Secure iButton

Project Secure iButton
Status	In progress
Contact	User:Bertrik Sikken
Last Update	2013-01-13

This project is about investigating and using the secure features of the iButton that people currently use for physical access to RevSpace.

Planned project phases are:

• phase 1: investigate possbilities of the iButton and experiment with it
• phase 2: write software for the iButton functionality and package it into a library
• phase 3: apply knowledge and software for application within RevSpace

iButton investigation

The iButton used at RevSpace is the DS1961, with the following features:

• 4 pages of 32-byte user data each
• an 8-byte write-only "secret"
• SHA-1 algorithm that can calculate a hash over a 32-byte user data page, the 8-byte secret, a 3-byte "challenge" and the unique iButton id.

Basically authentication could work like this:

• (the secret key has been installed on the iButton previously)
• user presents iButton to the reader
• reader reads the unique iButton id
• reader generates a random number and uses this as the challenge to read a 32-byte user page
• iButton sends the user page, followed by the SHA-1 hash
• reader also calculates SHA-1 hash and compares it with the iButton hash
• if the hash matches, the user data can be considered as authentic and the reader can interpret the user data as a "deelnemer id" (or something) for example.

Bus pirate experimentation

Investigation of the DS1961-specific commands can be done with a Bus Pirate. A bus pirate speaks the 1-wire protocol that any iButton-like device uses.

Examples of bus pirate commands to communicate with the DS1961:

• Initialise the bus pirate

  # (reset the bus pirate)
  M (select mode)
  2 (1-wire mode)
  W (enable power)

• Reset the iButton and get iButton unique id

  (240)

• Write scratchpad (command 0x0F) at address 0,0 with data 1 2 3 4 5 6 7 8

  0x0f 0 0 1 2 3 4 5 6 7 8 r:2

• Read scratchpad (command 0xAA)

  0xaa r:3 r:8 r:2

• Read auth page (command 0xA5), which is the secure read command

  0xa5 0 0 r:32 r r:2 &:1500 r:20 r:2

Example response:

1-WIRE>0xa5 0 0 r:32 r r:2 &:1500 r:20 r:2
WRITE: 0xA5
WRITE: 0x00
WRITE: 0x00
READ 0x20 BYTES:
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
READ: 0xFF
READ 0x02 BYTES:
0x6D 0x0D
DELAY 0xDCuS
READ 0x14 BYTES:
0x45 0x31 0x40 0x42 0xAE 0xD3 0x4B 0xC5 0x18 0xCC 0x10 0x43 0x27 0x56 0x33 0xD0 0xEB 0x47 0xA7 0x81
READ 0x02 BYTES:
0xAA 0x6D

DS1961 library

This library provides an API for the DS1961 specific functions. It will be targeted at the Arduino microcontroller, to run on top of the existing 1-wire library.

Application

We could apply this to improve the security of RevSpace access for example. to be discussed.

Stuff to be considered for access control:

• an extra initialisation step to install the secret before use is needed
• how can we handle the transition of the current iButton system to a more secure system?
• how advanced do we make the authentication scheme / how is key management handled?
  • Simplest: one secret for all revspace iButtons. Disadvantage: cracking one iButton means that *all* revspace iButtons are cracked.
  • More advanced: different secret for each revspace iButton (e.g. calculated from a master secret and the iButton serial number, or just generated every time and stored)

Concrete proposal

The following protocol extension for communication with the Arduino is proposed.

On the Arduino side, the following command/response is added:

• an extra command to send the 3-byte Challenge (in hex-ascii), example

C AB34EF

• a corresponding Response to return the 32-byte data and the 20-byte hash, example

R 0000000000000000000000000000000000000000000000000000000000000000 D54A4731DC5EDF675D7B0A23A6D83534FAE12E3F

TODO: what to respond if communication with the ibutton fails somehow?

On the server side, the access control sequence is extended as follows:

• receive an ibutton id from the Arduino
• lookup the id in the access control list
• if the id is not present, don't open the door
• if the id has no associated secret, open the door
• if the id has an associated secret, generate the 3-byte challenge and perform the challenge/response sequence
• if the response matches the expected response, open the door

External Links

Maxim iButton 1-wire public domain kit