Is it possible to control a SRAM module via the GPIO pins?

Question

I would like to attach a SRAM module to the RasPi.

My idea is to store sensitive (encrypted) data on the SD card but keep the key on a SRAM module. It would have its own backup battery (or supercapacitor) and an anti-tamper switch that would erase the contents if the enclosure was opened.

Is there any way to interface to the SRAM with only the 8 available GPIO lines?

The SRAM module itself would need a capacity of at least 128 bits (the size of the smallest AES key) but even a 1-bit wide memory would seem to require 7 address lines and one data line plus 2 or 3 control lines.

Can it be done? I am thinking it may be possible to cheat by reusing an I/O line that was designed for something else.

Answer 1

To install SRAM, I would suggest using an I2C/SPI module (example, instead of using the GPIO lines. Although potentially slower, it means you have the other GPIO lines for other things and your SRAM can be quite large.

There is a big disadvantage to using SRAM though. Suppose you store some really valuable information on your RPi and I'm really mean. Suppose I could hack into your RPi and run arbitrary code, then I could retrieve your key from the SRAM, which is required to decrypt the data. This is a security risk. Alternatively, you could use a AVR/PIC coprocessor, which you communicate which over I2C/SPI/UART. Instead of doing the encryption/decryption on the RPi, you would send your data to the coprocessor and ensure that there is no instruction for sending the key to your RPi. There can, however, be an instruction for generating a new key, and of course, you could have several keys stored on the coprocessor. This could make a cool little project.

Answer 2

You could just use a small microcontroller to emulate the RAM over SPI or I2C.

All but the very smallest micros would have 128 bits of RAM to store the key

eg

MSP430G2001 has 128 bytes of RAM
PIC12F508 has 50 bytes of RAM (16 bytes for key, 34 bytes free)
PIC10F202 has 24 bytes of RAM (16 bytes for key, 8 bytes free)

If you really prefer serial RAM, they also exist

23K640 is 64kbits/2.7-3.6V

Answer 3

You are assuming that there are only 8 gpio lines available on the Pi. In fact it is possible to have up to 17.

I don't know the details of how to access them, but you can find more information here

http://elinux.org/RPi_Low-level_peripherals

Answer 4

You can use I²C SRAM modules, like the PCF8570P £1.50ish 256x8Bit SRAM. It operates between 2.5V and 6.0V, has a power saving mode that draws as little as 50nA and can be stacked to create more RAM if needed.

Hook it up to I2C directly or to a Master for multiple I2C device. Only uses 2 GPIO, Power and Negative, with the option of MCU power off RAM retention using a small battery.

Alex Chamberlains security concern should be taken seriously if you wanted to protect the Private Key. Possibly thinking about creating some kind of isolation in your code which requires escalation to access to the SRAM.