Hello,
  I use your library StreamSec2.3 successfully, but for the import of keys and certificates, is it possible to work with another format than PFX (pem, der, ...)
  I can not really find an example in our documentation
  I want to use the certificates of https://letsencrypt.org/ and the PFX format is not usable easily ....
  Thank you in advance for your assistance.
Vincent

If you got an OpenSSL compatible PEM file that includes your server certificate, your private key and any intermediate CA certificates, you could relatively easily convert those to a PFX file. There are utilities and tutorials online.

Hello,
  Thank you for your answer, now it works.
  But it's not easy to manage and understand the many data formats and file extensions of the SSL system, which are often redundant, and the certificate providers each have their own way of providing the data ...
Vincent

Indeed, and ST 4.0 supports more formats, including the proprietary OpenSSL PEM formats for keys and certificates.