Goldman Sachs Group programmer Sergey Aleynikov has an interesting defence to the charge of stealing software from his employer: he was only trying to download open-source software.

According to a report published in the New York Times, Aleynikov told FBI investigators that he had inadvertently taken about 32MB of proprietary Goldman Sachs while taking open source code that can be used freely by anyone.

Aleynikov, a high-level developer for Goldman Sachs, was arrested by the FBI on 3 July on charges of stealing computer code that automates the firm's high-volume trading on stock and commodities markets.

Aleynikov, who is now free on bail, told the FBI he had not used the code at his new job nor given it to anyone else, according to the Times story. The complaint does not include such charges.

The case raises many intriguing questions, such as what exactly is the 'secret sauce' behind the high-speed trading software that some experts told the Times is used by Wall Street firms to generate huge revenues - some $8 billion (£4.95bn) this year. Experts also say the software could be giving the large trading firms an unfair advantage over regular investors.

Observers also wonder why Aleynikov didn't simply download the unnamed open-source code from any of its free repositories rather than from Goldman Sachs systems. And programmers and open-source users are left wondering whether Aleynikov can be found guilty of stealing the code that belongs to the programming community.

Actually, he can, according to legal and open-source experts who cite the terms and conditions of the General Public License (GPL), which is used to govern the use of about two-thirds of open-source software. ""This is a common misconception," said Brett Smith, license compliance engineer at the Free Software Foundation (FSF), which oversees use of the GPL.

Though the FSF has long argued that all software and source code should be free - it recently launched a campaign against the "sins" of Microsoft's proprietary Windows 7 operating system - the terms of the GPL does include some restrictions.

For example, the GPL states that companies that modify open-source software for internal use aren't required to share code changes with the open source world, said Smith. "You never have to provide the source code to an upstream developer or the general public if you don't want to," he said.