Advanced Encryption Standard (AES) Implementation in C/C++ with comments. Part 2: Decryption

<< Part 1: Encryption

The AES decryption source code in C/C++ is given below. Comments are given in separate color.

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/*

******************************************************************

**       Advanced Encryption Standard implementation in C.      **

**       By Niyaz PK                                            **

**       E-mail: niyazlife@gmail.com                            **

**       Downloaded from Website: www.hoozi.com                 **

******************************************************************

This is the source code for decryption using the latest AES algorithm.

AES algorithm is also called Rijndael algorithm. AES algorithm is

recommended for non-classified use by the National Institute of Standards

and Technology(NIST), USA. Now-a-days AES is being used for almost

all encryption applications all around the world.

 

THE MAIN FEATURE OF THIS AES ENCRYPTION PROGRAM IS NOT EFFICIENCY; IT

IS SIMPLICITY AND READABILITY. THIS SOURCE CODE IS PROVIDED FOR ALL

TO UNDERSTAND THE AES ALGORITHM.

 

Comments are provided as needed to understand the program. But the

user must read some AES documentation to understand the underlying

theory correctly.

 

It is not possible to describe the complete AES algorithm in detail

here. For the complete description of the algorithm, point your

browser to:

http://www.csrc.nist.gov/publications/fips/fips197/fips-197.pdf

 

Find the Wikipedia page of AES at:

http://en.wikipedia.org/wiki/Advanced_Encryption_Standard

******************************************************************

*/

 

// Include stdio.h for standard input/output.

// Used for giving output to the screen.

#include<stdio.h>

 

// The number of columns comprising a state in AES. This is a constant in AES. Value=4

#define Nb 4

 

// The number of rounds in AES Cipher. It is simply initiated to zero. The actual value is recieved in the program.

int Nr=0;

 

// The number of 32 bit words in the key. It is simply initiated to zero. The actual value is recieved in the program.

int Nk=0;

 

// in – it is the array that holds the CipherText to be decrypted.

// out – it is the array that holds the output of the for decryption.

// state – the array that holds the intermediate results during decryption.

unsigned char in[16], out[16], state[4][4];

 

// The array that stores the round keys.

unsigned char RoundKey[240];

 

// The Key input to the AES Program

unsigned char Key[32];

 

int getSBoxInvert(int num)

{

int rsbox[256] =

{ 0x52, 0x09, 0x6a, 0xd5, 0x30, 0x36, 0xa5, 0x38, 0xbf, 0x40, 0xa3, 0x9e, 0x81, 0xf3, 0xd7, 0xfb

, 0x7c, 0xe3, 0x39, 0x82, 0x9b, 0x2f, 0xff, 0x87, 0x34, 0x8e, 0x43, 0x44, 0xc4, 0xde, 0xe9, 0xcb

, 0x54, 0x7b, 0x94, 0x32, 0xa6, 0xc2, 0x23, 0x3d, 0xee, 0x4c, 0x95, 0x0b, 0x42, 0xfa, 0xc3, 0x4e

, 0x08, 0x2e, 0xa1, 0x66, 0x28, 0xd9, 0x24, 0xb2, 0x76, 0x5b, 0xa2, 0x49, 0x6d, 0x8b, 0xd1, 0x25

, 0x72, 0xf8, 0xf6, 0x64, 0x86, 0x68, 0x98, 0x16, 0xd4, 0xa4, 0x5c, 0xcc, 0x5d, 0x65, 0xb6, 0x92

, 0x6c, 0x70, 0x48, 0x50, 0xfd, 0xed, 0xb9, 0xda, 0x5e, 0x15, 0x46, 0x57, 0xa7, 0x8d, 0x9d, 0x84

, 0x90, 0xd8, 0xab, 0x00, 0x8c, 0xbc, 0xd3, 0x0a, 0xf7, 0xe4, 0x58, 0x05, 0xb8, 0xb3, 0x45, 0x06

