The OpenSSL Project is a collaborative effort to develop a robust, commercial-grade, fully featured, and Open Source toolkit implementing the Transport Layer Security (TLS) protocols (including SSLv3) as well as a full-strength general purpose cryptographic library.

OpenSSL is descended from the SSLeay library developed by Eric A. Young and Tim J. Hudson. The OpenSSL toolkit is licensed under a dual-license (the OpenSSL license plus the SSLeay license), which means that you are free to get and use it for commercial and non-commercial purposes as long as you fulfill the conditions of both licenses.

Source code
Available on GitHub:
OpenSSL, SSLeay
Copyright (c) 1998-2018 The OpenSSL Project
Copyright (c) 1995-1998 Eric A. Young, Tim J. Hudson
Download OpenSSL 1.1.2-dev Windows archives here below.
Windows binary packages for OpenSSL are available for Visual Studio 2013, 2015 and 2017 ABIs, 32bit and 64bit, for MinGW 5.3.0 32 bit and MinGW 7.3.0 64 bit.

MSVC2013 - Visual studio 12.0

MSVC2015 - Visual studio 13.0

MSVC2017 - Visual studio 14.0

MinGW 5.3.0 - 32 bits only

MinGW 7.3.0 - 64 bits only