What does the highlighted expression mean
Looks like E is a function with arguement M. Let M be 00. Hence, E(00) equals to plaintext letter A. But how can the following expression can be true: A ≡ 3 (modulo 26) ? Can you please explain that?
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2$E$ maps $0\to 3,,$ i.e. $A\to D$. As they say, encryption $E$ cyclically (right) rotates the alphabet by $3$ places $(A\mapsto D,, B\mapsto E,\ldots),$ so its inverse decryption $D$ cyclically (left) rotates by $3,,$ Note that each letter is encoded by its index in the alphabet, and the cryption functions work with the indices. – Bill Dubuque Dec 31 '21 at 10:30
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@BillDubuque and I still cannot understand, sorry. Function $E(00)$ returns plaintext letter A. But the right side of highlighted expression is a number. How can the letter and a number be compared? – MathsEnjoyer Dec 31 '21 at 11:31
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No, as I said the encryption and decription functions work with integers mod $26$ which are the codes representing the letters (here their index in the alphabet). Here $E(0) = 3$ – Bill Dubuque Dec 31 '21 at 18:06