Kiberxavfsizlik asoslari

2. Asosiy qism. Simmetrik shifrlash usullarini axborotni himoyalashda ishlatish.
2.1. A5/1 shifrlash algoritmi haqida (oqimli)
GSM tarmog‘ida ovozli so‘zlashuvlarni shifrlash uchun foydalaniladi.
A5/1 oqimli shifrlash algoritmida 64 bitli 𝐾 kalitdan foydalaniladi.
A5/1 shifrlash algoritmi uchta chiziqli siljitish registorlaridan iborat bo‘lib, kiritilgan kalit 𝐾 ularga bo‘lib yoziladi:
𝑋 registor o‘zida 19 bit (𝑥₀,𝑥₁,…,𝑥₁₈)
𝑌 registor esa 22 bit (𝑦₀,𝑦₁,…,𝑦₂₁)
𝑍 registor esa 23 bit (𝑧₀,𝑧₁,…,𝑧₂₂)
Tasodifiy ketma-ketliklarni generatsiyalash
Berilgan uchta bit 𝑥₈,₁₀,𝑧₁₀ asosida 𝑚=𝑚𝑎𝑗(𝑥₈,𝑦₁₀,𝑧₁₀ ) funksiya qiymati hisoblanadi.
Masalan, (1,0,0)=0,𝑚𝑎𝑗(1,1,0)=1
Agar 𝑥₈=𝑚 ga teng bo‘lsa, 𝑋 siljitiladi:
𝑡=𝑥₁₃⨁𝑥₁₆⨁𝑥₁₇⨁𝑥₁₈
𝑖=18,17,16,…,1 uchun 𝑥_i=𝑥_(i-1)
𝑥₀=𝑡
Agar 𝑦₁₀=𝑚 ga teng bo‘lsa, 𝑌 siljitiladi:
𝑡=𝑦₂₀⨁𝑦₂₁
𝑖=21,20,19,…,1 uchun 𝑦_i=𝑦_(i-1)
𝑦₀=𝑡
Tasodifiy ketma-ketliklarni generatsiyalash
Agar 𝑧₁₀=𝑚 ga teng bo‘lsa, 𝑍 siljitiladi:
𝑡=𝑧₇⨁𝑧₂₀⨁𝑧₂₁⨁𝑧₂₂
𝑖=22,21,20,…,1 uchun 𝑧_i=𝑧_(i-1)
𝑧₀=𝑡
Shundan so‘ng tasodifiy ketma-ketlikning bir biti quyidagicha hisoblanadi:
𝑠=𝑥₁₈⨁𝑦₂₁⨁𝑧₂₂
Yuqoridagi ketma-ketlik ochiq matnni uzunligiga teng bitlarni generatsiya qilguncha takrorlanadi.

Bunga misol tariqasida Ismimning bosh harfini shifrlaydigan bo’lsam:
T = E
K = XORAZMIY
T = 0100
K = 0101100001001111010100100100000101011010010011010100100101011001
X: 0101100001001111010
Y: 1001001000001010110100
Z: 10011010100100101011001
1:
maj(0,0,0) = 0
X, Y va Z registorlar suriladi
t = = 1 0 1 0 = 0
t = = 0 0 = 0
t = = 0 0 0 1 = 1
X: 0010110000100111101
Y: 0100100100000101011010
Z: 11001101010010010101100
C = 1 0 0 0 = 1
2:
maj(0,0,0) = 0
X, Y va Z registorlar suriladi
t = = 1 1 0 1 = 1
t = = 1 0 = 1
t = = 1 1 0 0 = 0

X: 1001011000010011110
Y: 1010010010000010101101
Z: 01100110101001001010110
C = 0 1 0 1 = 0
3:
maj(0,0,1) = 0
X, Y registorlar suriladi
t = = 0 1 1 0 = 0
t = = 0 1 = 1
X: 0001011000010011110
Y: 1010010010000010101101
Z: 11001101010010010101100
C = 0 1 0 0 = 1
4:
maj(0,0,0) = 0
X, Y va Z registorlar suriladi
t = = 0 1 1 0 = 0
t = = 0 1 = 1
t = = 1 1 0 0 = 0

X: 0001011000010011110
Y: 1010010010000010101101
Z: 01100110101001001010110
C = 0 1 0 0 = 1
C = 1011