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I'm really in trouble! I'm trying to understand how the T-tables in AES encryption work. But I don't know if I get the point. What I understood is that they are used to reduce the whole computation of the iteration of AES just looking at the T-boxes and the XOR operation. Now, what I got is that:

    State                 M Matrix            
┏             ┓       ┏             ┓       
┃ d4 e0 b8 1e ┃       ┃ 02 03 01 01 ┃        
┃ bf b4 41 27 ┃       ┃ 01 02 03 01 ┃        
┃ 5d 52 11 98 ┃       ┃ 01 01 02 03 ┃        
┃ 30 ae f1 e5 ┃       ┃ 03 01 01 02 ┃        
┗             ┛       ┗             ┛       

I have my State matrix and M matrix composed by 4X4 bytes. Each t-table should receive in input one byte and give in output 4 bytes. Now, in order to create the t-tables i will have in the first table:

T0=(d4*02)+(d4*01)+(d4*01)+(d4*03)
   (e0*02)+(e0*01)+(e0*01)+(e0*03)
   (b8*02)+(b8*01)+(b8*01)+(b8*03)
   (1e*02)+(1e*01)+(1e*01)+(1e*03)

Then the second table should be:

T0=(bf*03)+(bf*02)+(bf*01)+(bf*01)
   (b4*03)+(b4*02)+(b4*01)+(b4*01)
   (41*03)+(41*02)+(41*01)+(41*01)
   (27*03)+(27*02)+(27*01)+(27*01)

And so on...until I'll have 4 tables. Am I right? Now...How can I use these tables?

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1 Answer 1

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(You should take a look at page 18 of FIPS 197 where it describes the MixColumns transform).

You're close. Swap the order of your matrices so that you have:

|02 03 01 01| |d4 e0 b8 1e|
|01 02 03 01| |bf b4 41 27|
|01 01 02 03| |5d 52 11 98|  
|03 01 01 02| |30 ae f1 e5|

And then you compute the new columns. i.e. the new first column is:

|02 03 01 01| |d4|
|01 02 03 01| |bf|
|01 01 02 03| |5d|
|03 01 01 02| |30|

Which equals:

|02*d4 + 03*bf + 01*5d + 01*30|
|01*d4 + 02*bf + 03*5d + 01*30|
|01*d4 + 01*bf + 02*5d + 03*30|
|03*d4 + 01*bf + 01*5d + 02*30|

And so on.

Do you See the pattern in the sum? Each column in the matrix is multiplied by an element in the column and then they are summed. We can precompute these columns, and compute the above sum using 4 table lookups and 32-bit XOR operations.

The tables will be arrays of 32-bit values which look like this ("|" denotes byte concatenation):

T0[00] = 02*00 | 01*00 | 01*00 | 03*00
T0[01] = 02*01 | 01*01 | 01*01 | 03*00
...
T0[FF] = 02*FF | 01*FF | 01*FF | 03*FF


T1[00] = 03*00 | 02*00 | 01*00 | 01*00
T1[01] = 03*01 | 02*01 | 01*01 | 01*01
...
T1[FF] = 03*FF | 02*FF | 01*FF | 01*FF


T2[00] = 01*00 | 03*00 | 02*00 | 01*00
T2[01] = 01*01 | 03*00 | 02*00 | 01*00
...
T2[FF] = 01*FF | 03*FF | 02*FF | 01*FF


T3[00] = 01*00 | 01*00 | 03*00 | 02*00
T3[01] = 01*01 | 01*01 | 03*01 | 02*01
...
T3[FF] = 01*FF | 01*FF | 03*FF | 02*FF

Using four tables, you can compute the new state matrix with 16 table look-ups and 12 32-bit XOR operations.

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