// Certificate Constants
private const string SIGNING_CERTIFICATE_NAME = "CN=localhost";
private const string ENCRYPTING_CERTIFICATE_NAME = "CN=localhost";

private SigningCredentials _signingCreds;
private EncryptingCredentials _encryptingCreds;
// Used for validating applies to address, set to URI used in RP app of application, could also have been done via config
private string _addressExpected = "http://localhost:19851/";

/// <summary>
/// This method returns the configuration for the token issuance request. The configuration
/// is represented by the Scope class. In our case, we are only capable of issuing a token to a
/// single RP identity represented by the _encryptingCreds field.
/// </summary>
/// <param name="principal">The caller's principal</param>
/// <param name="request">The incoming RST</param>
/// <returns></returns>
protected override Scope GetScope(ClaimsPrincipal principal, RequestSecurityToken request)
{
    // Validate the AppliesTo address
    ValidateAppliesTo( request.AppliesTo );

    // Create the scope using the request AppliesTo address and the RP identity
    Scope scope = new Scope( request.AppliesTo.Uri.AbsoluteUri, _signingCreds );

    if (Uri.IsWellFormedUriString(request.ReplyTo, UriKind.Absolute))
    {
        if (request.AppliesTo.Uri.Host != new Uri(request.ReplyTo).Host)
            scope.ReplyToAddress = request.AppliesTo.Uri.AbsoluteUri;
        else
            scope.ReplyToAddress = request.ReplyTo;
    }
    else
    {
        Uri resultUri = null;
        if (Uri.TryCreate(request.AppliesTo.Uri, request.ReplyTo, out resultUri))
            scope.ReplyToAddress = resultUri.AbsoluteUri;
        else
            scope.ReplyToAddress = request.AppliesTo.Uri.ToString() ;
    }

    // Note: In this sample app only a single RP identity is shown, which is localhost, and the certificate of that RP is 
    // populated as _encryptingCreds
    // If you have multiple RPs for the STS you would select the certificate that is specific to 
    // the RP that requests the token and then use that for _encryptingCreds
    scope.EncryptingCredentials = _encryptingCreds;

    return scope;
}

/// <summary>
/// Validates the appliesTo and throws an exception if the appliesTo is null or appliesTo contains some unexpected address.
/// </summary>
/// <param name="appliesTo">The AppliesTo parameter in the request that came in (RST)</param>
/// <returns></returns>
void ValidateAppliesTo(EndpointReference appliesTo)
{
    if (appliesTo == null)
    {
        throw new InvalidRequestException("The appliesTo is null.");
    }

    if (!appliesTo.Uri.Equals(new Uri(_addressExpected)))
    {
        throw new InvalidRequestException(String.Format("The relying party address is not valid. Expected value is {0}, the actual value is {1}.", _addressExpected, appliesTo.Uri.AbsoluteUri));
    }
}