We address the problem of securing sequences of SOAP messages exchanged between web services and their clients. The WS-Security standard defines basic mechanisms to secure SOAP traffic, one message at a time. For typical web services, however, using WS-Security independently for each message is rather inefficient; moreover, it is often important to secure the integrity of a whole session, as well as each message. To these ends, recent specifications provide further SOAP-level mechanisms. WS-SecureConversation defines security contexts, which can be used to secure sessions between two parties. WS-Trust specifies how security contexts are issued and obtained. We develop a semantics for the main mechanisms of WS-Trust and WS-SecureConversation, expressed as a library for TulaFale, a formal scripting language for security protocols. We model typical protocols relying on these mechanisms and automatically prove their main security properties. We also informally discuss some pitfalls and limitations of these specifications.
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