Is it possible to export the private key from the forward-untrust certificate to view in wireshark ?

[DaxVC]

I want to check a specific HTTP request that is send to a webserver and which is currently blocked by one of our vulnerability checks to verify if the signature is correct.

But I need to be able to view the decrypted data on the exported capture, therfor I have to import the private key of the forward-untrust certificate into wireshark....

But I have bo success in doing this 

[gwesson]

You can, but there's a large caveat that is more and more common:

You will not be able to decrypt the traffic if a high security cipher (such as DHE or ECC) is used. 

That said, if you want to do it and are using a simple RSA cipher, there are a few steps:

1. Install OpenSSL

2. Export the cert and private key. I recommend PKCS12 because there will be fewer steps to do the conversion. Provide a passphrase with 6 characters minimum. Save it as something (default is something like cert_Forward-Untrust.p12).

3. In OpenSSL, enter the following. It will prompt you for the password from step 2:

openssl pkcs12 -in cert_Forward-Untrust.p12 -out PrivAndPub.pem -nodes

4. Open PrivAndPub.pem in a plain text editor. You'll see a section heading of -----BEGIN PRIVATE KEY-----. Save this until the end as a new file (private.key for example).

You can load the private.key file into Wireshark. You won't need a password, because the OpenSSL command outputs it unencrypted.