In the context of cloud security, the responsibility is shared between the cloud provider and the cloud consumer. This is known as the shared responsibility model. The cloud provider is responsible for securing the infrastructure that runs all of the services offered in the cloud. On the other hand, the cloud consumer is responsible for managing the security of their data, applications, and operating systems that they run on the cloud infrastructure. The specific responsibilities can vary depending on the service model being used (IaaS, PaaS, SaaS), but the underlying principle is that both parties have a role to play in ensuring the security of cloud services.

In the context of email encryption and digital signatures, authenticity is typically ensured through the use of a sender's digital signature. Dan would use his private key to create a digital signature on his emails. This signature is unique to both the sender and the email content. Alex, on the other hand, would use Dan's public key to verify the digital signature. If the verification process confirms that the signature was created with Dan's private key and that the email has not been altered, Alex can be assured of the email's authenticity. This process does not involve encrypting the entire email with a private key, as that would make it unreadable to anyone except the holder of the corresponding private key, which is not shared. Instead, encryption of the email content is typically done using symmetric encryption, where both Dan and Alex would use a shared secret key.