This is a maintenance release that enhances support for RSA with OAEP and PSS:
Improved RSA/OAEP and RSA/PSS support
We added support for RSA/OAEP with label (input parameter). RSAES-OAEP and RSASSA-PSS structures with mismatched hash algorithms are supported as well. This makes Rebex Secure Mail and Rebex Security fully compatible with third-party CMS and S/MIME libraries and and applications such as OpenSSL or OpenGPG's
For a complete list of changes in 2017 R6.3, see the release history.