.NET Core and Azure Active Directory integration

I am using the token authentication (instead of cookie) with Azure Active Directory.

Based on this article: https://www.itunity.com/article/angular-2-openid-connect-azure-active-directory-3093

I was able to get it working on the client side.

   public validateSignature(token): Observable<boolean> {
        /* Retrieve from federated metadata endpoint.
        In this sample, the document was downloaded locally */
        return this.httpService.get("metadata/metadata.xml")
            .map((res: Response) => {
                let dom = (new DOMParser()).parseFromString(res.text(), "text/xml");
                let json = xml2json(dom, "");
                let cert = "-----BEGIN CERTIFICATE-----" + 
                JSON.parse(json).EntityDescriptor[0]["ds:Signature"]
                    ["KeyInfo"]["X509Data"]["X509Certificate"] + 
                 "-----END CERTIFICATE-----";
                 let key = KEYUTIL.getKey(cert);
                return KJUR.jws.JWS.verifyJWT(token, key, { alg: ['RS256'] });
            })
        } 

I was trying to re-implement the above method in the .NET Core 1.0.3.

Based on this article: how to sign and verify signature with net and a certificate

The following line won't compile on .NET Core:

RSACryptoServiceProvider csp = (RSACryptoServiceProvider)cert.PublicKey.Key;

I am not sure what is correct way to verify the token based on the certificate in .NET Core.