path: root/aes.c

/*

This is the implementation of the AES128 algorithm, specifically ECB mode.

The implementation is verified against the test vectors in:
	National Institute of Standards and Technology Special Publication 800-38A 2001 ED

ECB-AES128
----------

	plain-text:
		6bc1bee22e409f96e93d7e117393172a
		ae2d8a571e03ac9c9eb76fac45af8e51
		30c81c46a35ce411e5fbc1191a0a52ef
		f69f2445df4f9b17ad2b417be66c3710

	key:
		2b7e151628aed2a6abf7158809cf4f3c

	resulting cipher
		50fe67cc996d32b6da0937e99bafec60
		d9a4dada0892239f6b8b3d7680e15674
		a78819583f0308e7a6bf36b1386abf23
		c6d3416d29165c6fcb8e51a227ba994e


Use aes_encrypt_string() to encrypt or decrypt a string

NOTE: 	String length must be evenly divisible by 16byte (str_len % 16 == 0)
				You should pad the end of the string with zeros if this is not the case.

*/

#ifndef _AES_C_
#define _AES_C_


/*****************************************************************************/
/* Includes:                                                                 */
/*****************************************************************************/
#include <string.h>
#include <stdint.h>
#include "aes.h"


/*****************************************************************************/
/* Defines:                                                                  */
/*****************************************************************************/
// The number of columns comprising a state in AES. This is a constant in AES. Value=4
#define Nb 4
// The number of 32 bit words in a key.
#define Nk 4
// Key length in bytes [128 bit]
#define keyln 16
// The number of rounds in AES Cipher.
#define Nr 10


/*****************************************************************************/
/* Private variables:                                                        */
/*****************************************************************************/
// in    - pointer to the CipherText to be decrypted.
// out   - pointer to buffer to hold output of the decryption.
// state - array holding the intermediate results during decryption.
static uint8_t* in, *out, state[4][4];

// The array that stores the round keys.
static uint8_t RoundKey[176];

// The Key input to the AES Program
static uint8_t* Key;

// Marked const so it can be placed in read-only storage instead of RAM
// The numbers below can be computed dynamically trading ROM for RAM - 
// This can be useful in (embedded) bootloader applications, where ROM is often limited.

static const uint8_t sbox[256] =   {
	//0     1    2      3     4    5     6     7      8    9     A      B    C     D     E     F
	0x63, 0x7c, 0x77, 0x7b, 0xf2, 0x6b, 0x6f, 0xc5, 0x30, 0x01, 0x67, 0x2b, 0xfe, 0xd7, 0xab, 0x76,
	0xca, 0x82, 0xc9, 0x7d, 0xfa, 0x59, 0x47, 0xf0, 0xad, 0xd4, 0xa2, 0xaf, 0x9c, 0xa4, 0x72, 0xc0,
	0xb7, 0xfd, 0x93, 0x26, 0x36, 0x3f, 0xf7, 0xcc, 0x34, 0xa5, 0xe5, 0xf1, 0x71, 0xd8, 0x31, 0x15,
	0x04, 0xc7, 0x23, 0xc3, 0x18, 0x96, 0x05, 0x9a, 0x07, 0x12, 0x80, 0xe2, 0xeb, 0x27, 0xb2, 0x75,
	0x09, 0x83, 0x2c, 0x1a, 0x1b, 0x6e, 0x5a, 0xa0, 0x52, 0x3b, 0xd6, 0xb3, 0x29, 0xe3, 0x2f, 0x84,
	0x53, 0xd1, 0x00, 0xed, 0x20, 0xfc, 0xb1, 0x5b, 0x6a, 0xcb, 0xbe, 0x39, 0x4a, 0x4c, 0x58, 0xcf,
	0xd0, 0xef, 0xaa, 0xfb, 0x43, 0x4d, 0x33, 0x85, 0x45, 0xf9, 0x02, 0x7f, 0x50, 0x3c, 0x9f, 0xa8,
	0x51, 0xa3, 0x40, 0x8f, 0x92, 0x9d, 0x38, 0xf5, 0xbc, 0xb6, 0xda, 0x21, 0x10, 0xff, 0xf3, 0xd2,
	0xcd, 0x0c, 0x13, 0xec, 0x5f, 0x97, 0x44, 0x17, 0xc4, 0xa7, 0x7e, 0x3d, 0x64, 0x5d, 0x19, 0x73,
	0x60, 0x81, 0x4f, 0xdc, 0x22, 0x2a, 0x90, 0x88, 0x46, 0xee, 0xb8, 0x14, 0xde, 0x5e, 0x0b, 0xdb,
	0xe0, 0x32, 0x3a, 0x0a, 0x49, 0x06, 0x24, 0x5c, 0xc2, 0xd3, 0xac, 0x62, 0x91, 0x95, 0xe4, 0x79,
	0xe7, 0xc8, 0x37, 0x6d, 0x8d, 0xd5, 0x4e, 0xa9, 0x6c, 0x56, 0xf4, 0xea, 0x65, 0x7a, 0xae, 0x08,
	0xba, 0x78, 0x25, 0x2e, 0x1c, 0xa6, 0xb4, 0xc6, 0xe8, 0xdd, 0x74, 0x1f, 0x4b, 0xbd, 0x8b, 0x8a,
	0x70, 0x3e, 0xb5, 0x66, 0x48, 0x03, 0xf6, 0x0e, 0x61, 0x35, 0x57, 0xb9, 0x86, 0xc1, 0x1d, 0x9e,
	0xe1, 0xf8, 0x98, 0x11, 0x69, 0xd9, 0x8e, 0x94, 0x9b, 0x1e, 0x87, 0xe9, 0xce, 0x55, 0x28, 0xdf,
	0x8c, 0xa1, 0x89, 0x0d, 0xbf, 0xe6, 0x42, 0x68, 0x41, 0x99, 0x2d, 0x0f, 0xb0, 0x54, 0xbb, 0x16 };

