Web services are self-describing, modular and autonomous application components that enable cross-application provision and utilisation of functions via the Internet Mobility, Security and Web Services, Wiehler G., ed. Siemens AG Berlin and Munich, Publicis Corporate Publishing Verl., Erlangen 2004. The web service paradigm is based on a service-oriented architecture that represents an environment for distributed processing. SOA aims to provide the resources of a system on demand. These resources are made available to the participants in a network as independent services that can be used in a standardised manner. This approach serves as the basis for the loose coupling of resources Looselycoupled.com: Loosely Coupled - connecting with webservices, 2004. Web services can help to reduce the complexity and costs of software applications due to their standardised provision and flexible reusability.

One of the main problems when considering integration aspects in the context of external web services is data security, as the parameters and return values of function calls no longer only move within the framework of a computer or a shielded network, but are sent via the Internet. The most important security threats that jeopardise web service-based applications are

• Loss of confidentiality of messages (e.g. through unauthorised interception of IP packets)
• Loss of message integrity (e.g. through unintentional or intentional manipulation of the message content)
• Invalidation of the authenticity of messages and senders (e.g. by forging the sender address)
• Loss of availability of web services (e.g. due to overloading of a web service with a flood of dubious requests)
• Non-traceability of web service calls and responses (e.g. due to inadequate authentication mechanisms)

The special feature of the security assessment of web service-based applications is that XML as a message format makes it possible to use standardised security scenarios that relate to the security of individual messages. In contrast to conventional encryption on the lower layers of the OSI reference model, e.g. SSL encryption, it is possible to use mechanisms to ensure confidentiality, integrity and authenticity, but these measures are not exclusively related to a message to be sent. As SSL or TLS is used to secure the transport layer (layer 4), authentication, integrity assurance and encryption functions are used to establish secure point-to-point connections. At the application level (layer 7), this security-relevant information no longer exists, meaning that a sent message cannot be directly linked to the security data described. Problems arise when messages are to be forwarded or archived for verification purposes, as security measures are required again at this point. IPsec (layer 3) is another security standard that has similar disadvantages to SSL.

Web Services Security is based on the fact that SOAP messages (layer 7) are extended by so-called security tokens in order to create a direct link between security-relevant information (such as signatures) and the actual message. In this way, the following advantages are achieved compared to SSL-based security concepts:

• Encryption is contained directly in the message so that it can also be forwarded directly to the actual recipient via several stations without the need for additional encryption of the communication channels (end-to-end encryption, as the security-critical message parts are encrypted for the actual recipient).
• Proofs of integrity (such as signed hash values) are directly linked to the message and are not provided at protocol level. The security-relevant data is also saved when the message is archived. Forwarded messages enable the subsequent recipient to check the integrity without additional effort.
• Authentication objects (such as signatures or passwords) are also contained directly in a message, so that third parties can also verify the authentication information directly with the message (e.g. archiving for verifiability/non-repudiation of transactions).

The aim of the Diplom thesis is the scientific investigation of existing security standards and tools in the web service environment for their usability within web service-based operational application systems. The following subtasks should be worked on:

• Classification of security threats for distributed operational information systems based on web services
• Development of security models taking into account the stated security objectives
• Analysis of available security tools in the area of web service security and evaluation according to a catalogue of requirements to be drawn up
• Prototypical development of a sample application based on Secure Web Services

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