Question

The DotNetOpenAuth OAuth 2 Library requires RSAParameters to access public and private keys (example in DotNetOpenAuth OAuth 2 – UriStyleMessageFormatter which uses RSAParameters to construct an RSACryptoServiceProvider).

I came across an Azure Security Whitepaper which noted that Azure installs certificates in a “certificate store with a flag indicating that the private key can be used but not exported”. Which I believe may be at the heart of this problem.

While I have been able extract the public and private keys from the cert while developing and debugging locally by referencing the certificate by it’s thumbprint (example below) I have had no luck getting the same code running in Azure.

The following code gives the error: “Key not valid for use in specified state” in Azure

    public class Global : System.Web.HttpApplication, IContainerAccessor
    {
        private static string thumbPrint = "<<my certificate thumbprint>>";
        public static readonly RSAParameters AuthorizationServerSigningPublicKey = OAuthUtil.GetPublicKey(thumbPrint);
        internal static readonly RSAParameters ResourceServerEncryptionPrivateKey = OAuthUtil.GetPrivateKey(thumbPrint); 

//....... unnecessary code omitted ..... //

    public static class OAuthUtil
        {
            public static RSAParameters GetPublicKey(string thumbPrint)
            {
                X509Store store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
                store.Open(OpenFlags.OpenExistingOnly | OpenFlags.ReadOnly);
                var cert = store.Certificates.Find(X509FindType.FindByThumbprint, thumbPrint, true)[0];
                var rsaParams = ((RSACryptoServiceProvider) cert.PublicKey.Key).ExportParameters(false);
                return rsaParams;            
            }

            public static RSAParameters GetPrivateKey(string thumbPrint)
            {
                X509Store store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
                store.Open(OpenFlags.OpenExistingOnly | OpenFlags.ReadOnly);
                var cert = store.Certificates.Find(X509FindType.FindByThumbprint, thumbPrint, true)[0];
                var rsaParams = ((RSACryptoServiceProvider) cert.PrivateKey).ExportParameters(true);
                return rsaParams;
            }
        }

Encryption / decryption code in Azure based off of the same certificate (example below) which does not require exporting the key works fine:

    public class Certificate
    {
        public string FriendlyName { get; set; }
        public string IssuedBy { get; set; }
        public string IssuedTo { get; set; }
        public string ExpirationDate { get; set; }
        public string PublicKey { get; set; }
        public string PrivateKey { get; set; }
    }

    public ActionResult Keys()
    {
        X509Certificate2Collection selectedCerts = new X509Certificate2Collection();
        var certList = new List<Certificate>();
        X509Store store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
        try
        {
            store.Open(OpenFlags.OpenExistingOnly | OpenFlags.ReadWrite);
            foreach (X509Certificate2 cert in store.Certificates)
            {
                // Encrypt string "hello world"
                CspParameters CSPParam = new CspParameters();
                CSPParam.Flags = CspProviderFlags.UseMachineKeyStore;
                string PlainString = "hello world";
                byte[] cipherbytes = ASCIIEncoding.ASCII.GetBytes(PlainString);
                RSACryptoServiceProvider rsa = (RSACryptoServiceProvider)cert.PublicKey.Key;
                byte[] cipher = rsa.Encrypt(cipherbytes, false);
                var encryptedString = Convert.ToBase64String(cipher);

                var cert2 = cert;
                string decryptedString = "verify = " + cert2.Verify() ;
                if (cert2.HasPrivateKey && cert2.Verify())
                {
                    // Decrypt encrypted string..
                    RSACryptoServiceProvider rsaDecrypt = (RSACryptoServiceProvider)cert2.PrivateKey;
                    byte[] cipherbytes2 = Convert.FromBase64String(encryptedString);
                    byte[] plainbytes = rsaDecrypt.Decrypt(cipherbytes2, false);
                    System.Text.ASCIIEncoding enc = new System.Text.ASCIIEncoding();
                    decryptedString = enc.GetString(plainbytes);  
                }
                var certItem = new Certificate
                                   {
                                       FriendlyName = cert.FriendlyName,
                                       IssuedBy = cert.Issuer,
                                       IssuedTo = cert.SubjectName.Name,
                                       ExpirationDate = cert.NotAfter.ToString("d"),
                                       PublicKey =
                                           "Public Key: " + cert.GetPublicKeyString() + "<br/>Encrypted String: " + encryptedString + "<br/>Decrypted String: " +
                                           decryptedString,
                                       PrivateKey =
                                           "cert has private key?: " + cert.HasPrivateKey + "<br/> key algo:" +
                                           cert.GetKeyAlgorithm()
                                   };
                certList.Add(certItem);
            }
        }
        finally
        {
            store.Close();
        }
        return View(certList);
    }

Aside from rewriting the OAuth 2 library to use RSACryptoServiceProvider references instead of RSAParameters is there any way I could get this to work in Azure?

Is anyone else experiencing the same issue with DotNetOpenAuth OAuth 2 and Azure when reading certificates out of the store?

I would like to avoid hacks such as installing the certificate with export privileges using a start up task (due to security concerns).

Answer 1

Great feedback. I’ve filed a ticket for it.Please check