Digital signatures

A digital signature shows two things: that something was signed by whoever holds a particular private key, and that it has not been altered since.

What it does not do is hide anything. A signed document is still perfectly readable by everybody; signing and encrypting are separate operations that people routinely confuse. It is also worth being precise about what is proved. The signature shows the key was used. Whether the right person was holding that key at the time is a different question, and it is the one that fails in practice.