OpenSSL-net DllNotFoundException: Assets/Plugins/OpenSSL/libeay32.dll

I am trying to use OpenSSL.net in Unity. I've cloned the git repository, build an AnyCPU release built in visual studio which made a directory containing: libeay32.dll, ManagedOpenSsl.dll and ssleay32.dll. I've dragged them in my project in Assets\Plugins\OpenSSL and I can now access OpenSSL from script:
using OpenSSL.Crypto;
I'm using a small script to try and use OpenSSL functionality:

 byte[] Encrypt(byte[] bytes)
     {
         using (CipherContext cc = new CipherContext(Cipher.AES_256_CBC))
         {
             return cc.Encrypt(bytes, testKey, testIV);
         }
     }

When I run the method, I get an error in the unity console:

DllNotFoundException: Assets/Plugins/OpenSSL/libeay32.dll
OpenSSL.Core.Version.get_Library ()
OpenSSL.Core.Native..cctor ()
Rethrow as TypeInitializationException: An exception was thrown by the type initializer for OpenSSL.Core.Native
OpenSSL.Crypto.Cipher..cctor ()
Rethrow as TypeInitializationException: An exception was thrown by the type initializer for OpenSSL.Crypto.Cipher
TestSerialization.Update () (at Assets/Scripts/SerializedClasses/TestSerialization.cs:51)

On line 51, the encrypt method is called. I'm targeting Android and in a later stage IOS but it seems to not even work on Windows 10.

Can someone tell me how this happens?

Kind regards,

Yorick