How do I make a digital self-destructing Mission: Impossible tape?
May 17, 2013 9:46 AM   Subscribe

I want to build a device that will play an audio file once and only once - basically the self-destructing tape from Mission: Impossible, but updated to the digital age. Bonus points for a setup that ends with an audible sizzle and a puff of smoke, yet isn't a federal offense to put through the USPS. Is it possible to do this with a microcontroller like Arduino or Raspberry Pi? I'm a complete beginner to this kind of 'Maker' stuff, so I'm aware I may not even be asking the right questions. Here's a link to instructions about an analog version:
posted by boghead to Computers & Internet (7 answers total) 1 user marked this as a favorite
Best answer: If I was going to make something like this, I might start with something like an ISD ChipCorder. These are little integrated flash-codec-controller chips designed to record and play audio for toys and similar things. Digikey sells eval boards.

If you want failsafe secure erasure, you'll need to do more work. The usual method is to store encrypted audio on an external flash chip, and the decryption key in battery-backed SRAM on your controller. When finished playing (or if an anti-tampering sensor is triggered), the controller wipes the key from SRAM. This is much faster and much more secure than trying to erase flash memory, which is a pretty slow, involved process.
posted by ryanrs at 10:08 AM on May 17, 2013 [1 favorite]

Best answer: You could certainly use an Arduino with an attached MP3 shield. They have enough example code and projects there to probably let you do this right out of the box.

As for the one-time-play, you probably don't need to get as heavy with decryption and stuff. Ship the device with a certain flash/EEPROM location set to 1 and have the playback code check for the 1 before playing. At the end of a complete playback, change the location to a zero. That's probably good long as you epoxy the SD card in place. =) Or, muck around with the MP3 payload (like XORing every byte with a pattern) so that the MP3 can't be extracted off the card and played by anything else.

The flame trick I'd maybe do with a model rocket ignitor and a transistor to switch the voltage from a battery. But that's making actual flame/smoke which would probably be a bit dangerous. Maybe a piezo transducer with a bit of baby powder stuffed inside?
posted by JoeZydeco at 10:22 AM on May 17, 2013 [1 favorite]

So it looks to me like you could adapt the Minty MP3 to ryanrs's suggestion: Encrypt an MP3 file, and decrypt it as you feed it to the STA013. It shouldn't be too hard to adapt that to an SD card.

Could you do the smoke by just having enough battery that you can pull a lot of current through a couple of components and burn them? I'd guess that a TIP120 between a battery with relatively low internal resistance and ground would go up in a satisfyingly awful smelling puff of smoke if you fed a little current into its collector... Might have to be careful to not take the battery with it...
posted by straw at 10:22 AM on May 17, 2013

Dumping some power into a couple 1/4 W carbon film resistors will generate noxious smoke.

Note that if it goes off before delivery or is delivered to the wrong person, it will most likely be treated as a terrorist bomb / incendiary device. Also it will meet the legal definition of such a device, so you'll be in serious trouble with the feds. (Not joking about this part.)
posted by ryanrs at 10:30 AM on May 17, 2013 [2 favorites]

A flash unit with the output of the capacitors connected to your flash memory instead of a flash tube will burn out the flash memory without incorporating incendiaries in a postal delivery. Any sort of timer or anti tamper switch could be used to trigger the strobe unit
posted by Mitheral at 10:48 AM on May 17, 2013

Or send an erase command to the flash memory chip. That works too.
posted by ryanrs at 10:54 AM on May 17, 2013

Response by poster: Thanks for the info everyone. This is going on the back burner now as a big secret project; once life calms down a bit I'll pick back up on it again.
posted by boghead at 6:04 AM on June 24, 2013

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