Ensuring that someone seeking to use some computer service is actually who they claim to be, or that the provider
of the service is actually who it claims to be. Such schemes work with a shared secret such as a password or a
private key. In some schemes, the actual secret need not be exchanged, just proof that other end knows the
secret, e.g. by encrypting a random message with the private key. You can do web authentication with basic
authentication where the browser brings up a dialog box for user name and password, form-based where the user
fills in a form with username and password and perhaps other information, certificate based where the browser
presents an X.509 certificate to the server to request access, and digest authorisation where the password is
digested before being sent to the server to avoid it being snooped on.
Basic Scheme Authentication
when your Applet or JWS (Java Web Start) application tries to access something restricted on the server it gets a message back
from the server telling it the realm (domain or subdomain) and the name of the password scheme the server wants
to use, e.g. basic or digest. Your Applet then retries the request embedding the userid and password information
in the requested way.
In Java version 1.2 or later, in your client code, you can use the java.net.Authenticator class to
handle the details. You extend the class overriding the getPasswordAuthentication
method like this:
Then you then register your custom Authenticator with
Authenticator.setDefault( new MyAuthenticator() );
You then do your GETs ignoring logons! Your Authenticator magically kicks in
when needed and logs you into the server. See the File I/O
amanuensis or the CMP HTTP package for how. The technique reputedly
works for HTTP (Hypertext Transfer Protocol) and proxies. It may work for HTTPS (Hypertext Transfer Protocol over SSL (Secure Socket Layer)). It even works for digest passwords. I don’t see how it
could work for certificate style authentication, however, but who knows…
Java version 1.1-
If you are using an older Java, you will have to do it the Smith-Barney way (obscure reference to the late
Digest Scheme Authentication
For the more secure digest-style authentication, the protocol is more complex. It requires nine subfields. It
is described in RFC 2617. Java Authenticator uses this method when the server specifies scheme=
digest. It works by sending an MD5 digest with each transaction, and changing the digest periodically. Your Applet does not need to get involved with the details of how it works. Authenticator handles it all transparently. You can fine tune how it works with networking properties:
Sun does does not have a method to tell you which schemes Authenticator supports or
what the official scheme names are. It may support others besides basic, digest, ntlm and spnego (Kerberos). It does not
work with cookies or forms.
Under the Hood
You might wonder how after the login is complete that the server can tell if messages coming in from the
Internet are from people who are already logged in. There are a number of ways of doing it. Some you might think
would work don’t.
- By IP (Internet Protocol). You might think the server could just check if an IP in a message header was from someone logged in.
This does not work because IPs (Internet Protocols)
are shared. Everyone in your home on the LAN (Local Area Network), when the access the Internet comes
from the same IP, the IP of your router.
- By TCP/IP (Transmission Control Protocol/Internet Protocol) session. You might think the server would just check that the message came in on the same TCP/IP
session as the user logged in on. This won’t work since you often connect with multiple sessions, and you
would not want to have to relogin just because a session tanked.
- Basic. The server sends you id/password with every request that is restricted. This method is not secure
since the id/password pair is in plain text for any snoop to see.
- NTLM (New Technology LAN Manager protocol). This is a Microsoft proprietary protocol than will only work with Microsoft servers and clients. I
don’t know how it works. Java supports it.
- By Cookie. The server sends a cookie at login time, and the user includes this cookie with each message to
the server. This method is not secure since anyone snooping can spoof the user by just copying the invariant
cookie. Further, the client’s browser must be configured to accept cookies, a practice which invites all
manner of malicious spying.
- URL (Uniform Resource Locator) rewriting. The server sends unique URLs (Uniform Resource Locators)
to different users. When they come back the server knows they
could only have come from the client they were sent to.
- By HTTP auth digest.RFC 2617 Here each incoming message is
digitally signed in an unforgeable way. The disadvantage of this approach is it takes a bit more CPU (Central Processing Unit) time to
compute the digests and requires the more transmission overhead. The advantage is it is the most secure method
without resorting to a fully encrypted data stream.
Oracle’s Javadoc on Authenticator
class : available:
Oracle’s Javadoc on PasswordAuthentication
class : available:
Oracle’s Technote Guide on Authentication