Exposure of Friet to Cryptanalysis and Side Channel Attacks. During a session, the outer state serves for in- and output and the inner state remains secret. A feature setting duplex apart from block cipher modes is the absence of a fixed key during operation. The state does depend on the key K, but evolves. Doing statistical (side channel) attacks, such as differential and linear cryptanalysis or DPA, require starting many sessions. If diversifier uniqueness is respected, these attacks are limited to absorbing of the diversifier D. In typical use cases, Friet would secure communication between devices that may both be accessible to attackers, such as IoT devices. We assume the two devices share a secret key K and can keep track of a session counter that serves as diversifier when a new session needs to be started. Whenever a session is started, one device (master) initiates the session and determines the session counter D and the other device (slave) follows and just must accept the session counter D. Consequently, the slave can be forced in starting a session multiple times with the same diversifier D. The slave can only be sure the session request comes from a valid device when verifying the session startup tag. If this tag is invalid, it can be a part of a denial of service attack, a statistical attack, or just corrupt due to a noisy communication channel. One typically offers protection against such attacks by having the slave keep track of two counters. The first of these two is the session counter and the slave only accepts session startup requests that have a higher session counter than any previously successful session. The second is a session retry counter. A successful session startup increments the session counter by 1 and resets the session retry counter to 0. An unsuccessful session startup just increments the session retry counter. If the session retry counter reaches some limit, the slave device refuses to use the key any longer. This limit shall be set to a value small enough to prevent an adversary to collect enough traces to conduct a statistical attack but large enough to still keep the session robust in the presence of noise communication. Another attack vector on the slave device is a fault attack. In such an attack, an adversary forces a slave to start multiple sessions with the same diversifier D and injects faults in at least one of it. She can then mount a differential fault attacks to extract information about the secret inner state from a single faultless output and faulted ones. This is w...
