MICKEY

In cryptography, Mutual Irregular Clocking KEYstream generator (MICKEY) is a stream cipher algorithm developed by Steve Babbage and Matthew Dodd.^[1] The cipher is designed to be used in hardware platforms with limited resources, and was one of the three ciphers accepted into Profile 2 of the eSTREAM portfolio. The algorithm is not patented and is free for any use.^[2]

The cipher maps an 80-bit key and a variable length initialization vector (0 to 80 bits) to a keystream with a maximum length of 2⁴⁰ bits.

The keystream generator makes use of two registers R and S (100 bits each). The registers are updated in a non-linear manner using the control variables: INPUT BIT R, INPUT BIT S, CONTROL BIT R, CONTROL BIT S. As referred to earlier, any implementation of the cipher contains flip-flops for the R, S registers and the 4 control variables. Furthermore, there must be 7 flip-flops for the counter register to keep track of the number of rounds in the Preclock stage. The keystream production stage in MICKEY 2.0 is preceded by the three stages:- IV Loading, Key Loading and Preclock. Initially the R, S registers are initialized to the all zero state.

Unlike Trivium, MICKEY 2.0 ^[3] does not allow direct loading of Key and IV bits on to the state register. As mentioned earlier, initially the R, S registers are initialized to the all zero state. Then a variable length IV and the 80 bit Key is used to update the state by successively executing CLOCK KG routine.

MICKEY 2.0 can be protected by an XOR-CHAIN structure. The attacker has the following advantages:

• He knows the algorithm of MICKEY 2.0
• He can use Initial Vectors of his own choice.
• The key remains secret.
• He can SCAN-IN and SCAN-OUT vectors as per his choice.

To hide the mapping between the scan cells and the actual variables of a cipher is what drove the previous single-feedback and Double-Feedback XOR-Chain schemes. As this is also falling prey to cryptanalysis, as shown in the previous section, we move towards a further secure architecture, named as random XOR-Chain (rXOR-Chain) structure.

The Flipped-Scan countermeasure technique to protect scan-chains was proposed earlier. This involved placing inverters at random points in the scan-chain. Security stemmed from the fact that an adversary could not guess the number and positions of the inverters. This technique was cryptanalyzed using a RESET attack. It was shown that if all flip-flops in the scan-chain are initially RESET, then the positions of the inverters can be completely determined by the 0 → 1 and 1 → 0 transitions in the scanned-out vector. As an alternative, the XOR-CHAIN based countermeasure was proposed. The technique involves placing XOR gates at random points of the chain.^[4] Security again stems from the fact that an adversary is unable to guess the number and positions of the XOR gates.

Scan-based DFT is the most widely used DFT scheme for integrated circuit testing as it is simple and yields high fault coverage. The advantage of scan-based testing is that it provides full observability and controllability of the internal nodes of the IC.

As of 2013, a differential fault attack has been reported against MICKEY 2.0 by Subhadeep Banik and Subhamoy Maitra.^[5]

• eStream page on MICKEY
• A Differential Fault Attack on MICKEY 2.0
• Scan-chain based Attacks
• Hardware implementation
• FPGA implementations

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