How can the encryption/decryption operation work if the tables are irreversible? (Justify the answer clearly)
The substitution tables in a Feistel network may be irreversible.
1. True/False. Justify your answer.
2. How can the encryption/decryption operation work if the tables are irreversible? (Justify the answer clearly)
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Ans :
a) GCD of 2938 and 183 using Eucledian
2938=183⋅16+10
183=10⋅18+3
10=3⋅3+1
3=1⋅3+0
Hence GCD(2938,183)=1
b)a) GCD of 93182 and 246 using Eucledian algorithm
93182=246⋅378+194
246=194⋅1+52
194=52⋅3+38
52=38⋅1+14
38=14⋅2+10
14=10⋅1+4
10=4⋅2+2
4=2⋅2+0
GCD(93182,246)=2
-> a inverse b exist in case of first option because there GCD(2938,183)=1 it will be
55≡2938−1(mod183)
55≡2938−1(mod183)
55×2938≡1(mod183)
Hence 55 is the inverse
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