Giving governments and other international agencies access to Microsoft's source code could improve security and is unlikely to increase the risk of code falling into the hands of hackers, according to experts.
The UK government has become the latest signatory to Microsoft's Government Security Program, under which the software giant provides access to source code in an effort to ease fears over the security of the Windows operating system.
The agreement builds on Microsoft's shared source initiative launched in 2001, and covers access to code for Windows 2000, XP, Server 2003 and CE.
Andrew Schulman, a software litigation consultant, insisted that more people looking at the software would improve security, and that the risk of hackers gaining access is low.
"Application Programming Interfaces [APIs] are not the greatest source for hackers," he said.
"Database expert David Litchfield did not have access to source code but found serious vulnerabilities with SQL Server.
"A large school of thought believes that security through obscurity is not the answer and that increasing the number of eyeballs scanning code increases the likelihood of finding and solving problems."
Stuart Okin, chief security officer for Microsoft UK, explained that the source code would be held on a secure website accessed via a smart card.
"Cryptographic APIs will be distributed to government in a controlled environment," he explained.
"There is always a chance that some hacker could get hold of a smart card, but the potential increased threat is low.
"There are many different techniques for hackers to use, such as public vulnerabilities or firewalls not configured correctly.
"Hackers are unlikely to gain a benefit by viewing 10 million lines of code in Windows."
Okin stressed that the programme is only open to governments and other international agencies, and not to independent software vendors.
But he admitted that the knowledge might leak as government developers leave for other jobs in the private sector.
"You can't eliminate people from taking knowledge with them. We have made no legal requirement for individuals to hide information from their next employers," explained Okin.
A Cabinet Office spokeswoman said: "The agreement allows a small number of civil servants to have access to the code and they have signed the Official Secrets Act.
"It is a trust issue and Microsoft faces the same potential problems."
E-envoy Andrew Pinder maintained that the partnership, which will also see government developers accessing Microsoft training and visiting Redmond to meet developers and programme managers, is "key to the risk management of the national infrastructure".
"By allowing us access to the source code, we will be using the knowledge gained, together with the rest of our experience, to make sure that a greater range of products meet the UK government's assurance needs," he said.
Pinder added that, in his role as central sponsor for information assurance for the UK, he would be seeking similar agreements with other software vendors.
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