aes

Verilog implementation of the symmetric block cipher AES (NIST FIPS 197).

Status

The core is completed, has been used in several FPGA and ASIC designs. The core is well tested and mature.

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.

Branches

There are several branches available that provides different versions of the core. The branches are not planned to be merged into master. The branches available that provides versions of the core are:

on-the-fly-keygen

This version of AES implements the key expansion using an on-the-fly mechanism. This allows the initial key expansion to be removed. This saves a number of cycles and also remove almost 1800 registers needed to store the round keys. Note that this version of AES only supports encryption. On-the-fly key generation does not work with decryption. Decryption must be handled by the block cipher mode - for example CTR.

dual-keys

This version of AES supports two separate banks of expanded keys to allow fast key switching between two keys. This is useful for example in an AEAD mode with CBC + CMAC implemented using a single AES core.

cmt-sbox

An experimental version of the core in which the S-box is implemented using circuit minimized logic functions of a ROM table. The specific table used is the 113 gate circuit by the CMT team at Yale.

Some area and performance results using the cmt_sbox compared to master.

Altera

• Tool: Quartus Prime 19.1.0

• Device: Cyclone V (5CGXFC7C7F23C8)

• master (S-box implemented with a table)

  • ALMs: 2599
  • Regs: 3184
  • Fmax: 93 MHz
  • aes_sbox: 160 ALUTs

• cmt_sbox

  • ALMs: 2759
  • Regs: 3147
  • Fmax: 69 MHz
  • aes_sbox: 363 ALUTs

Xilinx

• Tool: Vivado 2019.2

• Device: Kintex-7 (7k70tfbv676-1)

• master:

  • LUTs: 3020
  • FFs: 2992
  • Fmax: 125 MHz

• cmt_sbox:

  • LUTs: 2955
  • FFs: 2992
  • Fmax: 105 MHz

Core Usage

Usage sequence:

1. Load the key to be used by writing to the key register words.
2. Set the key length by writing to the config register.
3. Initialize key expansion by writing a one to the init bit in the control register.
4. Wait for the ready bit in the status register to be cleared and then to be set again. This means that the key expansion has been completed.
5. Write the cleartext block to the block registers.
6. Start block processing by writing a one to the next bit in the control register.
7. Wait for the ready bit in the status register to be cleared and then to be set again. This means that the data block has been processed.
8. Read out the ciphertext block from the result registers.

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

Microchip IGLOO 2

• Tool: Libero v 12.4
• Device: M2GL090TS-1FG484I
• LUTs: 6335
• SLEs: 1376
• BRAMs: 8
• Fmax: 98.5 MHz

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