recently I have been curious about 51% attacks so I created a fork of litecoin and want to do a 51% attack on the coin but don't know how to. please can you tell me how to do it.
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This question is somewhat like asking, "how can I steal money from myself?" Since you are the only person participating in the consensus of your altcoin, you control 100% of the hashpower. A 51% attack creates an alternative ledger, breaking consensus on which coins have been transferred, allowing the attacker to create a double spend.
First mine two competing chains, Chain A and Chain B.
In chain A, mine a block with a transaction that sends your coins to an address.
In chain B, mine a block with a similar but different transaction that uses the same transaction inputs as the first transaction but sends the coins to a different address.
There two transactions claim to transfer the same UTXO to two different addresses. There are two incompatible versions of the ledger, only one will be accepted. Eventually the one will with the most hashpower will have more work, and the other will be rejected (See chain reogranization).
A merchant or exchange should wait for sufficient block confirmations before accepting a payment, such that it is more profitable for a attacker to mine honestly, rather than investing the vast effort usually required to create an alternate version of the ledger in which the exchange/merchant has been defrauded. (See confirmations)