Which cryptographic key is contained in an X.509 certificate?

An X.509 certificate contains a public key, which is fundamental to the operation of public key infrastructure (PKI). Public keys are used in asymmetric cryptography, where two keys—a public key and a private key—are used in tandem for encryption and decryption processes.

The significance of the public key in an X.509 certificate lies in its role in ensuring secure communications over networks. When someone wants to send a secure message to a recipient, they can use the recipient's public key (contained within the X.509 certificate) to encrypt the message. Only the recipient, who possesses the corresponding private key, can then decrypt it. This relationship underscores why the public key is critical for establishing trust and securing communications within various applications like SSL/TLS.

While asymmetric (the method involving both public and private keys) plays a role here, the certificate specifically includes the public key, thus making it the correct answer. The other types of keys mentioned—symmetric and private—are either not included in the certificate itself or are part of different cryptographic processes.