// The round constant word array, Rcon[i], contains the values given by 
// x to th e power (i-1) being powers of x (x is denoted as {02}) in the field GF(2^8)
// Note that i starts at 1, not 0).
static const uint8_t Rcon[255] = {
	0x8d, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0x1b, 0x36, 0x6c, 0xd8, 0xab, 0x4d, 0x9a, 
	0x2f, 0x5e, 0xbc, 0x63, 0xc6, 0x97, 0x35, 0x6a, 0xd4, 0xb3, 0x7d, 0xfa, 0xef, 0xc5, 0x91, 0x39, 
	0x72, 0xe4, 0xd3, 0xbd, 0x61, 0xc2, 0x9f, 0x25, 0x4a, 0x94, 0x33, 0x66, 0xcc, 0x83, 0x1d, 0x3a, 
	0x74, 0xe8, 0xcb, 0x8d, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0x1b, 0x36, 0x6c, 0xd8, 
	0xab, 0x4d, 0x9a, 0x2f, 0x5e, 0xbc, 0x63, 0xc6, 0x97, 0x35, 0x6a, 0xd4, 0xb3, 0x7d, 0xfa, 0xef, 
	0xc5, 0x91, 0x39, 0x72, 0xe4, 0xd3, 0xbd, 0x61, 0xc2, 0x9f, 0x25, 0x4a, 0x94, 0x33, 0x66, 0xcc, 
	0x83, 0x1d, 0x3a, 0x74, 0xe8, 0xcb, 0x8d, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80, 0x1b, 
	0x36, 0x6c, 0xd8, 0xab, 0x4d, 0x9a, 0x2f, 0x5e, 0xbc, 0x63, 0xc6, 0x97, 0x35, 0x6a, 0xd4, 0xb3, 
	0x7d, 0xfa, 0xef, 0xc5, 0x91, 0x39, 0x72, 0xe4, 0xd3, 0xbd, 0x61, 0xc2, 0x9f, 0x25, 0x4a, 0x94, 
	0x33, 0x66, 0xcc, 0x83, 0x1d, 0x3a, 0x74, 0xe8, 0xcb, 0x8d, 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 
	0x40, 0x80, 0x1b, 0x36, 0x6c, 0xd8, 0xab, 0x4d, 0x9a, 0x2f, 0x5e, 0xbc, 0x63, 0xc6, 0x97, 0x35, 
	0x6a, 0xd4, 0xb3, 0x7d, 0xfa, 0xef, 0xc5, 0x91, 0x39, 0x72, 0xe4, 0xd3, 0xbd, 0x61, 0xc2, 0x9f, 
	0x25, 0x4a, 0x94, 0x33, 0x66, 0xcc, 0x83, 0x1d, 0x3a, 0x74, 0xe8, 0xcb, 0x8d, 0x01, 0x02, 0x04, 
	0x08, 0x10, 0x20, 0x40, 0x80, 0x1b, 0x36, 0x6c, 0xd8, 0xab, 0x4d, 0x9a, 0x2f, 0x5e, 0xbc, 0x63, 
	0xc6, 0x97, 0x35, 0x6a, 0xd4, 0xb3, 0x7d, 0xfa, 0xef, 0xc5, 0x91, 0x39, 0x72, 0xe4, 0xd3, 0xbd, 
	0x61, 0xc2, 0x9f, 0x25, 0x4a, 0x94, 0x33, 0x66, 0xcc, 0x83, 0x1d, 0x3a, 0x74, 0xe8, 0xcb  };


