MIDTERM EXAM PART 2

THE ENCRYPTION

The final stages of EAS Encryption

When encrypting with AES, we only need to multiply by the Galois fields for 1, 2 and 3. The interesting part about this multiplication, it is already done for us. It is everywhere on the internet. I will show you the tables in a few minutes.

A few things to remember –

1. Multiply a Galois field by a Galois field for 1 – you get the same thing. It is like multiplying a number by 1. You get the same number back. 2. Multiply by two or three are different. The original values des change. We can multiply or we can use the look up tables. 3. There are two tables that are available MUL2 and MUL3. 4. Remember the resulting values have to be MOD 100011011 Do you remember why? This is the reducer to make the result fit in a byte

Let’s look at the word ‘what’ and apply AES to it

W is a 57 in HEX – represented by 0x57 h is a 68 in HEX – represented by 0x68 a is a 61 in HEX – represented by 0x61 t is a 74 in HEX - – represented by 0x74

Now we have to take the DOT PRODUCT and it is complex. 0x57 0x68 [ 0x02 0x03 0x01 0x01 ] 0x61 0x74

5 = 0101 (in bits) 7 = 0111 (in bits) 57 = 01010111 (Or use your scientific calculator)

NOW How about the 02? -> (10)

Now multiple the 01010111 by 10 (0x57 times 0x02 - HEX) (remember you did this before)

X^6 + x^4 + x^2 + x^1 + x^0 multiply by X^1

0 1 1 2 2 3 4 5 6 7

Now we count How many

7 – 1

6 -- 0 5 -- 1 4 -- 0 3 -- 1 2 -- 1 1 – 0

So 0x57 DOT 0x02 = 1 0 1 0 1 1 0

Now do you need to reduce

THE ANSWER IS NO.

Why because the bits fit in a BYTE

NO REDUCER NEEDED.