Description
Trivium's 288-bit internal state consists of three shift registers of different lengths. At each round, a bit is shifted into each of the three shift registers using a non-linear combination of taps from that and one other register; one bit of output is produced. To initialize the cipher, the key and IV are written into two of the shift registers, with the remaining bits starting in a fixed pattern; the cipher state is then updated 4 × 288 = 1152 times, so that every bit of the internal state depends on every bit of the key and of the IV in a complex nonlinear way.
No taps appear on the first 65 bits of each shift register, so each novel state bit is not used until at least 65 rounds after it is generated. This is the key to Trivium's software performance and flexibility in hardware.
Read more about this topic: Trivium (cipher)
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