/*****************************************************************************/
/* Private functions:                                                        */
/*****************************************************************************/
static uint8_t getSBoxValue(uint8_t num)
{
	return sbox[num];
}

// This function produces Nb(Nr+1) round keys. The round keys are used in each round to decrypt the states. 
static void KeyExpansion()
{
	uint8_t i, j, k;
	uint8_t tempa[4]; // used for the column/row operations
	
	// The first round key is the key itself.
	for(i = 0; i < Nk; ++i)
	{
		RoundKey[ i * 4 ]     = Key[ i * 4 ];
		RoundKey[(i * 4) + 1] = Key[(i * 4) + 1];
		RoundKey[(i * 4) + 2] = Key[(i * 4) + 2];
		RoundKey[(i * 4) + 3] = Key[(i * 4) + 3];
	}

	// All other round keys are found from the previous round keys.
	for(; (i < (Nb * Nr) + 1); ++i)
	{
		for(j = 0; j < 4; ++j)
		{
			tempa[j]=RoundKey[(i-1) * 4 + j];
		}
		if (i % Nk == 0)
		{
			// This function rotates the 4 bytes in a word to the left once.
			// [a0,a1,a2,a3] becomes [a1,a2,a3,a0]

			// Function RotWord()
			{
				k = tempa[0];
				tempa[0] = tempa[1];
				tempa[1] = tempa[2];
				tempa[2] = tempa[3];
				tempa[3] = k;
			}

			// SubWord() is a function that takes a four-byte input word and 
			// applies the S-box to each of the four bytes to produce an output word.

			// Function Subword()
			{
				tempa[0] = getSBoxValue(tempa[0]);
				tempa[1] = getSBoxValue(tempa[1]);
				tempa[2] = getSBoxValue(tempa[2]);
				tempa[3] = getSBoxValue(tempa[3]);
			}

			tempa[0] =  tempa[0] ^ Rcon[i/Nk];
		}
		else if (Nk > 6 && i % Nk == 4)
		{
			// Function Subword()
			{
				tempa[0] = getSBoxValue(tempa[0]);
				tempa[1] = getSBoxValue(tempa[1]);
				tempa[2] = getSBoxValue(tempa[2]);
				tempa[3] = getSBoxValue(tempa[3]);
			}
		}
		RoundKey[ i * 4 ]     = RoundKey[ i - Nk  * 4 ]      ^ tempa[0];
		RoundKey[(i * 4) + 1] = RoundKey[(i - Nk) * (4 + 1)] ^ tempa[1];
		RoundKey[(i * 4) + 2] = RoundKey[(i - Nk) * (4 + 2)] ^ tempa[2];
		RoundKey[(i * 4) + 3] = RoundKey[(i - Nk) * (4 + 3)] ^ tempa[3];
	}
}

