MOSQUITO

The MOSQUITO cipher has eight registers of varying lengths, let's call the register CCSR -${\displaystyle a^{{\mathcal {h}}0{\mathcal {i}}}}$, the first register -${\displaystyle a^{{\mathcal {h}}1{\mathcal {i}}}}$, second -${\displaystyle a^{{\mathcal {h}}2{\mathcal {i}}}}$ and so on up to the seventh register -${\displaystyle a^{{\mathcal {h}}7{\mathcal {i}}}}$. We will designate the i-th position of register j as follows: ${\displaystyle a_{i}^{{\mathcal {h}}j{\mathcal {i}}}}$. Register lengths:

CCSR — 128 bits;

${\displaystyle a^{{\mathcal {h}}1{\mathcal {i}}}}$ — ${\displaystyle a^{{\mathcal {h}}5{\mathcal {i}}}}$ 53 bits;

${\displaystyle a^{{\mathcal {h}}6{\mathcal {i}}}}$ — 12 bits;

${\displaystyle a^{{\mathcal {h}}7{\mathcal {i}}}}$ — 3 bits.

The essence of the cipher operation is to calculate, for each clock cycle, the bits of any of the registers (except CCSR) based on some combination of bits of the previous register. The CCSR register works as a shift register: the register elements are shifted, and a bit of the encrypted text (from the cipher output) is written to the zero position of the CCSR register. Let us denote by ${\displaystyle G_{i}^{j}}$ the rule by which the bit in the i-th position in register j is calculated. Then:

${\displaystyle G_{4i~mod~53}^{1}=a_{128-i}^{{\mathcal {h}}0{\mathcal {i}}}+a_{18+i}^{{\mathcal {h}}0{\mathcal {i}}}+a_{113-i}^{{\mathcal {h}}0{\mathcal {i}}}(a_{1+i}^{{\mathcal {h}}0{\mathcal {i}}}+1)+1}$, где ${\displaystyle 0\leqslant i<53}$;

${\displaystyle G_{4i~mod~53}^{j}=a_{i}^{{\mathcal {h}}j-1{\mathcal {i}}}+a_{3+i}^{{\mathcal {h}}j-1{\mathcal {i}}}+a_{1+i}^{{\mathcal {h}}j-1{\mathcal {i}}}(a_{2+i}^{{\mathcal {h}}j-1{\mathcal {i}}}+1)+1}$, где ${\displaystyle 0\leqslant i<53}$ и ${\displaystyle 2\leqslant j\leqslant 5}$, if the subscript of any element on the right side of the equality becomes greater than 53, then this element is replaced by 0;

${\displaystyle G_{i}^{6}=a_{4i}^{{\mathcal {h}}5{\mathcal {i}}}+a_{3+4i}^{{\mathcal {h}}5{\mathcal {i}}}+a_{1+4i}^{{\mathcal {h}}5{\mathcal {i}}}+a_{2+4i}^{{\mathcal {h}}5{\mathcal {i}}}}$, где ${\displaystyle 0\leqslant i<12}$;

${\displaystyle G_{i}^{7}=a_{4i}^{{\mathcal {h}}6{\mathcal {i}}}+a_{3+4i}^{{\mathcal {h}}6{\mathcal {i}}}+a_{1+4i}^{{\mathcal {h}}6{\mathcal {i}}}(a_{2+4i}^{{\mathcal {h}}6{\mathcal {i}}}+1)+1}$, где ${\displaystyle 0\leqslant i<3}$;

and finally the keystream bit ${\displaystyle z=a_{0}^{{\mathcal {h}}7{\mathcal {i}}}+a_{1}^{{\mathcal {h}}7{\mathcal {i}}}+a_{2}^{{\mathcal {h}}7{\mathcal {i}}}}$.

It is worth noting that the calculation of register bits is performed using combinational logic, and the shift, naturally, using register logic, which means that in order to prevent incorrect operation of the pipeline, when the bits from the register do not have time to be processed by combinational logic, it is necessary that the function ${\displaystyle G_{i}^{j}}$, implementing the calculations was relatively simple.