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ma01c09rec5-7

# ma01c09rec5-7 - MA1C ANALYTIC RECITATION 1 AES I am not...

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MA1C ANALYTIC RECITATION 5/7/09 1. AES I am not sure whether you learned about vector spaces over fields other than R in math 1b. I thought you might be interested to know about the Advanced Encryption Standard (AES). Notice that the set { 0 , 1 } under regular multiplication and addition mod 2 is a field (often called GF(2)). Therefore, we can consider vector spaces over this field, and we can do linear algebra. Everything you learned in 1b is still true for linear algebra over GF(2), except stuff with eigenvalues can be weird. Among all computers in the world doing linear algebra right now, by far the majority will be doing linear algebra over GF(2) or another finite field like GF(2 8 ). One of the reasons for this is cryptographic computations, and in particular AES. If you know about RSA or Diffie-Hellman (public key cryptosystems) you may wonder why there is a need for a regular, symmetric key cryptosystem. The reason is that public key systems tend to be extremely costly computationally. Therefore, they are used for an initial exchange of secret information, from which a key is generated and used in a symmetric system, like AES. The way AES works is the following: the key is 128 bits, and data is encrypted in 128-bit blocks; think of it as a four by four array of bytes. A round of AES consists of doing an S-box, a shiftrows, a mixcolumns, and an addkey which are: S-box: This is a nonlinear function (usually done by table lookup) on each of the 16 bytes individually. Shiftrows: In this step, the rows of the 4 by 4 array are shifted to the left by their row index, i.e. the first (zeroth) row is left unchanged, the second row is shifted left one, the third twice, and the fourth 3 times. Mixcolumns: This is where the linear algebra comes in; each column of the array is thought of as a vector in GF(2 8 ) 4 and is multiplied by a matrix, the result of which replaces that column. Addkey: In the final step of the round, the key is added (remember this is mod 2, or xor) to the array bitwise.

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ma01c09rec5-7 - MA1C ANALYTIC RECITATION 1 AES I am not...

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