, 0xd0, 0x2c, 0x1e, 0x8f, 0xca, 0x3f, 0x0f, 0x02, 0xc1, 0xaf, 0xbd, 0x03, 0x01, 0x13, 0x8a, 0x6b

, 0x3a, 0x91, 0x11, 0x41, 0x4f, 0x67, 0xdc, 0xea, 0x97, 0xf2, 0xcf, 0xce, 0xf0, 0xb4, 0xe6, 0x73

, 0x96, 0xac, 0x74, 0x22, 0xe7, 0xad, 0x35, 0x85, 0xe2, 0xf9, 0x37, 0xe8, 0x1c, 0x75, 0xdf, 0x6e

, 0x47, 0xf1, 0x1a, 0x71, 0x1d, 0x29, 0xc5, 0x89, 0x6f, 0xb7, 0x62, 0x0e, 0xaa, 0x18, 0xbe, 0x1b

, 0xfc, 0x56, 0x3e, 0x4b, 0xc6, 0xd2, 0x79, 0x20, 0x9a, 0xdb, 0xc0, 0xfe, 0x78, 0xcd, 0x5a, 0xf4

, 0x1f, 0xdd, 0xa8, 0x33, 0x88, 0x07, 0xc7, 0x31, 0xb1, 0x12, 0x10, 0x59, 0x27, 0x80, 0xec, 0x5f

, 0x60, 0x51, 0x7f, 0xa9, 0x19, 0xb5, 0x4a, 0x0d, 0x2d, 0xe5, 0x7a, 0x9f, 0x93, 0xc9, 0x9c, 0xef

, 0xa0, 0xe0, 0x3b, 0x4d, 0xae, 0x2a, 0xf5, 0xb0, 0xc8, 0xeb, 0xbb, 0x3c, 0x83, 0x53, 0x99, 0x61

, 0x17, 0x2b, 0x04, 0x7e, 0xba, 0x77, 0xd6, 0x26, 0xe1, 0x69, 0x14, 0x63, 0x55, 0x21, 0x0c, 0x7d };

 

return rsbox[num];

}

 

int getSBoxValue(int num)

{

int 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 };

return sbox[num];

}

 

// 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).

int 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  };

 

// This function produces Nb(Nr+1) round keys. The round keys are used in each round to decrypt the states.

void KeyExpansion()

{

int i,j;

unsigned char temp[4],k;

 

// 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.

while (i < (Nb * (Nr+1)))

{

for(j=0;j<4;j++)

{

temp[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 = temp[0];

temp[0] = temp[1];

temp[1] = temp[2];

temp[2] = temp[3];

temp[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()

{

temp[0]=getSBoxValue(temp[0]);

temp[1]=getSBoxValue(temp[1]);

temp[2]=getSBoxValue(temp[2]);

temp[3]=getSBoxValue(temp[3]);

}

 

temp[0] =  temp[0] ^ Rcon[i/Nk];

}

else if (Nk > 6 && i % Nk == 4)

{

// Function Subword()

{

temp[0]=getSBoxValue(temp[0]);

temp[1]=getSBoxValue(temp[1]);

temp[2]=getSBoxValue(temp[2]);

temp[3]=getSBoxValue(temp[3]);

}

}

RoundKey[i*4+0] = RoundKey[(i-Nk)*4+0] ^ temp[0];

RoundKey[i*4+1] = RoundKey[(i-Nk)*4+1] ^ temp[1];

RoundKey[i*4+2] = RoundKey[(i-Nk)*4+2] ^ temp[2];

RoundKey[i*4+3] = RoundKey[(i-Nk)*4+3] ^ temp[3];

i++;

}

}

 

// This function adds the round key to state.

