A New Secure Stream Cipher for Scan Chain Encryption

Abstract

The accessibility to the internal IP cores of Systems on Chip (SoC) provided by the testing infrastructures is a serious security threat. It has been known for many years that the scan chains can be exploited to retrieve secret keys of cryptoprocessors. Encryption of the scan chain content is one of the proposed techniques to overtake this threat. Many proposals are based on stream ciphers, due to their moderate area cost compared to that of block ciphers. Stream ciphers encrypt data serially with a keystream generated from an Initialization Vector (IV) and a secret key. Stream ciphers have a crucial limitation concerning the encryption of different data with the same keystream, called two times pad. Not enough caution in the IV and secret key management has been exercised in previous proposed works. In this paper, we show how the existing implementations can be exploited to perform a scan attack bypassing the encryption of the scan data. We also present a new implementation of scan chain encryption with a stream cipher, based on the IV generation by a True Random Number Generator (TRNG). Finally, we show that this new implementation is robust against the aforementioned attack.

ScanSAT: Unlocking Static and Dynamic Scan Obfuscation

“…Further, ScanSAT can be thwarted by encrypting/decrypting the stimulus and the response by using provably secure techniques [37]; but this requires using dedicated, and thus, costly on-chip cipher blocks. Another approach would be to utilize PUFs as in [38] at the expense of reliability concerns associated with PUFs.…”

“…Bias PUF based scan [38] ScanSAT-resilient large circuits such as s38584 and b19 (even with a key-size of 128). For s38584, the only benchmark common to our and their work, the NSAA tool reportedly crashed [24].…”

Stream vs block ciphers for scan encryption

“…In this section, we first present a SESC [19] implementation that does not present the vulnerability of the state-of-the-art solutions discussed so far (see Section V.A). After that, we describe a SEBC [23] implementation using lightweight block ciphers.…”

“…The present SESC countermeasure has been proposed in [19]. Compared to previous solutions, the IV value is randomly generated by a True Random Number Generator (TRNG) at every circuit reset, thus preventing two-time pad attacks.…”

“…The choice must be driven by performance and security trade-offs. The preferred mechanism in the literature ( [9], [17]- [19]) for encrypting the scan network is stream ciphering, because block ciphers present a larger area and have to be adapted to cope with the serial nature of the exchanged data with the ATE (Automatic Test Equipment). Nevertheless, as shown in Section IV, stream ciphers may introduce vulnerabilities.…”