Consider AES with a 128-bit block length and a 128-bit key length. What is the output of the first round of AES if the plaintext consists of 128 ones and the first subkey also consists of 128 zeros?

Question

High School

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Answer :

ZaraArmstrong ZaraArmstrong · Answer 1 · 2025-03-30T16:43:20-04:00

your data is all 1s each byte is FF

so we arrange it in 4X4 matrix

FF FF FF FF

first we do a subbytes so in AES we look in S-Box substitution table. FF substitution is 16

so after subbytes data becomes

16 16 16 16

now we do a shift row. however since the data is same in all rows. the output is

16 16 16 16

now we do a shift column. however since the data is same in all columns. the output is

16 16 16 16

the KEY is

FF FF FF FF

now to complete the first round we do a add round key which is bitwise XOr

E9 E9 E9 E9

Answer is E9 E9 E9 E9 E9 E9 E9 E9 E9 E9 E9 E9 E9 E9 E9 E9

What is a matrix?

In mathematics, a matrix is a rectangular array of numbers or symbols arranged in rows and columns.
Matrices are widely used in various fields of mathematics, science, engineering, and computer science.

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