Final answer:
Signing an email using a private key assures non-repudiation. Confidentiality, availability, and authentication are not directly assured.
Step-by-step explanation:
When a user signs an email using a private key, it assures non-repudiation which means the sender cannot deny sending the email. This is because the private key used for signing is unique to the sender and can be verified by the recipient using the corresponding public key.
Confidentiality, availability, and authentication are not directly assured by signing an email using a private key. Confidentiality refers to keeping the content of the email private, which can be achieved by encrypting the email content. Availability refers to ensuring the email service is accessible, and authentication refers to verifying the identity of the sender, both of which are not directly related to signing an email using a private key.