Acksess_2.0

This is the second version of the Acksess project.
The goal is to create an RFID based door access system with MiFare classic 1K cards.
It also implements a mitigation of the vulnerability of MiFare classic.

Installation

Setting the masterKey

In acksess_2.0.ino, uncomment the masterKey variable and change it to something secret. All keys to lock the tags are derived from this master key, so don't lose it. Without it you can't recover your tags anymore should you want to use them for something else.

Bootstrapping the flash storage with the first admin tag

To bootstrap the flash storage, adjust your loop() in acksess_2.0.ino See the comments there for more details.

Securely flashing the firmware

Once you've got a tag added and have confirmed everything is working, it's a good idea to lock your Arduino so the firmware can't be read back anymore. Otherwise you'll be able to read the masterKey by connecting it via USB and dumping the Arduino flash. You need and ISP for this. The USBASP is pretty cheap on aliexpress, or you could use a second arduino (https://www.arduino.cc/en/pmwiki.php?n=Tutorial/ArduinoISP). The below examples assume you're using an USBASP

Open the Arduino Sketch
Sketch -> Export compiled binary
Browse terminal to arduino sketch location
Flash the hex using avrdude and usbasp: WITHOUT THE BOOTLOADER. Chip must be erased, so don't include -D switch!
e.g.: avrdude -c usbasp -v -p atmega328p -P usb -U flash:w:acksess_2.0.ino.eightanaloginputs.hex:i
Lock the flash using avrdude.
e.g.: avrdude -c usbasp -v -p atmega328p -P usb -D -U lock:w:0xC0:m
To reset lock bits (which also erases the chip):
e.g.: avrdude -c usbasp -v -p atmega328p -P usb -U lock:w:0xFF:m

Usage

As a user

Scan your tag to open the door. It's that easy!

As an admin

Connect via USB to the arduino with a serial console. (baud rate: 115200)
(We recommend one which has local echo and local line editing capabilities, or the Arduino Serial Monitor)
Send an empty command in order to receive the command list
Send "1" to log in as admin
Scan a tag with admin access in order to enter the command menu.
Send an empty command again to receive the admin command list. It should be pretty self explanatory.

Process flow when a tag is presented

A normal scan:

Tag is presented to reader
UID is read and looked up in flash storage
When found, the key is calculated (see below) in order to unlock block 1 of the tag
Block 1 gets unlocked
The first 4 bytes of the block (the secret) are read and compared to the value stored in flash
If all ok, a new 4 byte value is generated and written to both flash and tag
The door is opened.

Scan but UID not found

Tag is presented to reader
UID is read, but after searching through the entire flash storage (Takes a few seconds!), it comes up empty
Reader waits 5 seconds before accepting a new scan

Scan but unable to unlock block (or tag was removed during the process)

Tag is presented to reader
UID is read and looked up in flash storage
When found, the key is calculated (see below) in order to unlock block 1 of the tag
Block 1 fails to unlock
Readers waits 5 seconds before accepting new scan

Scan but secret failed to verify (or tag was removed during the process)

Tag is presented to reader
UID is read and looked up in flash storage
When found, the key is calculated (see below) in order to unlock block 1 of the tag
Block 1 gets unlocked
The first 4 bytes of the block are read and compared to the value stored in flash
Values to not match. Do not open the door.

Everything is OK, but the new secret failed to update

Tag is presented to reader
UID is read and looked up in flash storage
When found, the key is calculated (see below) in order to unlock block 1 of the tag
Block 1 gets unlocked
The first 4 bytes of the block (the secret) are read and compared to the value stored in flash
All ok, attempt to update secret on tag
Fails, write 0x00000000 as secret in flash
Do not open door.
Tag is presented to reader again
When found, the key is calculated (see below) in order to unlock block 1 of the tag
Block 1 gets unlocked
Reader sees that secret in flash is set to 0x00000000 and skips the check
New secret is generated and written to tag and flash
The door is opened

Key calculation

The key to be used it calculated by concatenating the master password with the UID of the tag and the sector number that is being locked. It then grabs a 6-byte SHA-512 hash from this.
This is used to lock every block of the tag in order to make it more time consuming to maliciously clone the tag

Beepcodes

* = short high beep
- = short medium beep
_ = short low beep
+ = medium low beep
0 = long low beep

* * * = Access granted
* 0 = UID not found. Access denied.
* _ _ _ _ = Unlocking block failed. Access denied.
* _ = Reading secret from tag failed. Access denied.
* + + + + = Secret on tag does not match secreton flash.
* * - = Tag authenticated, but new information failed to write. Access denied
* * - * = Read a tag which failed to update last time. Fixed it and access granted
* = Timeout passed and new tag can be scanned. Door closes if it was opened.

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acksess_2.0's Introduction