How does Monero prevent double spending?

Monero prevents double spending through a combination of cryptographic techniques, consensus mechanisms, and network validation rules. Here's how Monero ensures that each transaction is valid and cannot be spent more than once:

1. **Unspent Transaction Outputs (UTXOs)**: Like Bitcoin, Monero utilizes a UTXO model to track the ownership and spending of coins. Each transaction creates new UTXOs, which represent spendable coins, and consumes existing UTXOs as inputs. To prevent double spending, Monero ensures that each UTXO can only be spent once.

2. **Ring Signatures**: Monero uses ring signatures to obfuscate the true input used in a transaction. When a transaction is created, it includes a ring signature that combines the spender's input with several other inputs from the blockchain, making it computationally infeasible to determine the actual spender.

3. **Stealth Addresses**: Monero employs stealth addresses to enhance recipient privacy. Each transaction includes a unique one-time stealth address derived from the recipient's public address and a random value. This ensures that the recipient's address cannot be linked to previous transactions, preventing the recipient from spending the same funds multiple times.

4. **Blockchain Consensus**: Monero's consensus mechanism, based on proof-of-work (PoW), ensures that all nodes in the network agree on the validity of transactions and the state of the blockchain. Miners validate transactions, group them into blocks, and compete to add new blocks to the blockchain. Consensus rules dictate that only valid transactions can be included in the blockchain, preventing double spending attempts.

5. **Blockchain Immaturity**: Monero introduces a maturity requirement for newly mined coins before they can be spent. Newly mined coins must mature for a certain number of blocks (typically 60 blocks) before they can be used in transactions. This prevents miners from immediately spending newly minted coins and provides time for the network to validate transactions and detect any attempts at double spending.

6. **Network Propagation and Confirmation**: When a transaction is broadcast to the Monero network, it propagates to other nodes, which validate and relay it to their peers. As more nodes confirm the transaction's validity and include it in their copy of the blockchain, the likelihood of double spending decreases. Once a transaction has been confirmed by a sufficient number of blocks (typically six blocks), it is considered irreversible, and the spent coins cannot be double spent.

Overall, Monero's combination of cryptographic privacy features, consensus mechanisms, and network validation rules ensures that transactions are secure, valid, and resistant to double spending attempts.