Signature methods are always closely tied to public key cryptography. RSA is the best-known method for digital signatures. A signature is the code containing the private key for the message. The sender “signs” their message with RSA and, in this way, adds a layer of security. They can then send the message. The authenticity of the message and identity of the sender are verified by the recipient using their public key.
RSA is regarded as an old but proven signature method. A couple of alternatives that generate and recognize digital signatures using a similar method are DSA (Digital Signature Algorithm) and ElGamal.
A concrete example of cryptographic protocols is encryption with SSL/TLS. This network protocol ensures secure communication, such as between the web server and browser. Simultaneously, it verifies the server’s authenticity. To do so, SSL/TLS uses hybridencryption (i.e. both asymmetric and symmetric methods). The public key is signed by a certificate authority, and the resulting certificate is encrypted. The certificate can then only be opened by the certificate authority’s public key. To do so, the web server sends its certified public key to the browser, for example, which checks the certificate. If the certificate is valid, the browser generates a symmetric key and sends it to the web server. Both will then use the same shared key for the rest of the SSL/TLS session for the symmetric encryption of their data traffic.