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)