At Amboss.space , we use a digital signature for logging in to the platform. When you go to sign a message, you include the message, the signature, and the public key. With this information, you can mathematically verify that the signature matches the message and the public key. If the signature was anything else, it would fail verification.

The public key is derived from a private key and a private key is necessary to make a valid signature. There are lots of signed messages out there, but the digital signature is a function of the message it is signing. So, having a valid digital signature to fresh message content proves that whoever signed is the owner of the private key.