The SCTProvider

Now that we've covered the TokenProvider interface, let's look at an implementation that is shipped with the STS. The SCTProvider is used to provide a token known as a SecurityContextToken, that is defined in the WS-SecureConversation specification. A SecurityContextToken essentially consists of a String Identifier which is associated with a particular secret key. If a service provider receives a SOAP message with a digital signature which refers to a SecurityContextToken in the KeyInfo of the signature, then the service provider knows that it must somehow obtain a secret key associated with that particular Identifier to verify the signature. How this is done is "out of band".

To request a SecurityContextToken, the client must use one of the following Token Types:

• http://schemas.xmlsoap.org/ws/2005/02/sc/
• http://docs.oasis-open.org/ws-sx/ws-secureconversation/200512

Two properties can be configured on the SCTProvider directly:

• long lifetime - The lifetime of the generated SCT. The default is 30 minutes.
• boolean returnEntropy - Whether to return any entropy bytes to the client or not. The default is true.

The SCTProvider generates a secret key using the KeyRequirements object that was supplied, and constructs a SecurityContextToken with a random Identifier. It creates a CXF SecurityToken object that wraps this information, and stores it in the supplied cache using the given lifetime. The SecurityContextToken element is then returned, along with the appropriate references, lifetime element, entropy, etc.

When requesting a token from an STS, the client will typically present some entropy along with a computed key algorithm. The STS will generate some entropy of its own, and combine it with the client entropy using the computed key algorithm to generate the secret key. Alternatively, the client will present no entropy, and the STS will supply all of the entropy. Any entropy the STS generates is then returned to the client, who can recreate the secret key using its own entropy, the STS entropy, and the computed key algorithm.

This secret key is then used for the SCT use-case to encrypt/sign some part of a message. The SecurityContextToken is placed in the security header of the message, and referred to in the KeyInfo element of the signed/encrypted structure. As noted earlier, the service provider must obtain somehow the secret key corresponding to the SecurityContextToken identifier. Perhaps the service provider shares a (secured) distributed cache with an STS instance. Or perhaps the service provider sends the SCT to an STS instance to "validate" it, and receives a SAML token in response with the embedded (encrypted) secret key.