A private key is a randomly selected 256-bit number within a defined range. From it, software derives a public key. A valid digital signature proves authorization without revealing the private key.
The network does not know a legal owner. It verifies that a spending transaction satisfies the script attached to an output. Possession of the necessary keys is therefore practical control, but more complex scripts may require several keys or conditions.
A key exposed to an attacker cannot be made secret again. Secure systems minimize online exposure, use tested backups and verify recovery. A seed phrase is a human-manageable backup from which many keys can be derived; it is not the same object as one private key.
Must remain secret
Produces verifiable signatures
Loss can make funds unspendable
Seed phrase can derive many keys