“Invalid provider type specified” CryptographicException when trying to load private key of certificate

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I had the same problem on Windows 8 and Server 2012/2012 R2 with two new certificates I recently received. On Windows 10, the problem no longer occurs (but that does not help me, as the code manipulating the certificate is used on a server). While the solution of Joe Strommen in principle works, the different private key model would require massive change to the code using the certificates. I find that a better solution is to convert the private key from CNG to RSA, as explained by Remy Blok here.

Remy uses OpenSSL and two older tools to accomplish the private key conversion, we wanted to automate it and developed an OpenSSL-only solution. Given MYCERT.pfx with private key password MYPWD in CNG format, these are the steps to get a new CONVERTED.pfx with private key in RSA format and same password:

  1. Extract public keys, full certificate chain:
OpenSSL pkcs12 -in "MYCERT.pfx" -nokeys -out "MYCERT.cer" -passin "pass:MYPWD"
  1. Extract private key:
OpenSSL pkcs12 -in "MYCERT.pfx" -nocerts -out "MYCERT.pem" -passin "pass:MYPWD" -passout "pass:MYPWD"
  1. Convert private key to RSA format:
OpenSSL rsa -inform PEM -in "MYCERT.pem" -out "MYCERT.rsa" -passin "pass:MYPWD" -passout "pass:MYPWD"
  1. Merge public keys with RSA private key to new PFX:
OpenSSL pkcs12 -export -in "MYCERT.cer" -inkey "MYCERT.rsa" -out "CONVERTED.pfx" -passin "pass:MYPWD" -passout "pass:MYPWD"

If you load the converted pfx or import it in the Windows certificate store instead of the CNG format pfx, the problem goes away and the C# code does not need to change.

One additional gotcha that I encountered when automating this: we use long generated passwords for the private key and the password may contain ". For the OpenSSL command line, " characters inside the password must be escaped as "".

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