What should an engineer use to aid the trusted exchange of public keys between user...

 In the context of public key infrastructure (PKI), a trusted certificate authority (CA) is responsible for issuing digital certificates that verify a digital entity’s identity on the internet. The CA acts as a trusted third party between the user (in this case, tom0411976943) and the recipient (dan1968754032), ensuring that the public keys are indeed who they claim to be. The CA verifies the identity of the users and then issues a certificate containing the public key and a variety of other identification information. The trusted CA can then vouch for the authenticity of each user to the other.

Understanding Cisco Cybersecurity Operations Fundamentals (CBROPS)