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][1] has 128 _bytes_ of RAM  
[PIC12F508][2] has 50 bytes of RAM (16 bytes for key, 34 bytes free)  
[PIC10F202][3] has 24 bytes of RAM (16 bytes for key, 8 bytes free)


  [1]: http://www.ti.com/product/msp430g2001
  [2]: http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en020094
  [3]: http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en020030