Comparison of the relative merits and appropriate scenarios for the supported WCF authentication credentials

I'm trying to select the correct authentication mechanism for a WCF service. This article explains what the supported credentials are - I just don't understand how to decide between them.

Please could somebody supply an explanation of when each of the authentication options are appropriate to use, and the advantages/disadvantages of each over the others.

For reference, the authentication options are:

  • None
  • Basic
  • Digest
  • Ntlm
  • Windows
  • Certificate
  • Password

Note: I have seen a great MSDN article on this, but I cannot for the life of me find it anymore.

Answers


None: Pretty straightforward - use this when you don't want to identify or authenticate your users.

Basic and Digest: These authentication types aren't used much any more, but occasionally you might need to connect to an older web service hosted in IIS, which might be configured to use Basic or Digest authentication. Traffic won't be encrypted. For Basic, the password will be sent in plain text, and for Digest the password will be sent in a poorly encrypted form. Avoid using these authentication types.

NTLM and Windows: NTLM uses the NT LAN Manager to control security. Windows, by default, will use Kerberos (ie Active Directory) to control security. If Kerberos is not available, it will default to NTLM. Only use NTLM if you specifically need to avoid Kerberos (I cannot think of a scenario where you would want to do this, but part of WCFs greatness is its flexibility).

Certificate: If your users have their own certificates which can be used to identify them, you might consider using this authentication mode. Passwords can be guessed - it is very hard to guess a certificate, so this is a pretty secure mode of authentication (provided the certificate itself is secure).

Password: Use Password when you want to create your own method of validating a users username and password. This might involve accessing an existing user credential store in a custom database. You will need to write your own UserNamePasswordValidator - example at http://nayyeri.net/custom-username-and-password-authentication-in-wcf-3-5.

To summarize, I usually choose Windows as the authentication mode. It is secure and simple and works for most people in an enterprise environment. If you're creating a new service and for some reason Windows cannot be used, go for Certificate or Password. If you're hooking into an older SOAP service hosted in IIS, you may need to look at using None, Basic or Digest.


Although I asked this question I will add what I know about Certificates as this is something I have worked with - though if my understanding is imperfect, I'd be happy to be corrected.

An X.509 certificate consists of a Public Key and a Private Key and issued by a trusted Certificate Authority (you can self-sign a certificate, this limits the usefulness of the certificate to trusted sources only). The CA will charge a fee for the certificate, and it will expire after a period of time.

The Public Key does not need to be protected and is given to the server. The Private Key is kept private and is used by the client. Messages encrypted by the Private Key can be decrypted by the Public Key.

A request from a client has a digital signature embedded in the SOAP. This is generated using the Private Key. When the server receives the request, it uses the Public Key to read the signature. If the signature can be read, then the request is considered genuine.

Within Active Directory, there is a mechanism whereby an X.509 certificate can be mapped to a user account, so you can impersonate a user with a certain permission set based on the X.509 authentication.

The benefits of X.509, are:

  1. that it can be used across domains; and
  2. that the Private Key can be securely stored in the client's Certificate Store. So there is no .config file containing credentials.

Failure to control the distribution and storage of a Private Key is akin to writing a username/password on a post-it note and sticking it on your monitor.


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