Last summer, the BBC tried to sneak "digital rights management" into its high-def digital broadcasts.

Now, generally speaking, the BBC isn't allowed to encrypt or restrict its broadcasts: the licence fee payer pays for these broadcasts, and no licence fee payer woke up today wishing that the BBC had added restrictions to its programming.

But the BBC tried to get around this, asking Ofcom for permission to encrypt the "metadata" on its broadcasts – including the assistive information used by deaf and blind people and the "tables" used by receivers to play back the video. The BBC couched this as a minor technical change, and Ofcom held a very short, very quiet consultation, but was overwhelmed by a flood of negative submissions from the public and from technologists who understood the implications of this move.

Fundamentally, the BBC is trying to leverage its broadcast licence into control over the devices that can receive broadcasts. That is, in addition to deciding what shows to put on the air, the Beeb wants the power to decide what kinds of tellies and set-top boxes will be able to display and record those shows – and it wants the power to control the design of all the devices that might be plugged into a TV or set-top box. This is an unprecedented amount of power for a broadcaster to have.

As Ofcom gears up to a second consultation the issue, there's one important question that the BBC must answer if the implications of this move are to be fully explored, namely: How can free/open source software co-exist with a plan to put DRM on broadcasts?

A brief backgrounder on how this system is meant to work: the BBC will encrypt a small, critical piece of the signal. To get a key to decrypt the scrambled data, you will need to sign onto an agreement governed by a consortium called the Digital Transmission Licensing Administrator (some of the agreement is public, but other parts are themselves under seal of confidentiality, which means that the public literally isn't allowed to know all the terms under which BBC signals will be licensed).

DTLA licenses a wide variety of devices to move, display, record, and make limited copies of video. Which programmes can be recorded, how many copies, how long recordings can last and other restrictions are set within the system. To receive a licence, manufacturers must promise to honour these restrictions. Manufacturers also must promise to design their devices so that they will not pass video onto unapproved or unlicenced devices – only DTLA-approved boxes can touch or manipulate or play the video.

DTLA enforces these rules through a system of penalties for non-compliant vendors. It also has the power to "revoke" devices after they are sold to you, so that the BBC's signals will refuse to play on your set-top box if the DTLA determines that its security is inadequate and they pass it a revocation message (even though you always used your box in accordance with the law).

With DTLA devices, the integrity and usefulness of your home theatre is subject to the ongoing approval of the consortium, and they can switch it off if they decide, at any time in the future, that they don't trust it any more.

The entire DTLA system relies on the keys necessary to authenticate devices and unscramble video being kept secret, and on the rules governing the use of keys being inviolable. To that end, the DTLA "Compliance and Robustness Agreement" (presented as "Annex C" to the DTLA agreement) has a number of requirements aimed at ensuring that every DTLA-approved device is armoured against user modification. Keys must be hidden. Steps must be taken to ensure that the code running on the device isn't modified. Failure to take adequate protection against user modification will result in DTLA approval being withheld or revoked.

This is where the conflict with free/open source software arises.

Free/open source software, such as the GNU/Linux operating system that runs many set-top boxes, is created cooperatively among many programmers (thousands, in some cases). Unlike proprietary software, such as the Windows operating system or the iPhone's operating system, free software authors publish their code and allow any other programmer to examine it, make improvements to it, and publish those improvements. This has proven to be a powerful means of quickly building profitable new businesses and devices, from the TomTomGo GPSes to Google's Android phones to the Humax Freeview box you can buy tonight at Argos for around £130.

Because it can be adapted by anyone, free software is an incredible source of innovative new ideas. Because it can be used without charge, it has allowed unparalleled competition, dramatically lowering the cost of entering electronics markets. In short, free software is good for business, it's good for the public, it's good for progress, and it's good for competition.

But free software is bad for DTLA compliance.

Free software is intended to be examined and modified by all comers.

Generally, the licence terms for free software require that it is licensed for public examination and adaptation. It is literally impossible for a device to be both "open" and for it to prevent its users from retrieving keys hidden in its guts, or from changing the code that runs on it. This, of course, is totally incompatible with the DTLA requirement to hide keys and prevent modification of code.

And so, when the BBC threatens to infect its high-def broadcasts with DTLA, it also threatens to remove free/open software from consideration for any device that can play, record, or manipulate the video that the licence fee pays for. It means that you can't use a GNU/Linux phone to watch a show, or an open video player like VLC on your laptop. It means that your kids can't use free/open video-editing software to cut some of last night's news into a presentation for class.

It means that British entrants into the DTV device market can't avail themselves of the free software that their competitors all over the world are using, and will have to spend fortunes reinventing the wheel, creating operating systems and programs that do the same things as their free counterparts, but in such a way as to enforce restrictions against the device's owner.

Ofcom is meant to guard the public interest in matters such as these. If the public interest is to be upheld here, the BBC must explain how it intends to do the impossible: add DRM without banning free/open source development.