Had trouble using the Twitter oAuth API recently as they seem to have started to retire the HTTP GET method and associated API endpoints that the DotNetOpenAuth library was expecting, resulting in 401 unauthorised response from the API.

To fix this you need to rebuild the DotNetOpenAuth.ApplicationBlock project with the following changes to TwitterConsumer.cs to point to the updated API endpoints and to use HTTP POST for a couple of the methods instead of GET. Otherwise your API call will be redirected to https and lose the oauth variables in the process:

public static readonly ServiceProviderDescription ServiceDescription = new ServiceProviderDescription {
            RequestTokenEndpoint = new MessageReceivingEndpoint("https://api.twitter.com/oauth/request_token", HttpDeliveryMethods.GetRequest | HttpDeliveryMethods.AuthorizationHeaderRequest),
            UserAuthorizationEndpoint = new MessageReceivingEndpoint("https://api.twitter.com/oauth/authorize", HttpDeliveryMethods.GetRequest | HttpDeliveryMethods.AuthorizationHeaderRequest),
            AccessTokenEndpoint = new MessageReceivingEndpoint("https://api.twitter.com/oauth/access_token", HttpDeliveryMethods.PostRequest | HttpDeliveryMethods.AuthorizationHeaderRequest),
TamperProtectionElements = new ITamperProtectionChannelBindingElement[] { new HmacSha1SigningBindingElement() },
};

/// <summary>
/// The description of Twitter's OAuth protocol URIs for use with their "Sign in with Twitter" feature.
/// </summary>
public static readonly ServiceProviderDescription SignInWithTwitterServiceDescription = new ServiceProviderDescription {
            RequestTokenEndpoint = new MessageReceivingEndpoint("https://api.twitter.com/oauth/request_token", HttpDeliveryMethods.GetRequest | HttpDeliveryMethods.AuthorizationHeaderRequest),
            UserAuthorizationEndpoint = new MessageReceivingEndpoint("https://api.twitter.com/oauth/authenticate", HttpDeliveryMethods.GetRequest | HttpDeliveryMethods.AuthorizationHeaderRequest),
            AccessTokenEndpoint = new MessageReceivingEndpoint("https://api.twitter.com/oauth/access_token", HttpDeliveryMethods.PostRequest | HttpDeliveryMethods.AuthorizationHeaderRequest),
TamperProtectionElements = new ITamperProtectionChannelBindingElement[] { new HmacSha1SigningBindingElement() },
};