Apps similar to GnuTLS include OpenSSL, which is free and open source. Other options are LibreSSL, mbed TLS, Themis and SecureBlackbox®. In total there are eight Cryptography Libraries like GnuTLS for Windows, Linux, Mac, Android and iPhone.
OpenSSL is a framework which implements the secure socket layer. It ships with a program/library which gives you access to all those functions.
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LibreSSL is a version of the TLS/crypto stack forked from OpenSSL in 2014, with goals of modernizing the codebase, improving security, and applying best practice development processes.
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.
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.
Software components for Data Protection, Secure Storage, and Secure Transfer. Trusted in high security, mission-critical applications for decades.
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.
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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.
wolfSSL (formerly CyaSSL) is a small, fast, portable implementation of TLS/SSL for embedded devices to the cloud. http://www.wolfssl.com.
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