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With such a small block size there is no way to employ RSA padding modes such as PKCS#1 v1.5 padding or OAEP. You could however see the encryption as ECB mode encryption. In that case you could apply padding mechanisms that have been constructed for symmetric block ciphers. Those padding modes however have been defined for bytes rather than characters. ...


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First since E is an encryption algorithm, it has a Decryption counterpart, lets name it D. From the correcntess equation we get that $E(x,D(x,c)) = c$ for every $x$. Then we can easily see that if $$y= x \oplus D(x,c)$$ then $$f_2(x,y) = E(x, y \oplus x) = E(x, x \oplus D(x,c) \oplus x) = E(x,D(x,c)) = c$$ Again this holds for every $x$. So we have just ...



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