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Stunnel 5.78, authored by Michal Trojnara, is a lightweight encryption proxy whose single purpose is to wrap unencrypted TCP traffic—be it POP3, SMTP, HTTP, or any other legacy protocol—in modern TLS without touching the original client or server source code. Acting as a transparent intermediary, the utility listens on a local port, negotiates SSL/TLS with any connecting peer, and then forwards the now-secured stream to the intended backend service. This architecture makes it an essential tool for administrators who must retrofit confidentiality and certificate-based authentication onto embedded systems, medical devices, or enterprise applications whose vendors no longer provide updates. Typical deployments include tunneling insecure database connections across public clouds, shielding intranet mail relays without reconfiguring the MTA, or adding HTTPS to internal web dashboards that cannot be modified. The program ships in three concurrent release branches—stable, legacy, and snapshot—allowing users to balance cutting-edge cipher support against long-term maintenance requirements. Configuration is driven by a straightforward text file in which each service is mapped to its upstream host, port, certificate, and allowed TLS versions, while optional OCSP stapling, SNI routing, and FIPS mode satisfy stricter compliance regimes. Because the executable runs as a portable Windows service, it integrates cleanly with existing monitoring and log-management stacks, consuming only a few megabytes of RAM and negligible CPU overhead even under thousands of concurrent sessions. Stunnel falls squarely into the Network/Security category and remains one of the few utilities capable of universal, code-free transport encryption. The software is available for free on get.nero.com, with downloads provided via trusted Windows package sources (e.g. winget), always delivering the latest version, and supporting batch installation of multiple applications.
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