XML Reference Guide

Mar 14, 2003

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It's not surprising that web services, with its promise of remote computing and platform independance, also has its share of security concenrs. After all, any technology specifically designed to let outsiders interact with your company has got to have its own set of concerns built in.

In fact, for a long time the question of how to secure web services was a major sticking point it its wider adoption, as companies tried to figure out how to make good use of it without cutting their own throats.

Web services security generally takes into account four issues: access control, confidentiality, data integrity, and non-repudiation.

Access control

How do you make sure that only those permitted to access a web service can do so? We're dealing with a situation in which web services messages are designed to be widely accessible. How do we limit that? Two specifications that attempt to address this problem are SAML and XACML.

Security Assertion Markup Language (SAML) is an XML vocabulary for specifying "assertions," such as the fact that a certain person, or subject, is permitted to access a particular resource at certain times of the day. A SAML-enabled system compares the various assertions with the given situation and makes an authorization decision, which is then relayed back using SAML.

XML Access Control Markup Language (XACML) seems on the surface as though it is a competitor for SAML, but in fact both are being managed within OASIS, and XACML complements SAML. XACML provides a way to specify groups of subjects who have specific privileges, authentication methods that must be used, and other information needed for a SAML solution to work properly.

Encryption and its benefits

The other major issues can all be managed by encryption of one sort or another, so let's look at how they figure into the equation.

Confidentiality

How do you make sure that nobody reads the web services message besides the sender and intended recipient? Web services messages are simply XML that travels the same hops through the Internet as email and web page requests; any intermediate recipient could conceivably read the data.

Data integrity

How do you make sure the message received is actually the message sent, and not some corrupted or tampered-with version? Those same intermediate recipients mentioned above could conceivably alter the data for their own (nefarious) purposes, such as redirecting replies to themselves.

Non-repudiation

How do you prove the alledged sender of the message is the actual sender? Think of this the same way you'd think of a signature on a credit card slip. How can you prove the requestor actually did the requesting?

So how can we solve all of these problems through the use of encryption? Well, it depends on what kind of encryption we're talking about.

XML Encryption is perhaps the most basic part of this picture, as a specification that enables you to use cryptographic techniques to encrypt some or all of an XML document and then include the encrypted information within the XML. It also enables you to encrypt different parts of a document with different keys, so that multiple recipients can retrieve only the data intended for them.

So that seems to take care of the confidentially issue, but what about data integrity and non-repudiation? It turns out that by using the proper encryption method, we can cover those, as well.

There are two types of encryption in general use on the web: symmetrical, or "shared key" encryption, and asymmetrical, or "public key/private key" encryption. In "shared key" encryption, the same secret code, or key, is used to encrypt and decrypt the message. Anyone who has that key can read or send messages in that particular conversation, but of course pains are taken to make sure that only the sender and receiver have that key.

In "public key/private key" encryption, the situation is a little bit different. In this case, you have a situation in which one party has a pair of keys. One is kept secret; theoretically, only the owner has it. The other is public, and everyone knows that it belongs to that person because it's published to some sort of registry. A message encrypted with one key can only be decrypted with the other, and vice versa. So it works like this:

You want to send me a message, but you don't want anyone else to read it. You look up my public key and use it to encrypt the message. Now that message is unreadable to anybody but me; I can only read it because I have the private key that matches the public key you used to encrypt it in the first place. On the other hand, when I send you a response, I encrypt it with my private key. That means that anybody with my public key -- which is to say, potentially anybody -- can read it, but the goal here wasn't secrecy, it was identification. That message had to come from me, because it was encrypted with my private key. You know that because you were able to decrypte it with my public key.

So what does this have to do with web services? First off, it takes care of the non-repudation problem. If you can decode a message with my public key, it had to have been encrypted with my private key, so it had to come from me. I can't deny sending it.

Second, there's an interesting side-effect to this process. The encrypted messages are extremely sensitive to changes, so if anybody tries to tamper with these messages, that'll show up in the decryption process. So that takes care of the data integrity issue.

OK, so how do we put this to work, in a practical sense?

As far as web services are concerned, we can "sign" messages using the XML Digital Signatures speficiation, which provides a vocabulary for representing the encrypted data and provding any data necessary for the recipient system to retrieve the appripriate key and decrypt the message.

Of course, all of these keys floating around have to be managed somehow, and the idea of Public Key Infrastructure (PKI) installations has been around in one form or another for some time. (In order for this scheme to work, you'd have to be able to get hold of my public key so you can encrypt the initial message to me.) All of this is moving into the XML world with the XML Key Management (XKMS) specification, which consists of the overall structure and two sub-specifications.

Confused yet? And I haven't even mentioned the numerous other web services security-related specifications!

Helping to pull it all together, we have specifications such as WS-Security, which doesn't actually specify how to protect something, but rather provides a way to explain how you've protected something. It defines a number of extensions that can be placed in the Head of a SOAP Envelope to specify all of this information.

In future entries, we'll look at each of these individual topics, but for now check out the resources for more information.

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