Pseudorandom Black Swans: Cache Attacks on CTR_DRBG

Abstract

Modern cryptography requires the ability to securely generate pseudorandom numbers. However, despite decades of work on side-channel attacks, there is little discussion of their application to pseudorandom number generators (PRGs). In this work we set out to address this gap, empirically evaluating the side-channel resistance of common PRG implementations.We find that hard-learned lessons about side-channel leakage from encryption primitives have not been applied to PRGs, at all abstraction levels. At the design level, the NIST-recommended CTR_DRBG does not have forward security if an attacker is able to compromise the state (e.g., via a side-channel). At the primitive level, popular implementations of CTR_DRBG such as OpenSSL’s FIPS module and NetBSD’s kernel use leaky T-table AES as their underlying cipher, enabling cache side-channel attacks. Finally, we find that many implementations make parameter choices that enable an attacker to fully exploit side-channels and recover secret keys from TLS connections.We empirically demonstrate our attack in two scenarios. First, we carry out a cache attack that recovers the private state from vulnerable CTR_DRBG implementations when the TLS client connects to an attacker-controlled server. We then subsequently use the recovered state to compute the client’s long-term authentication keys, thereby allowing the attacker to impersonate the client. In the second scenario, we show that an attacker can exploit the high temporal resolution provided by Intel SGX to carry out a blind attack to recover CTR_DRBG’s state within three AES encryptions, without viewing output, and thus decrypt passively collected TLS connections from the victim.