In the mix column stage when multiplying the byte value by 2 the following gives the correct answer. If the MSB of the byte value is 1, then left shift the byte value by 1 bit and then XOR the resultant with 27. What is the logic behind this number 27. How was it arrived at.
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2 Answers
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It is because the values being calculated are the byte representation of a polynomial in a finite field, and if the polynomial is too large to be in the field it must be reduced.
The reduction polynomial for Rijndael is $x^8 + x^4 + x^3 + x + 1$, which has a byte representation of 0x11b, and after you have shifted the byte by 1 bit, the 0x100 is no longer required so 0x1b or 27 is XOR'd against the value to complete the reduction.
answered Jun 28, 2018 at 8:11
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your question is unclear.
27 is represent as 0x1b. this is irreducible polynomial in GF(2^8).
what do you mean how was it arrived at? Do you the approach of choosing 0x1b as irreducible polynomial ? I suggest you read the book " The Design of Rijndael - AES - The Advanced Encryption Standard " to understand the multiplication in galois field.
