user77055: Why do I need a private key to send someone my bitcoins? 

user76726: A private key is the only way to assure everybody else you and only signed a specific transaction saying 'give X of my coins to Y'.
If you publish your private key, anybody can sign transactions like that and your money can be stolen. You should keep your private key private.

user5406: Standard Bitcoin addresses encode a user's public key. When someone sends bitcoins to such a "pay-to-public-key-hash" address, they sign them over to the owner of the specific public key. The public key hereby is part of a unique key pair. The public  key (as in public knowledge) can be shared freely, while the private key should be kept secret.
When the new owner of the funds wants to spend them, they have to prove their ownership. To that end, they provide a signature that corresponds to the public key that the funds were assigned to. This signature can only be produced with the knowledge of the corresponding private key, but can then be checked by everyone that knows the public key.
You therefore need the private key to prove your ownership in order to spend funds.

77055: <p>Why do I need a private key to send someone my bitcoins? </p>
 76726: <p>A private key is the only way to assure everybody else you and only signed a specific transaction saying 'give X of my coins to Y'.</p>

<p>If you publish your private key, anybody can sign transactions like that and your money can be stolen. You should keep your private key private.</p>
 5406: <p>Standard Bitcoin addresses encode a user's public key. When someone sends bitcoins to such a "pay-to-public-key-hash" address, they sign them over to the owner of the specific public key. The public key hereby is part of a unique key pair. The public  key (as in public knowledge) can be shared freely, while the private key should be kept secret.</p>

<p>When the new owner of the funds wants to spend them, they have to prove their ownership. To that end, they provide a signature that corresponds to the public key that the funds were assigned to. This signature can only be produced with the knowledge of the corresponding private key, but can then be checked by everyone that knows the public key.</p>

<p>You therefore need the private key to prove your ownership in order to spend funds.</p>
