421 QAMER CryptoUART: Encrypted UART with LED Status :: Quicker, easier and cheaper to make your own chip!

How it works

QAMER CryptoUART is an encrypted UART demonstration that receives serial data, echoes the received plaintext byte, and then transmits an encrypted version of the same byte.

The design operates at a default clock frequency of 50 MHz and a UART baud rate of 9600.

The 8-bit ui_in input is used as follows:

ui_in[0] is the UART RX input.
ui_in[7:1] provides a 7-bit key seed used to initialize the LFSR.

After reset, the LFSR is initialized from the 7-bit key seed. For each received UART character, the current 8-bit LFSR keystream value is XORed with the received byte:

ciphertext = plaintext XOR keystream

The design then:

Receives a UART byte on ui_in[0].
Generates the corresponding encrypted byte using the LFSR keystream.
Echoes the original plaintext byte through uo_out[0].
Transmits the encrypted ciphertext byte through uo_out[0].
Updates the LFSR for the next received character.
Updates the four LED status outputs after each received character.

The four LED outputs build up as characters are received:

Character 1: 0001
Character 2: 0011
Character 3: 0111
Character 4: 1111

After four characters, the LED sequence starts again with the next group.

Additional status signals are provided on uo_out:

uo_out[5]: RX valid pulse
uo_out[6]: UART TX busy status
uo_out[7]: Ciphertext valid pulse

The encrypted byte is also continuously available on uio_out[7:0]. The uio_oe signal is fixed to 8'hFF, so all eight uio pins operate as outputs.

How to test

Connect a UART transmitter to ui_in[0] and use the default UART settings:

Clock: 50 MHz
Baud rate: 9600
Data: 8 bits
Start bit: 1
Stop bit: 1

Provide the 7-bit encryption key through ui_in[7:1]. The key is sampled when the design initializes after reset.

For example, the supplied functional test uses:

KEY_SEED = 7'h55

and transmits the characters:

A (0x41)
B (0x42)

For each character, the design first sends the plaintext byte back through the UART TX output and then sends the corresponding encrypted byte.

The expected ciphertext is calculated using the LFSR keystream:

ciphertext = plaintext XOR keystream

The ciphertext can also be observed directly on uio_out[7:0].

The four LED outputs indicate the number of characters received within the current group of four. The RX-valid and ciphertext-valid signals can be used as one-cycle indicators when the corresponding data becomes available.

The automated Cocotb testbench verifies reset behavior, UART plaintext echo, ciphertext generation, LFSR progression, LED status, the live ciphertext output, and the uio_oe configuration.

External hardware

No external hardware is required for simulation.

For physical demonstration, a standard USB-to-UART adapter can be connected to the UART RX/TX signals. The four LED status outputs can be connected to LEDs through appropriate current-limiting circuitry if the target hardware does not already provide LEDs.

The encryption key is provided through the seven available ui_in[7:1] input pins.

#	Input	Output	Bidirectional
0	UART_RXD	UART_TXD	CIPHERTEXT[0]
1	KEY_SEED[0]	LED[0]	CIPHERTEXT[1]
2	KEY_SEED[1]	LED[1]	CIPHERTEXT[2]
3	KEY_SEED[2]	LED[2]	CIPHERTEXT[3]
4	KEY_SEED[3]	LED[3]	CIPHERTEXT[4]
5	KEY_SEED[4]	RX_VALID	CIPHERTEXT[5]
6	KEY_SEED[5]	TX_BUSY	CIPHERTEXT[6]
7	KEY_SEED[6]	CIPHER_VALID	CIPHERTEXT[7]

421 QAMER CryptoUART: Encrypted UART with LED Status