Let’s Take it Offline: Boosting Brute-Force Attacks on iPhone’s User Authentication through SCA2021
Research Hub B: Eingebettete Sicherheit
RC 6: Next-Generation Implementation Security
In recent years, smartphones have become an increasingly importantstorage facility for personal sensitive data ranging from photos and credentials upto financial and medical records like credit cards and person’s diseases. Trivially,it is critical to secure this information and only provide access to the genuine andauthenticated user. Smartphone vendors have already taken exceptional care toprotect user data by the means of various software and hardware security featureslike code signing, authenticated boot chain, dedicated co-processor and integratedcryptographic engines with hardware fused keys. Despite these obstacles, adversarieshave successfully broken through various software protections in the past, leavingonly the hardware as the last standing barrier between the attacker and user data.In this work, we build upon existing software vulnerabilities and break throughthe final barrier by performing the first publicly reported physical Side-ChannelAnalysis (SCA) attack on an iPhone in order to extract the hardware-fused device-specific User Identifier (UID) key. This key – once at hand – allows the adversary toperform an offline brute-force attack on the user passcode employing an optimizedand scalable implementation of the Key Derivation Function (KDF) on a GraphicsProcessing Unit (GPU) cluster. Once the passcode is revealed, the adversary has fullaccess to all user data stored on the device and possibly in the cloud.As the software exploit enables acquisition and processing of hundreds of millions oftraces, this work further shows that an attacker being able to query arbitrary manychosen-data encryption/decryption requests is a realistic model, even for compactsystems with advanced software protections, and emphasizes the need for assessingresilience against SCA for a very high number of traces.