If you want to use a micro (a small FPGA would work well also) then one with a dedicated peripheral would be good.
The STM32F407 series have a cryptographic processor peripheral capable of DES, Triple DES, AES 128, 192 or 256.
They also have a dedicated SD card peripheral (for 1, 4 or 8-bit interface) which allows transfer at up to 48MHz.
You can grab hold of the discovery F4 dev board very cheaply on Mouser (probably a few other places too)
Note I haven't used either of the peripherals yet, I've just noticed them whilst using the chip (which is very powerful for the price) I have used the PIC32 plenty also but this is a step up, my only grumble is ST don't provide an IDE (plenty to choose from though - Keil, IAR, Rowley, Atollic - we went for Raisonance) and their documentation/code examples are not as good as Microchips.