811 LED Bitserial Cipher :: Quicker, easier and cheaper to make your own chip!

How it works

tt09-led-serial is a nibble-serial implementation of the LED block cipher, proposed in 2012 and defined in The LED Block Cipher by J. Guo et. al. The cipher encrypts a 64-bit block of plaintext with a 128-bit key into a 64-bit block of ciphertext. The nibble-serial implementation enables a very compact implementation as most of the datapath logic can be reused over each nibble. The downside is that such nibble-serial implementations have a much larger latency. The nibble-serial architecture shown below was presented and analyzed earlier in Differential Fault Intensity Analysis on PRESENT and LED Block Ciphers by N. F. Galathy et. al.

To further reduce the I/O pinout constraints, this design also serializes the data-input (64 bit plaintext and 128 bit key) as well as the data-output (64 bit ciphertext).

Activity	Cycles
Load Plaintext	64
Load Key	128
Read Ciphertext	64
Encrypt	2045

The module is controlled through the bits of the input word ui_in. The serial data format is MSB to LSB. That is, given a block of plaintext 0x0123..., the bits would be shift in as in the bitstring 0b0000000100100011....

Bit	Name	Function
7-6	unused	NA
5	start	Assert to start encryption
4	getct	Assert to shift out ciphertext bit
3	loadkey	Assert to shift in key bit
2	loadpt	Assert to shift in plaintext bit
1	keyi	Key input bit
0	datai	Plaintext input bit

The results are generation on the output word uo_out.

Bit	Name	Function
7-2	unused	NA
1	done	1 indicates encryption complete
0	dataq	Ciphertext output bit

LIMITATIONS

This design forces the key bits to 0 upon loading, so that the effective key value of the cipher is always hardcoded to 00000000_00000000_00000000_00000000. This disables the use of the design as a cipher, yet it still demonstrates how a nibble-serial architecture can be designed.

How to test

This block could be tested with some integration on a Raspberry PI to control ui_in and uo_out. The typical sequence of operation is as follows.

Wait until done == 1, which indicates that the cipher is idle
Assert loadkey, and shift in key bits. Repeat 128 times. De-assert loadkey.
Assert loadpt, and shift in plaintext bits. Repeat 64 times. De-assert loadpt.
Assert start for one clock cycle.
Wait until done == 1.
Assert getct and shift out ciphertext bits. Repeat 64 times. De-assert getct.

Here are twotthree sample test vectors. Consult the testbench for additional test vectors.

Plaintext	Key	Ciphertext
0000000000000000	00000000000000000000000000000000	3decb2a0850cdba1
0123456789abcdef	00000000000000000000000000000000	da261393c73be9ce
12153524c0895e81	00000000000000000000000000000000	29db5fe262572f4e

External hardware

You will need external hardware to use the block cipher.

#	Input	Output
0	ui_in[0]	uo_out[0]
1	ui_in[1]	uo_out[1]
2	ui_in[2]	uo_out[2]
3	ui_in[3]	uo_out[3]
4	ui_in[4]	uo_out[4]
5	ui_in[5]	uo_out[5]
6	ui_in[6]	uo_out[6]
7	ui_in[7]	uo_out[7]

811 LED Bitserial Cipher