Steve was sharing his confidential file with John via an email that was digitally signed...

S/MIME (Secure/Multipurpose Internet Mail Extensions) is a protocol that provides security services for email messages, such as encryption, digital signature, authentication, and integrity. S/MIME is based on the MIME standard, which defines the format and structure of email messages. S/MIME uses public-key cryptography to encrypt and decrypt the message content and to sign and verify the message sender. S/MIME supports various algorithms for encryption and digital signature, such as Diffie-Hellman, DSS, RSA, and triple DES. S/MIME is widely used for secure email communication in various applications and platforms, such as Outlook, Gmail, and Thunderbird. S/MIME is the protocol that employs the features mentioned in the question, namely Diffie-Hellman (X9.42) with DSS for digital signature and triple DES for encryption.References:

S/MIME- Week 7: Email Security

S/MIME - Wikipedia

S/MIME Version 3.2 Message Specification