Payment channels allow for off-chain transactions between two parties to enable trustless, high-volume, low-cost micropayments on the Bitcoin blockchain. The key aspects are: (1) a multi-signature wallet is set up and funded, (2) off-chain balance updates occur via time-locked transactions, (3) the final on-chain transaction closes the channel by broadcasting the final signed state. Hashed Timelock Contracts (HTLCs) extend this to enable payments across multiple channels by using hash preimages and timelocks to guarantee delivery or refunds. Bidirectional channels allow payments in both directions through commitment transactions signed by both parties.

1. Payment Channel
Introduction
2017/10/03 blockchain.tokyo #1
@mosa_siru

2. @mosa_siru ( )
•
• TV
• Gunosy
PO

3. @mosa_siru as engineer
•
• (DeNA)
• (Gunosy)
• (Gunosy)

4. 1. Payment channel
2. HTLCs

5. bitcoin micro payment
•
• (API )
• bitcoin 1 1
• 2017/10 150
• ( ) 10
• 3.47

6. • platform
user
• platform user

7. • platform BTC
user 10
platform BTC
• user platform
•
• platform

8. Payment channel
• trustless ( )
• off-chain transaction(
broadcast )
• off-chain

10. Multisig
•
( ) of ( )
• 2 of 3 ( 3 2
• 1 of 2 ( )
• 2 of 2 ( )
P2SH Redeem script: OP_2 [A's pubkey] [B's pubkey] OP_2 OP_CHECKMULTISIG

11. CLTV(CheckLockTimeVerify)
• BIP65 (Final) (OP_CHECKLOCKTIMEVERIFY)
• unspentable
( BTC )
※ nLockTime
( UTXO )

12. CSV(CheckSequenceVerify)
• BIP112 (Final) (OP_CHECKSEQUENCEVERIFY)
• (
) unspentable
BTC (3 )
•
• CLTV CSV TimeLock

13. Payment Channel

14. Payment Channel
• trustless(A, B )
• trustless 2 of 2 multisig(A, B
)
• fund
• off-chain
(100 )

15. Payment Channel (1)
A → B micro payment
1. A 2 of 2(multisig) fund 1BTC deposite
broadcast
2. 1 txid 1 1BTC A
refund transaction
B CSV CLTV
3. 1 broadcast (channel open

16. Payment Channel (2)
4. deposite address A, B
A B
(off-chain)
deposite address => B(0.01BTC), A(0.99BTC)
deposite address => B(0.02BTC), A(0.98BTC)
…
5. B broadcast
(channel close)

17. Bob => Alice
https://bitcoin.org/en/developer-guide#micropayment-channel

18. Trustless (1)
• B
• refund transaction( ) broadcast 1 (CSV
or CLTV) A
• refund transaction B B
fund deposite
• B 1 A

19. Trustless (2)
• A
• B broadcast

20. Time Lock
• refund CLTV( )
channel close (B )
CSV( )
• CLTV nLockTime(
)
• deposite txid
refund transaction
segwit

21. •
• channel
• deposite
•

22. HTLC

23. HTLC
(Hashed TimeLock Contracts)
• CSV, CTLV TimeLock HashLock
• A => B => C
• channel
• B A
• Lightning Network

24. HTLC (1)
A => C micro payment
1. A => B , B => C payment channel
2. C A r
hash(r)
generate rOpen
channel
Open
channel

25. HTLC (2)
3. A => B 1BTC tx1 (off-chain)
IF
1. B r
2. 10 A (CLTV) ※refund .
4. Tx1 B => C 1BTC tx2 (off-chain)
1. C r
2. 5 B (CTLV) ← tx1
5. C r tx2 broadcast 1BTC
r B tx1 broadcast 1BTC

26. HTLC (1)
• trustless A => B => C
•
• tx2 tx1 refund B
CSV
CLTV tx2

27. HTLC (2)
• Tx1 (A => B) tx2
segwit
id
• r
pre-image

28. Payment Channel
(Bidirectional)

29. Payment Channel
(Bidirectional)
• payment channel
• channel 2
fund
• Time Lock, Hash Lock

30. Payment Channel
(Bidirectional)
1. A, B 5BTC 2 of 2 fund
opening transaction (off-chain)
2. A secret1, B secret2( )
hash

31. Payment Channel
(Bidirectional)
3. A commitment transaction tx2 B (off-chain)
4. B commitment transaction tx1 A (off-chain)
100block secret
opening transaction broadcast.

32. Payment Channel
(Bidirectional)
A => B 1BTC
5. tx3, tx4 secret
6. Secret 1, 2 hash tx1, tx2

33. transaction
• A => B 1BTC A tx1, tx2
secret
• A tx1 broadcast A 5BTC
100block B 5BTC
secret1 10BTC

34. Payment Channel
(Bidirectional)
7. transaction broadcast

36. •
layer2
• CTLV, CSV
•
trustless
• …

37. @mosa_siru