With Apple seemingly blocking apps that access UDID (unique device identifier) data from the App Store, developers and mobile advertising firms are scrambling to find alternative ways to track user behaviour.

Apple hasn't officially confirmed that it is blocking apps that use UDID data or given any reason why it doesn't want apps to access UDID data any more, but it is presumed that privacy concerns lie behind the decision.

Apple had told developers late last year that it planned to block the ability of iOS apps to access the UDID but in order to satisfy lawmakers who have put iOS under scrutiny as part of a wider investigation into how mobile apps impact on individual's privacy this policy appears to have been brought forward.

As Wes Biggs, the co-founder and CTO of mobile advertising firm Adfonic explains, UDID data is invaluable to developers and mobile advertising firms.

"As the iOS application ecosystem has grown in the past three years, UDID has emerged as the de facto way of enabling a host of functions that make developers’ lives easier. By virtue of its widespread adoption and usefulness in tracking how advertising impacts App Store downloads and purchases, it underpins the ad-funded iOS app economy."

However, as Biggs concedes, it does present some privacy problems. "The identifier is permanent and tied to a particular device in a way that is irrevocable and ultimately does not give the user choice over whether to share it with app developers, and by extension, ad networks and analytics companies."

Apple's decision has prompted an urgent hunt for an alternative way to track user behaviour in a way that is useful for developers and advertisers, but will satisfy Apple and privacy campaigners' concerns. Adfonic has outlined a number of new techniques that could be used.

The first is known as OpenUDID and is described as an "open source initiative for a universal and persistent UDID solution for iOS". It works by generating an identifier the first time it is requested by an application, not by taking an existing code, and then the same code is shared with any other app that requests it. However, in order to protect privacy, it can be removed or regenerated at the user's request.

"The main motivation here was to find a UDID replacement not owned by any single provider. It is easy to foresee a fragmented market where UDID management is operated by multiple providers with no cooperation between them. This open source initiative is to enable a better solution for thousands of other mobile app developers," wrote Yann Lechelle, the project's initiator and co-founder of Appsfire on the project's webpage.

But there are some solutions that are being proposed by single providers such as Madvertise. Its internal research and data science department has created a "highly accurate" unique device detection system (UUD) for tracking and analysing the mobile audience.

Though Madvertise hasn't gone into the finer details of how the system works, it says that there is "a scientific method being applied to measuring, in real-time, the value of information available from a device and how much information is required in order to identify a device uniquely".

The UUD gives Madvertise the ability to offer advertisers campaigns in which it can track Click Through Rates (CTR), download and conversion tracking, target specific audiences by criteria such as location, as well as offer other "sophisticated" methods of behavioral targeting.

"Data collection, analysis, and enrichment on mobile is rapidly changing. The days of cookies as a valid identification metric are over and any company still relying primarily on them as a basis for their technologies will be rapidly left behind," said Adam Drake, the Vice President of Research and Chief Data Scientist at Madvertise.

Meanwhile, Adfonic has been putting its weight behind ODIN, or the Open Device Identification Number project. Despite using part of a devices' MAC address to generating reusable device identifiers, its backers insist that it is an anonymous and secure technique.

"While we share the concerns of others in the industry about the long term viability of the MAC address as a tracking solution – it shares many of the same privacy issues as UDID – this alternative has some immediate traction with developers and exchanges, and is easily implementable for a variety of use cases. Adfonic will be supporting ODIN-1 for both publishers and advertisers, and working with the ODIN group on the evolution of the standard," said Biggs.

Another solution that Adfonic comes as part of its partnership with the developers of AD-X tracking technology to come up with what is "not specifically a UDID replacement" but does provide "a way for campaigns to be tracked across both mobile web and in-app inventory".

So while it isn't yet clear which or how many of these solutions will be adopted instead of UDID, it is clear that there are alternatives available that should please those on both sides of the debate.

"While this may seem like an uncertain time, it’s merely another exciting evolution point on the timeline of the app ecosystem. With cool heads and clear objectives, we’re confident the industry will converge on common practices that not only provide a replacement to UDIDs, but accelerate innovation and adoption within the mobile advertising space," Biggs said.