Which protocol would need to be available on a target in order for Nmap to identify services like IMAPS and POP3S?

Nmap's service and version detection (-sV) identifies services like IMAPS and POP3S by initiating a cryptographic handshake. These services are standard application protocols (IMAP, POP3) tunneled over a security layer. The modern and standard protocol for this security layer is Transport Layer Security (TLS). Nmap sends a TLS "ClientHello" probe to the target port. By analyzing the server's response, specifically the "ServerHello" message and the digital certificate presented, Nmap can accurately identify the underlying service (e.g., IMAP, POP3) running over the TLS-encrypted channel.

A. HTTPS: This is a specific application protocol (HTTP over TLS), not the foundational encryption protocol needed to identify other TLS-wrapped services.
B. SSL: Secure Sockets Layer (SSL) is the deprecated predecessor to TLS. While Nmap can interact with legacy SSL, modern systems exclusively use the more secure TLS protocol.
C. LDAP: The Lightweight Directory Access Protocol is used for accessing and maintaining directory services and is unrelated to the encryption of email protocols.

1. Lyon, G. (2009). Nmap Network Scanning: The Official Nmap Project Guide to Network Discovery and Security Scanning. Insecure.com LLC. Chapter 7, "Service and Application Version Detection," section "The nmap-service-probes File," describes the Probe TCP SSLv3 which sends a TLS/SSL ClientHello message to identify services. The text explicitly mentions this probe is used for "any SSL-wrapped TCP service such as https, smtps, pop3s, or imaps."

2. Rescorla, E. (2018). The Transport Layer Security (TLS) Protocol Version 1.3. IETF. RFC 8446. Section 1, "Introduction." This document defines the TLS protocol, which provides the security layer for application protocols. It is the standard protocol that must be available on the target for services like IMAPS to function. (DOI: https://doi.org/10.17487/RFC8446)

3. Melnikov, C., & Newman, C. (2018). Cleartext Considered Obsolete: Use of TLS for Email Submission and Access. IETF. RFC 8314. This standard explicitly recommends and describes the use of TLS for securing IMAP and POP3, creating the IMAPS and POP3S services. (DOI: https://doi.org/10.17487/RFC8314)

4. Rivest, R. (2017). Lecture 12: Network Security I. 6.857 Computer and Network Security, Fall 2017. MIT OpenCourseWare. The lecture notes detail the TLS handshake process (ClientHello, ServerHello, Certificate exchange), which is the fundamental mechanism Nmap's probes leverage to identify TLS-protected services.