# Master Key Memory Interface #
This core provides a 32-bit interface to a master key memory (MKM)
implemented using an external volatile memory. The memory targeted is
[Microchip 23K640](https://www.microchip.com/wwwproducts/en/23K640), a
serial SRAM with a SPI interface.


## Purpose and Functionality ##
The Master Key Memory is where a cryptographic master key is stored. the
key is used (for example) to cryptographically wrap other keys and
secrets. By wiping the MKM and thus the master key, the wrapped secrets
are protected against leakage to a local attacker that physically breaks
an active tamper detect shield.

The core will in future versions provide functionality to autonomously
protect against memory remanence effects by rotating bits in stored data
and moving data to different addresses in the external memory. The core
will also be able to autonomously zeroise the memory when given an alarm
signal.

The current version however simply provides an interface to the slower,
serial memory including initializing the memory in the correct mode. The
core supports three commands: read word, write word and initialize
memory.


## Limitations ##
The SPI clock is generated by the core clock (clk) divided by the
SPI clock divisor * 2 (the divisor is the half period in cycles). The
default divisor is set to generate an SPI clock of less than 1 MHz when
the core clock is 50 MHz. For other speeds and other
core frequencies the divisor will have to be adjusted.

The core will only read and write complete 32-bit words.

Commands given while the core is performing a read, write or
initialization operation will silently be ignored.


## Implementation ##
The implementation is divided into three parts:

- A SPI interface able to transmit a given number of bits at a given SPI
  clock rate. Data received are simultaneously collected and provided as
  read data. The SPI interface also generates the SPI clock and chip
  enable.

- A command handler core that read and write words as well as send
  init commands to the memory using the SPI interface.

- An API interface that provides the ability to configure the SPI clock
  speed, setting the address to be read or written and data access.


The current implementation will initiate the Microchip memory directly
after reset and set the memory in sequential mode. This means that it
would actually be possible to write a stream of data to the memory, but
since the API only handles a single 32-bit word, the mode is only used
to remove the need to update the address between bytes.


### Implementation Results ###

**Altera Cyclone IV E**

- Registers: 212
- Logic Elements: 289
- Fmax: 250 MHz


**Altera Cyclone V**

- Registers: 221
- ALMs: 113
- Fmax: 194 MHz


**Xilinx Spartan 6**

- Slice Registers: 206
- Slice LUTs: 185
- Fmax: 200 MHz


**Xilinx Artix 7**

- Slice Registers: 205
- Slice LUTs: 176
- Fmax: 383 MHz


## Status ##

**(2016-05-10)**

The core has now been verified in a Xilinx Spartan-6 FPGA and the target
Microchip memory connected to the FPGA.memory. Read and write access has
successfully been performed with SPI clock speeds from 300 Hz to 10 MHz.


**(2016-05-02)**

Functional development completed. Simulation based debugging
completed. Built design for both Altera and Xilinx FPGAs.


**(2016-04-25)**

Refactored core into top_-, core- and spi-modules. Made the design much
simpler. First implementation almost completed.


**(2016-04-21)**

Core implementation started.