// This function adds the round key to state.
// The round key is added to the state by an XOR function.
static void AddRoundKey(uint8_t round) 
{
	uint8_t i,j;
	for(i=0;i<4;i++)
	{
		for(j = 0; j < 4; ++j)
		{
			state[j][i] ^= RoundKey[round * Nb * 4 + i * Nb + j];
		}
	}
}

// The SubBytes Function Substitutes the values in the
// state matrix with values in an S-box.
static void SubBytes()
{
	uint8_t i, j;
	for(i = 0; i < 4; ++i)
	{
		for(j = 0; j < 4; ++j)
		{
			state[i][j] = getSBoxValue(state[i][j]);
		}
	}
}

// The ShiftRows() function shifts the rows in the state to the left.
// Each row is shifted with different offset.
// Offset = Row number. So the first row is not shifted.
static void ShiftRows()
{
	uint8_t temp;

	// Rotate first row 1 columns to left	
	temp        = state[1][0];
	state[1][0] = state[1][1];
	state[1][1] = state[1][2];
	state[1][2] = state[1][3];
	state[1][3] = temp;

	// Rotate second row 2 columns to left	
	temp        = state[2][0];
	state[2][0] = state[2][2];
	state[2][2] = temp;

	temp = state[2][1];
	state[2][1] = state[2][3];
	state[2][3] = temp;

	// Rotate third row 3 columns to left
	temp = state[3][0];
	state[3][0] = state[3][3];
	state[3][3] = state[3][2];
	state[3][2] = state[3][1];
	state[3][1] = temp;
}

static uint8_t xtime(uint8_t x)
{
	return ((x<<1) ^ (((x>>7) & 1) * 0x1b));
}

// MixColumns function mixes the columns of the state matrix
static void MixColumns()
{
	uint8_t i;
	uint8_t Tmp,Tm,t;
	for(i = 0; i < 4; ++i)
	{	
		t   = state[0][i];
		Tmp = state[0][i] ^ state[1][i] ^ state[2][i] ^ state[3][i] ;
		Tm  = state[0][i] ^ state[1][i] ; Tm = xtime(Tm); state[0][i] ^= Tm ^ Tmp ;
		Tm  = state[1][i] ^ state[2][i] ; Tm = xtime(Tm); state[1][i] ^= Tm ^ Tmp ;
		Tm  = state[2][i] ^ state[3][i] ; Tm = xtime(Tm); state[2][i] ^= Tm ^ Tmp ;
		Tm  = state[3][i] ^ t ; Tm = xtime(Tm); state[3][i] ^= Tm ^ Tmp ;
	}
}

// Cipher is the main function that encrypts the PlainText.
static void Cipher()
{
	uint8_t i, j, round = 0;

	//Copy the input PlainText to state array.
	for(i = 0; i < 4; ++i)
	{
		for(j = 0; j < 4 ; ++j)
		{
			state[j][i] = in[(i * 4) + j];
		}
	}

	// Add the First round key to the state before starting the rounds.
	AddRoundKey(0); 
	
	// There will be Nr rounds.
	// The first Nr-1 rounds are identical.
	// These Nr-1 rounds are executed in the loop below.
	for(round = 1; round < Nr; ++round)
	{
		SubBytes();
		ShiftRows();
		MixColumns();
		AddRoundKey(round);
	}
	
	// The last round is given below.
	// The MixColumns function is not here in the last round.
	SubBytes();
	ShiftRows();
	AddRoundKey(Nr);

	// The encryption process is over.
	// Copy the state array to output array.
	
	for(i = 0; i < 4; ++i)
	{
		for(j = 0; j < 4; ++j)
		{
			out[(i * 4) + j] = state[j][i];
		}
	}
}


/*****************************************************************************/
/* Public functions:                                                         */
/*****************************************************************************/

void AES128_ECB(uint8_t* input, uint8_t* key, uint8_t *output)
{
	// Copy the Key and CipherText
	Key = key;
	in = input;
	out = output;

	// The KeyExpansion routine must be called before encryption.
	KeyExpansion();

	// The next function call encrypts the PlainText with the Key using AES algorithm.
	Cipher();
}


#endif //_AES_C_