hash_part2 33110101000000
set A3 to @hash_part3 33120102000000
set A4 to @hash_part4 33130103000000
check A equals to B and store in @comparator 35270110000000
if comparator is not 0 goto :payout 1e100000000b
goto :txloop 1a34000000
:payout (0xb3)
set @secret_part1 to $secret_temp1 020b00000011000000
set @secret_part2 to $secret_temp2 020c00000012000000
set @secret_part3 to $secret_temp3 020d00000013000000
set @secret_part4 to $secret_temp4 020e00000014000000
set B1 to @address_part1 33160104000000
set B2 to @address_part2 33170105000000
set B3 to @address_part3 33180106000000
set B4 to @address_part4 33190107000000
send all to address in B 320304
finish 28
:refund (0xf7)
set B to address of the AT creator 320b03
send remaining balance to addres in B 320304
finish 28
Assembly Code
-------------
00000000* BNZ $0000000a :00000025
00000006 FUN @00000009 0x0301
0000000d SET @0000000a $00000009
00000016 FUN @00000009 0x0406 $00000009 $00000008
00000025 BGE $0000000a $00000009 :00000034
0000002f JMP :000000f7
00000034 FUN 0x0304 $0000000a
0000003b FUN @00000010 0x0125
00000042 FIZ @00000010
00000047 FUN @0000000f 0x0305
0000004e FUN @0000000a 0x0307
00000055 BNZ $0000000f :00000060
0000005b JMP :00000034
00000060 FUN 0x0309
00000063 FUN 0x0128
00000066 FUN 0x0204
00000069 FUN @00000011 0x0100
00000070 FUN @00000012 0x0101
00000077 FUN @00000013 0x0102
0000007e FUN @00000014 0x0103
00000085 FUN 0x0110 $00000000
0000008c FUN 0x0111 $00000001
00000093 FUN 0x0112 $00000002
0000009a FUN 0x0113 $00000003
000000a1 FUN @00000010 0x0127
000000a8 BNZ $00000010 :000000b3
000000ae JMP :00000034
000000b3 SET @0000000b $00000011
000000bc SET @0000000c $00000012
000000c5 SET @0000000d $00000013
000000ce SET @0000000e $00000014
000000d7 FUN 0x0116 $00000004
000000de FUN 0x0117 $00000005
000000e5 FUN 0x0118 $00000006
000000ec FUN 0x0119 $00000007
000000f3 FUN 0x0403
000000f6 FIN
000000f7 FUN 0x030b
000000fa FUN 0x0403
000000fd FIN
Machine Code
------------
1e0a0000002535010309000000020a00000009000000370604090000000900000008000000210a000000090000000f1af700000033040
30a0000003525011000000026100000003505030f0000003507030a0000001e0f0000000b1a3400000032090332280132040235000111
0000003501011200000035020113000000350301140000003310010000000033110101000000331201020000003313010300000035270
1100000001e100000000b1a34000000020b00000011000000020c00000012000000020d00000013000000020e00000014000000331601
0400000033170105000000331801060000003319010700000032030428320b0332030428
