A former government contractor has accused the FBI of installing a crypto backdoor in OpenBSD, the open source, Unix-derived operating system, circa 2000.

The allegation was made by Gregory Perry, former chief technology officer of FBI contractor Network Security Technology, who on Saturday emailed software engineer Theo de Raadt, the founder and leader of the OpenBSD project.

In his email, Perry said: "My NDA with the FBI has recently expired, and I wanted to make you aware of the fact that the FBI implemented a number of backdoors and side channel key leaking mechanisms into the [Open Cluster Framework], for the express purpose of monitoring the site to site VPN encryption system implemented by EOUSA, the parent organization to the FBI."

Perry also alleged that because of the backdoor, people on the FBI payroll -- such as virtualization expert Scott Lowe -- were continuing to advocate "the use of OpenBSD for VPN and firewalling implementations."

Perry's email was released by de Raadt, who said that "over 10 years, the IPSEC code" -- referring to OpenBSD's IPsec VPN capabilities -- "has gone through many changes and fixes, so it is unclear what the true impact of these allegations are."

While Perry's email was private, de Raadt said that he was releasing it publicly so that anyone in the know could audit the OpenBSD code, explore the allegations, or defend themselves, as appropriate. "The mail came in privately from a person I have not talked to for nearly 10 years. I refuse to become part of such a conspiracy, and will not be talking to Gregory Perry about this," he said.

Rebuttals have been swift. On Tuesday, Scott Lowe blogged: "Let's get right to the point and set the record straight: I am not, nor have I ever been, affiliated with or employed by the FBI or any other government agency."

On Wednesday, former FBI cyber special agent Ernest E.J. Hilbert said that while the FBI might examine software, it didn't add backdoors. "There is no way that the FBI paid anyone to create backdoors in OpenBSD and put it out there for the world to use. It just doesn't happen that way," he told Kaspersky Lab's Threatpost.

But in subsequent remarks, Perry defended his allegations: "In reality, the Clinton administration was very quietly working behind the scenes to embed backdoors in many areas of technology as a counter to their supposed relaxation of the Department of Commerce encryption export regulations."

For now, many security experts seem unconvinced. "Aside from the mailing list post, there is little if any hard evidence of such a backdoor," said SANS chief research officer Johannes Ullrich in a blog post. "My gut feeling is that this is FUD if not an outright fraud."