Secure SD card- 2 - Cassette tape copier
Cassette tapes may be obsolete nowadays, but their unique properties still make them almost perfect from the security point of view. Easy to erase, easy to monitor writing of every single sector, and very easy to destroy - everything that SSD devices ARE NOT!
SD cards are small, cheap, light and fast and they can hold gigabytes of data.
On the other hand, because of their inherent physical properties, they are far from perfect from the security standpoint. We explained almost everything here:
https://www.elektormagazine.com/news/secure-communications-an-interview-with-luka-matic
This device solves some problems of secure data management on SD cards:
https://www.elektormagazine.com/labs/secure-sd-card-2-sd-card-copier
Then why bother with obsolete cassette tape technology at all?
Thanks to the following properties, cassette tapes may still be more convenient than SDs:
1.) Easy to erase.
2.) Easy to monitor and control every sector of data.
3.) Easy to destroy.
The last property may be crucial in a spy scenario, where Alice must quickly destroy the codes when chased by Mallory or simply to prevent the dumpster-diving recovery of data by Eve! The PET-based tape melts at cca. 250°C and boils at 350°C, which is more practical than minimum 1300°C required for SSD.
Tape speed variation, wow and flutter are the main problems of cassette tape technology. This device uses a different approach - instead of precisely controlling the tape speed (which is very expensive in any aspect), the tape speed is constantly measured, and system clock frequency is varied accordingly. This way we can use any below-average tape deck, since this device can compensate for wow-flutter variations up to +/- 10% The only requirement is 200Hz-3000Hz frequency span which is achievable even with a dictaphone microcassette running at only 2.4 cm/s.
On the other hand, because of their inherent physical properties, they are far from perfect from the security standpoint. We explained almost everything here:
https://www.elektormagazine.com/news/secure-communications-an-interview-with-luka-matic
This device solves some problems of secure data management on SD cards:
https://www.elektormagazine.com/labs/secure-sd-card-2-sd-card-copier
Then why bother with obsolete cassette tape technology at all?
Thanks to the following properties, cassette tapes may still be more convenient than SDs:
1.) Easy to erase.
2.) Easy to monitor and control every sector of data.
3.) Easy to destroy.
The last property may be crucial in a spy scenario, where Alice must quickly destroy the codes when chased by Mallory or simply to prevent the dumpster-diving recovery of data by Eve! The PET-based tape melts at cca. 250°C and boils at 350°C, which is more practical than minimum 1300°C required for SSD.
Tape speed variation, wow and flutter are the main problems of cassette tape technology. This device uses a different approach - instead of precisely controlling the tape speed (which is very expensive in any aspect), the tape speed is constantly measured, and system clock frequency is varied accordingly. This way we can use any below-average tape deck, since this device can compensate for wow-flutter variations up to +/- 10% The only requirement is 200Hz-3000Hz frequency span which is achievable even with a dictaphone microcassette running at only 2.4 cm/s.
Discussion (1 comment)
Mark Harder 4 years ago
All I see here is the discussion of the features and advantages of storing SD to tape and links to an SD to SD secure copier and an interview with the author. But no specifics. How do I access the project description and explanation?
Reply
lux36 4 years ago
Dear Mark,
your interest for this project is appreciated. I have just uploaded firmware and schematics to "Project Elements" section. The article is not ready yet, because we are first planning to publish my SD-2-SD copier, and then this SD-2-tape copier.
As you can see on the picture, I made a fully working perfboard prototype, according to the schematics. The only thing yet to be completed is full FAT32 support.
For now, there is no PCB designed yet, but you can try building it on a perfboard or designing your own PCB.
