= Authentication
{disambiguate=cryptography}
In the context of cryptography, authentication means "ensuring that the message you got comes from who you think it did".
Authentication is how we prevent the <man-in-the-middle attack>.
Authentication is one of the hardest parts of cryptography, because the only truly secure way to do it is by driving to the other party yourself to establish a <pre-shared key> so you can do <message authentication code>. Or to share your <public key> with them if you are satisfied with the safety of <post-quantum cryptography>.