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InvMixColumn() optimization #159

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InvMixColumn() optimization #159

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51013b3
InvMixColumn() optimization
dmitrystu Mar 7, 2020
ea77d6f
Update feat/optimize_inv_mix from master
dmitrystu Dec 22, 2021
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62 changes: 23 additions & 39 deletions aes.c
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Original file line number Diff line number Diff line change
Expand Up @@ -55,16 +55,6 @@ NOTE: String length must be evenly divisible by 16byte (str_len % 16 == 0)
#define Nr 10 // The number of rounds in AES Cipher.
#endif

// jcallan@github points out that declaring Multiply as a function
// reduces code size considerably with the Keil ARM compiler.
// See this link for more information: https://github.com/kokke/tiny-AES-C/pull/3
#ifndef MULTIPLY_AS_A_FUNCTION
#define MULTIPLY_AS_A_FUNCTION 0
#endif




/*****************************************************************************/
/* Private variables: */
/*****************************************************************************/
Expand Down Expand Up @@ -312,29 +302,6 @@ static void MixColumns(state_t* state)
}
}

// Multiply is used to multiply numbers in the field GF(2^8)
// Note: The last call to xtime() is unneeded, but often ends up generating a smaller binary
// The compiler seems to be able to vectorize the operation better this way.
// See https://github.com/kokke/tiny-AES-c/pull/34
#if MULTIPLY_AS_A_FUNCTION
static uint8_t Multiply(uint8_t x, uint8_t y)
{
return (((y & 1) * x) ^
((y>>1 & 1) * xtime(x)) ^
((y>>2 & 1) * xtime(xtime(x))) ^
((y>>3 & 1) * xtime(xtime(xtime(x)))) ^
((y>>4 & 1) * xtime(xtime(xtime(xtime(x)))))); /* this last call to xtime() can be omitted */
}
#else
#define Multiply(x, y) \
( ((y & 1) * x) ^ \
((y>>1 & 1) * xtime(x)) ^ \
((y>>2 & 1) * xtime(xtime(x))) ^ \
((y>>3 & 1) * xtime(xtime(xtime(x)))) ^ \
((y>>4 & 1) * xtime(xtime(xtime(xtime(x)))))) \

#endif

#if (defined(CBC) && CBC == 1) || (defined(ECB) && ECB == 1)
/*
static uint8_t getSBoxInvert(uint8_t num)
Expand All @@ -350,18 +317,35 @@ static uint8_t getSBoxInvert(uint8_t num)
static void InvMixColumns(state_t* state)
{
int i;
uint8_t a, b, c, d;
uint8_t a, b, c, d, T, X;
for (i = 0; i < 4; ++i)
{
a = (*state)[i][0];
b = (*state)[i][1];
c = (*state)[i][2];
d = (*state)[i][3];

(*state)[i][0] = Multiply(a, 0x0e) ^ Multiply(b, 0x0b) ^ Multiply(c, 0x0d) ^ Multiply(d, 0x09);
(*state)[i][1] = Multiply(a, 0x09) ^ Multiply(b, 0x0e) ^ Multiply(c, 0x0b) ^ Multiply(d, 0x0d);
(*state)[i][2] = Multiply(a, 0x0d) ^ Multiply(b, 0x09) ^ Multiply(c, 0x0e) ^ Multiply(d, 0x0b);
(*state)[i][3] = Multiply(a, 0x0b) ^ Multiply(b, 0x0d) ^ Multiply(c, 0x09) ^ Multiply(d, 0x0e);
// Let "*" denotes polynomial multiplication modulo x^4+1 over GF(2^8)
// Let "+" denotes polynimial addition over GF(2^8) (XOR)
// xtime(X) is X*2
// A' = (A*8 + A*4 + A*2) + (B*8 + B*2 + B) + (C*8 + C*4 + C) + (D*8 + D)
// B' = (A*8 + A) + (B*8 + B*4 + B*2) + (C*8 + C*2 + C) + (D*8 + D*4 + D)
// C' = (A*8 + A*4 + A) + (B*8 + B) + (C*8 + C*4 + C*2) + (D*8 + D*2 + D)
// D' = (A*8 + A*2 + A) + (B*8 + B*4 + B) + (C*8 + C) + (D*8 + D*4 + D*2)
// Let T = (A + B + C + D)*8 + (A + B + C + D)
// Then
// A' = T + (A + C)*4 + (A + B)*2 + A
// B' = T + (B + D)*4 + (B + C)*2 + B
// C' = T + (C + A)*4 + (C + D)*2 + C
// D' = T + (D + B)*4 + (D + A)*2 + D
// Let X = (A + C)*4 or (B + D)*4
T = a ^ b ^ c ^ d;
T ^= xtime(xtime(xtime(T)));
X = xtime(xtime(a ^ c));
(*state)[i][0] = T ^ X ^ xtime(a ^ b) ^ a;
(*state)[i][2] = T ^ X ^ xtime(c ^ d) ^ c;
X = xtime(xtime(b ^ d));
(*state)[i][1] = T ^ X ^ xtime(b ^ c) ^ b;
(*state)[i][3] = T ^ X ^ xtime(d ^ a) ^ d;
}
}

Expand Down