Don’t Push It: Breaking iButton Security

Abstract

Maxims iButtons are small portable (steel) tokens that can be attached to objects (e.g., keys, fobs) and are deployed in various applications from access control to devices and buildings to asset management and electronic cash. So far, the security and privacy aspects of iButtons have been widely unexplored. The so-called Secure iButtons are advocated for security critical applications for e.g., micropayment, authentication or feature activation.

In this paper we present for the first time a detailed security analysis of the Secure iButtons DS1963S. Although no technical details are publicly available, Secure iButtons have a variety of physical and cryptographic built-in measures to protect against physical tampering as well as unauthorized access to cryptographic material. We developed methods to bypass all these protection mechanisms of the manufacturer. We present a differential fault attack and implementation attack on the SHA-1-enabled iButton (DS1963S chip). Beside the emulation and impersonation, our attacks succeed in extracting the secret keys stored in the iButton. Our methods allow an infinite rollback to the initial state, which is crucial when targeting micropayment systems based on iButtons. We also demonstrate our attacks on Maxims reference platform of a micropayment system. Our best attack requires a minimal financial invest and take less than ten minutes, including target preparation, while the pure attack on all eight 64-bit keys is completed in a few seconds.

Appendices

Attack Related Commands

For our attack, we need a set of relevant 1-wire low-level commands. Those commands are Read Scratchpad, Write Scratpad, Copy Scratchpad, Erase Scratchpad, Read Authenticated Page, Compute First Secret and Compute Next Secret.

Furthermore, the cryptographic engine has seven SHA-1 command functions, namely the Compute First Secret, Compute Next Secret command for generation of new secrets, SignDataPage for signing and Validate Dat aPagefor HMAC verification. Furthermore, the engine offers a set of function to perform random generator operations and generation of challenges. Finally, the command Read Authenticated Page combines a read operation with a CRC and generation of the corresponding HMAC result. For a detailed description of the commands, we refer the interested reader to the iButton standard [8] and [26].

Resolved Secrets S0-S7 (Example)

The following table give an example for resolved secret $S0$ - $S7$ (Fig. 10).

Fig. 10.

Resolved secrets S0-S7 from an iButton DS1963S device

Tamper Protection Milling Aparatus with iButton

Fig. 11.

Milling apparatus with iButton

Akbil Micropayment System for Electronic Ticketing

The Akbil system [29] is an integrated micropayment for electronic tickets used for fare payment in public transport of Istanbul, Turkey. The system is currently being phased out, but still in use. Figure 12 shows a SCU access control gate to the metropolitan transportation system.

Fig. 12.

Akbil SCU access control gate (left) and Ticketing Machine (right)

ROM-ID Reverse Engineering

The figure shows the reverse engineered ROM-ID of a Secure iButton. The ROM-ID layout is reconstructed by the following drawing. Purple spots indicate a laser burned bit lane, representing a set bit value.