ratings[, [e.g.]{},]{}such of good quality, is possible.

Content Protection for Push Services {#sec:Push}
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Workers occupied with ‘nomadic’ tasks depend on infrastructures for easy and swift access to required data. E-Mail push services like RIM’s Blackberry conquer this market with huge success, and aim at high availability and ease of use. Push services are characterised by the ability to notify end users of new content. For an e-mail service the end user device is activated by the mail server, gets new mail, and notifies the user. The basic method for this has been formulated for instance in the standards of the Open Mobile Alliance (OMA, see [@OMAPush2006]). Some providers have extended their range to enable access to company databases and implement loosely coupled work-flows incorporating nomadic workers.

Due to the high value of the exchanged data these systems are threatened by, even professional, attackers, raising the requirement to protect the distribution of pushed data. Security concerns can be grouped in two main areas. First, data has to be protected in transit to the device, and confidentiality is to be maintained. Second, after the data is delivered it has to be protected against unauthorised access. The latter problem is of practical 