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[The following is the conclusion of our series of excerpts from chapter 5 of the O'Reilly title, Apache: The Definitive Guide.]
A halfway house between complete encryption and none at all is digest authentication. The idea is that a one-way hash, or digest, is calculated from a password and various other bits of information. Rather than sending the lightly encoded password, as is done in basic authentication, the digest is sent. At the other end, the same function is calculated: if the numbers are not identical, something is wrong — and in this case, since all other factors should be the same, the "something" must be the password.
Digest authentication is applied in Apache to improve the security of passwords. MD5 is a cryptographic hash function written by Ronald Rivest and distributed free by RSA Data Security; with its help, the client and server use the hash of the password and other stuff. The point of this is that although many passwords lead to the same hash value, there is a very small chance that a wrong password will give the right hash value, if the hash function is intelligently chosen; it is also very difficult to construct a password leading to the same hash value (which is why these are sometimes referred to as one-way hashes). The advantage of using the hash value is that the password itself is not sent to the server, so it isn't visible to the Bad Guys. Just to make things more tiresome for them, MD5 adds a few other things into the mix: the URI, the method, and a nonce. A nonce is simply a number chosen by the server and told to the client, usually different each time. It ensures that the digest is different each time and protects against replay attacks.[2] The digest function looks like this:
MD5(MD5(<password>)+":"+<nonce>+":"+MD5(<method>+":"+<uri>))MD5 digest authentication can be invoked with the following line:
AuthType DigestThis plugs a nasty hole in the Internet's security. As we saw earlier — and almost unbelievably — the authentication procedures discussed up to now send the user's password in barely encoded text across the Web. A Bad Guy who intercepts the Internet traffic then knows the user's password. This is a Bad Thing.
You can either use SSL (see Chapter 11) to encrypt the password or Digest Authentication. Digest authentication works this way:
The client requests a URL.
Because that URL is protected, the server replies with error 401, "Authentication required," and among the headers, it sends a nonce.
The client combines the user's password, the nonce, the method, and the URL, as described previously, then sends the result back to the server. The server does the same thing with the hash of the user's password retrieved from the password file and checks that its result matches.[3]
A different nonce is sent the next time, so that the Bad Guy can't use the captured digest to gain access.
MD5 digest authentication is implemented in Apache, using
mod_auth_digest, for two reasons. First, it
provides one of the two fully compliant reference HTTP 1.1
implementations required for the standard to advance down the
standards track; second, it provides a test bed for browser
implementations. It should only be used for experimental purposes,
particularly since it makes no effort to check that the returned
nonce is the same as the one it chose in the first place.[4]
This makes it susceptible to a replay attack.
2. This is a method in which the Bad Guy simply monitors the Good Guy's session and reuses the headers for her own access. If there were no nonce, this would work every time! Back
3. Which is why MD5 is applied to the password, as well as to the whole thing: the server then doesn't have to store the actual password, just a digest of it. Back
4. It is unfortunate that the nonce must be returned as part of the client's digest authentication header, but since HTTP is a stateless protocol, there is little alternative. It is even more unfortunate that Apache simply believes it! An obvious way to protect against this is to include the time somewhere in the nonce and to refuse nonces older than some threshold. Back
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Created: March 10, 2003
Revised: March 10, 2003
URL: http://webreference.com/internet/apache/chap5/3/
