What Is a ‘Spent’ Transaction and How Does It Affect the Security of the Public Key?

A Bitcoin transaction is 'spent' when the funds associated with a specific public key (or address) are moved to a new address. When a transaction is spent, the full public key is revealed on the blockchain as part of the signature verification process.

Before this, only the public address (the hash of the public key) is visible. The security model relies on users generating a new key pair for each transaction to limit the time the public key is exposed for potential preimage attacks.