Stunnel is a proxy designed to add TLS encryption functionality to existing clients and servers without any changes in the programs code. Its architecture is optimized for security, portability, and scalability (including load-balancing), making it suitable for large deployments.
Stunnel uses the OpenSSL library for cryptography, so it supports whatever cryptographic algorithms are compiled into the library. It can benefit from the FIPS 140-2 validation of the OpenSSL FIPS Object Module, as long as the building process meets its Security Policy. A scanned FIPS 140-2 Validation Certificate document is available for download on the NIST web page. The Windows binary installer is compiled with FIPS 140-2 support. The FIPS mode of operation is no longer enabled by default since stunnel 5.00.