GnuTLS is described as '(the GNU Transport Layer Security Library) is a free software implementation of the SSL, TLS and DTLS protocols'. There are eight alternatives to GnuTLS for a variety of platforms, including Windows, Linux, Mac, Android and iPhone. The best alternative is OpenSSL, which is both free and Open Source. Other great apps like GnuTLS are LibreSSL (Free, Open Source), Themis (Free, Open Source), mbed TLS (Free) and SecureBlackbox® (Paid).
Themis is a cryptolibrary, providing high-quality security services for secure messaging and encrypted storage. Themis is aimed at modern development practices, with wrappers for Ruby, Python, PHP, Java / Android and iOS / OSX.
mbed TLS (formerly known as PolarSSL) makes it trivially easy for developers to include cryptographic and SSL/TLS capabilities in their (embedded) products, facilitating this functionality with a minimal coding footprint.
SecureBlackbox is a library (suite of components and classes) for software developers that allows them to provide secure data transfer and storage, digital signing and verification, encryption, and compression (please refer to /n software website to get the complete list of...
Network Security Services (NSS) comprises a set of libraries designed to support cross-platform development of security-enabled client and server applications with an option support for hardware SSL acceleration on the server side and hardware smart cards on the client side.
The SSL/TLS and Certificate tutorial for Windows enables you to fast track your understanding of the SSL/TLS protocol, Certificate Management, and secure communication. The download includes many C source code examples, including a secure WebSocket tutorial.