aes

Verilog implementation of the symmetric block cipher AES (Advanced Encryption Standard) as specified in the NIST document FIPS 197.

Introduction

This implementation supports 128 and 256 bit keys. The implementation is iterative and process one 128 block at a time. Blocks are processed on a word level with 4 S-boxes in the data path. The S-boxes for encryption are shared with the key expansion and the core can thus not do key update in parallel with block processing.

The encipher and decipher block processing datapaths are separated and basically self contained given access to a set of round keys and a block. This makes it possible to hard wire the core to only encipher or decipher operation. This allows the synthesis/build tools to optimize away the other functionality which will reduce the size to about 50%. This has been tested to verify that decryption is removed and the core still works.

For cipher modes such as CTR, CCM, CMAC, GCM the decryption functionality in the AES core will never be used and thus the decipher block processing can be removed.

This is a fairly compact implementation. Further reduction could be achived by just having a single S-box. Similarly the performane can be increased by having 8 or even 16 S-boxes which would reduce the number of cycles to two cycles for each round.

Status

The core is completed, has been used in FPGAs and in ASIC. The core is well tested and mature.

Implementation results - ASIC

The core has been implemented in standard cell ASIC processes.

TSMC 180 nm

Target frequency: 20 MHz Complete flow from RTL to placed gates. Automatic clock gating and scan insertion.

• 8 kCells
• Aera: 520 x 520 um
• Good timing margin with no big cells and buffers.

Implementation results - FPGA

The core has been implemented in Altera and Xilinx FPGA devices.

Altera Cyclone V GX

• 2624 ALMs
• 3123 Regs
• 96 MHz
• 46 cycles/block

Altera Cyclone IV GX

• 7426 LEs
• 2994 Regs
• 96 MHz fmax
• 46 cycles/block

This means that we can do more than 2 Mblocks/s or 256 Mbps performance.

Removing the decipher module yields:

• 5497 LEs
• 2855 Regs
• 106 MHz fmax
• 46 cycles/block

Xilinx Spartan6LX-3

• 2576 slices
• 3000 regs
• 100 MHz
• 46 cycles/block

Xilinx Artix 7 200T-3

• 2298 slices
• 2989 regs
• 97 MHz
• 46 cycles/block