The functions implemented in the present version of the firmware are as follows:
1 : SD card directory listing
5 : From/to terminal directly to/from TAPE
6 : Read file from SD to terminal
A : Copy file from SD to TAPE
9 : Read SD Memory Capacity (Total/Free)
A you can see, it is possible to copy a FAT32 file from SD to the TAPE (A:), and then read this file back to the terminal (6:) in text mode. You can also use (5:) for testing, sending text to the TAPE and reading it back to the terminal. I have the full FAT32.C set of functions, I only need to implement "B: Copy file form TAPE to SD", to record a FAT32 formatted file to the SD card. The present version of the firmware is all you need for testing and adjustments.
I hope you can see the description of operation in "Updates from the author" section. If you need any help (like for e.g. adjusting the trimmers), or additional explanations, don't hesitate to contact me through Labs any time.
your interest for this project is appreciated. I have just uploaded firmware and schematics to "Project Elements" section. The article is not ready yet, because we are first planning to publish my SD-2-SD copier, and then this SD-2-tape copier.
As you can see on the picture, I made a fully working perfboard prototype, according to the schematics. The only thing yet to be completed is full FAT32 support.
For now, there is no PCB designed yet, but you can try building it on a perfboard or designing your own PCB.
The functions implemented in the present version of the firmware are as follows:
1 : SD card directory listing
5 : From/to terminal directly to/from TAPE
6 : Read file from SD to terminal
A : Copy file from SD to TAPE
9 : Read SD Memory Capacity (Total/Free)
A you can see, it is possible to copy a FAT32 file from SD to the TAPE (A:), and then read this file back to the terminal (6:) in text mode. You can also use (5:) for testing, sending text to the TAPE and reading it back to the terminal. I have the full FAT32.C set of functions, I only need to implement "B: Copy file form TAPE to SD", to record a FAT32 formatted file to the SD card. The present version of the firmware is all you need for testing and adjustments.
I hope you can see the description of operation in "Updates from the author" section. If you need any help (like for e.g. adjusting the trimmers), or additional explanations, don't hesitate to contact me through Labs any time.
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Updates from the author
lux36 4 years ago
thank you for your interest. I have just uploaded schematics and software to "Project Elements" section, it should be OK now.
SD2tape_copier_328_2313_analog1.pdf (23kb)
DG4EK 4 years ago
thank you for uploading the soft and the circuit diagrams.
Will soon get into to create a printed board.
Best regards
Peter (DG4EK)
lux36 4 years ago
-ATmega328P x1 pcs - Heart of the system, takes commands from UART terminal and handles data exchange between SD and the tape, clocked from VCO.
-ATTiny2313 x1 pcs - measures the tape speed based on frequencies of audio signals recorded and quickly adjusts the VCO frequency- actually a simple PLL. Clocked by XTAL running at constant 10.00MHz, to enable precise real-time tape speed measurement and VCO adjustments.
-DAC0800 x1 pcs - reads 8 bits from ATTiny2313 outputs and sends analog input to VCO. A simple PWM on ATTiny2313 output without a DAC wouldn't work-it's too slow.
-74HC14 x1 pcs. - wired as a VCO RC oscillator provides 4.0-5.0 MHz system clock for ATmega328P and the FSK modem.
-TCM3105 x1 pcs. - FSK modem, clocked from VCO
-LM324 x3 pcs - used for analog signal processing - several analog filters and AGC - also needed to create a 200 Hz squarewave, a so the ATTiny2313 can measure the tape speed in real time.
When recording a file to the tape, one 200Hz sinewave signal is mixed with FSK signal carrying the data (700Hz-2700Hz span). When reading this file from the tape, 200Hz is filtered with a 2-stage peak filter built around the IC1-LM324 (frequency span 180Hz-220Hz), and brought to ATTiny2313 input PD2. A notch filter built around the IC2-LM324 will suppress the 200Hz signal, leaving the pure FSK at its output. IC3-LM324 is an AGC (automatic gain control) circuit used to stabilize the amplitude of FSK before passing it to TCM3105 for demodulation.
With tape speed variations well compensated, the reliable data storage is achieved.
SD2tape_copier_328_2313_analog1.pdf (23kb)
DG4EK 4 years ago
where can I get the software?
Thanks and
regards
Peter