  Lightning Network can execute unlimited number of off-chain payments, without
incurring the cost of recording each of them in the blockchain. However,
conditional payments in such networks is susceptible to Griefing Attack. In
this attack, an adversary doesn't resolve the payment with the intention of
blocking channel capacity of the network. We propose an efficient
countermeasure for the attack, known as Griefing-Penalty. If any party in the
network mounts a griefing attack, it needs to pay a penalty proportional to the
collateral cost of executing a payment. The penalty is used for compensating
affected parties in the network. We propose a new payment protocol HTLC-GP or
Hashed Timelock Contract with Griefing-Penalty to demonstrate the utility of
the countermeasure. Upon comparing our protocol with existing payment protocol
Hashed Timelock Contract, we observe that the average revenue earned by the
attacker decreases substantially for HTLC-GP as compared to HTLC. We also study
the impact of path length for routing a transaction and rate of
griefing-penalty on the budget invested by an adversary for mounting the
attack. The budget needed for mounting griefing attack in HTLC-GP is 12 times
more than the budget needed by attacker in HTLC, given that each payment
instance being routed via path length of hop count 20.
