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How it works

The Tiny Hardware OTP Authenticator is a self-contained digital authentication system implemented on the Tiny Tapeout platform.
It generates, stores, and verifies a 4-digit (16-bit) One-Time Password (OTP) using a Linear Feedback Shift Register (LFSR) and a Finite State Machine (FSM) that controls all authentication, retry, and timeout operations.

Functional Flow

The LFSR runs continuously, generating a new 16-bit pseudo-random OTP on every clock cycle.
When the otp_latch signal is pulsed, the current 16-bit value from the LFSR is stored as the active OTP for that session.
The user enters a 4-digit OTP attempt using user_in[3:0], one digit at a time.
Each digit is captured when user_latch is pulsed.
Once all 4 digits are entered, the FSM compares the entered OTP with the latched OTP and determines the outcome:
- ✅ U (Unlocked): Correct OTP entered
- ❌ L (Locked): Incorrect OTP (3 failed attempts)
- ⏰ E (Expired): 50 seconds of inactivity
The system also includes a display driver that alternates between OTP and system status every 2 seconds.

Display Information

Right 4 digits: User-entered OTP (USER_OTP)
Left 4 digits: Toggles every 2 seconds between:
- The generated LFSR OTP (for reference)
- The status format "A#-L/U/E", where:
  - A# → Attempt number (A1, A2, A3)
  - L/U/E → Lock / Unlock / Expire status

All information (LFSR OTP, user OTP, attempt count, and state) is shown on 8 multiplexed 7-segment displays, making this a standalone hardware authenticator.

Finite State Machine (FSM)

The FSM manages OTP generation, user input, comparison, and timeout handling.

State Encoding

State	Code	Description
IDLE	`2'b00`	Default/reset state; initializes all signals.
GENERATE_OTP	`2'b01`	Reads OTP from LFSR; stores when `otp_latch` is asserted.
ENTER_OTP	`2'b10`	Accepts 4-digit user input; detects expiry after 50s inactivity.
UNLOCK	`2'b11`	Compares OTPs and determines lock/unlock; increments attempt counter.

State Transitions

From	Condition	To
IDLE	`reset = 1`	`GENERATE_OTP`
GENERATE_OTP	`otp_latch = 1`	`ENTER_OTP`
ENTER_OTP	4 user entries complete	`UNLOCK`
ENTER_OTP	Timeout > 50s	`IDLE (EXPIRED)`
UNLOCK	OTP match	`IDLE (UNLOCKED)` after 5s
UNLOCK	OTP mismatch < 3 attempts	`ENTER_OTP`
UNLOCK	3rd mismatch	`IDLE (LOCKED)` after 5s

At any state, if reset = 0, the system resets to IDLE.

Timing and Clock Details

Signal	Frequency	Description
clk (Master Clock)	50 MHz	Drives LFSR and FSM logic
Display Multiplexing Clock	2 kHz (0.5 ms)	Controls 7-segment display scanning
Display Toggle Clock	0.5 Hz (2 sec)	Toggles between OTP and status display
Expire Timer	Derived from master clock	Detects 50-second inactivity
Hold Timer	Derived from master clock	Holds result for 5 seconds for readability

How to test

Power on / Reset → The LFSR starts generating random 16-bit OTPs.
Latch OTP: Pulse otp_latch to store the current OTP for authentication.
Enter User OTP:
- Use user_in[3:0] to set each 4-bit digit (0–9).
- Pulse user_latch to confirm each digit entry.
After entering all four digits:
- FSM compares the OTP and displays the result on the left 4 displays:
  - A#-U → Unlocked
  - A#-L → Locked
  - A#-E → Expired
The system automatically returns to IDLE after 5 seconds of displaying results.
Reset the system or latch a new OTP to restart.

External hardware

Component	Description	Purpose
8× Seven-Segment Display	Common cathode, connected to `lfsr_out`, `user_out`, and `an`	Displays OTP and user input/status
Push Buttons / Switches	Connected to `otp_latch` and `user_latch`	For OTP latch and digit entry
Clock (50 MHz)	Onboard or external	Main system timing
Reset Button	Connected to `reset_n`	Resets FSM and regenerates OTP

Tip: During simulation, display values can be viewed in waveform format instead of physical LEDs.

Signal Specifications

Signal	Direction	Width	Description
`clk`	Input	1	System clock
`reset_n`	Input	1	Active-low asynchronous reset
`user_in`	Input	4	4-bit user digit input
`otp_latch`	Input	1	Captures current LFSR OTP
`user_latch`	Input	1	Captures user-entered digit
`lfsr_out`	Output	7	Decoded LFSR output for 7-segment display
`user_out`	Output	7	Decoded user output for 7-segment display
`an`	Output	2	Anode control for multiplexed display

End of Document

#	Input	Output	Bidirectional
0	user_in[0]	lfsr_out[0]	user_out[0]
1	user_in[1]	lfsr_out[1]	user_out[1]
2	user_in[2]	lfsr_out[2]	user_out[2]
3	user_in[3]	lfsr_out[3]	user_out[3]
4	otp_latch	lfsr_out[4]	user_out[4]
5	user_latch	lfsr_out[5]	user_out[5]
6	Unused	lfsr_out[6]	user_out[6]
7	Unused	an[1]	an[0]

620 Tiny Hardware Authentication Engine

620 : Tiny Hardware Authentication Engine