// The round key is added to the state by an XOR function.

void AddRoundKey(int round)

{

int 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.

void InvSubBytes()

{

int i,j;

for(i=0;i<4;i++)

{

for(j=0;j<4;j++)

{

state[i][j] = getSBoxInvert(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.

void InvShiftRows()

{

unsigned char temp;

 

// Rotate first row 1 columns to right

temp=state[1][3];

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

state[1][2]=state[1][1];

state[1][1]=state[1][0];

state[1][0]=temp;

 

// Rotate second row 2 columns to right

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 right

temp=state[3][0];

state[3][0]=state[3][1];

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

state[3][2]=state[3][3];

state[3][3]=temp;

}

 

// xtime is a macro that finds the product of {02} and the argument to xtime modulo {1b}

#define xtime(x)   ((x<<1) ^ (((x>>7) & 1) * 0x1b))

 

// Multiplty is a macro used to multiply numbers in the field GF(2^8)

#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))))))

 

// MixColumns function mixes the columns of the state matrix.

// The method used to multiply may be difficult to understand for beginners.

// Please use the references to gain more information.

void InvMixColumns()

{

int i;

unsigned char a,b,c,d;

for(i=0;i<4;i++)

{

 

a = state[0][i];

b = state[1][i];

c = state[2][i];

d = state[3][i];

 

 

state[0][i] = Multiply(a, 0x0e) ^ Multiply(b, 0x0b) ^ Multiply(c, 0x0d) ^ Multiply(d, 0x09);

state[1][i] = Multiply(a, 0x09) ^ Multiply(b, 0x0e) ^ Multiply(c, 0x0b) ^ Multiply(d, 0x0d);

state[2][i] = Multiply(a, 0x0d) ^ Multiply(b, 0x09) ^ Multiply(c, 0x0e) ^ Multiply(d, 0x0b);

state[3][i] = Multiply(a, 0x0b) ^ Multiply(b, 0x0d) ^ Multiply(c, 0x09) ^ Multiply(d, 0x0e);

}

}

 

// InvCipher is the main function that decrypts the CipherText.

void InvCipher()

{

int i,j,round=0;

 

//Copy the input CipherText 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(Nr);

 

 

 

// There will be Nr rounds.

// The first Nr-1 rounds are identical.

// These Nr-1 rounds are executed in the loop below.

for(round=Nr-1;round>0;round–)

{

InvShiftRows();

InvSubBytes();

AddRoundKey(round);

InvMixColumns();

}

 

// The last round is given below.

// The MixColumns function is not here in the last round.

InvShiftRows();

InvSubBytes();

AddRoundKey(0);

 

// The decryption 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];

}

}

}

void main()

{

int i;

 

// Receive the length of key here.

while(Nr!=128 && Nr!=192 && Nr!=256)

{

printf(“Enter the length of Key(128, 192 or 256 only): “);

scanf(“%d”,&Nr);

}

// Calculate Nk and Nr from the received value.

Nk = Nr / 32;

Nr = Nk + 6;

 

 

 

// Part 1 is for demonstrative purpose. The key and plaintext are given in the program itself.

//     Part 1: ********************************************************

 

// The array temp stores the key.

// The array temp2 stores the plaintext.

unsigned char temp[32] = {0x00  ,0x01  ,0x02  ,0x03  ,0x04  ,0x05  ,0x06  ,0x07  ,0x08  ,0x09  ,0x0a  ,0x0b  ,0x0c  ,0x0d  ,0x0e  ,0x0f};

unsigned char temp2[16]= {0x69  ,0xc4  ,0xe0  ,0xd8  ,0x6a  ,0x7b  ,0x04  ,0x30  ,0xd8  ,0xcd  ,0xb7  ,0x80  ,0x70  ,0xb4  ,0xc5  ,0x5a};

 

// Copy the Key and CipherText

for(i=0;i<Nk*4;i++)

{

Key[i]=temp[i];

in[i]=temp2[i];

}

 

//           *********************************************************

 

 

 

 

// Uncomment Part 2 if you need to read Key and CipherText from the keyboard.

//     Part 2: ********************************************************

/*

//Clear the input buffer

flushall();

 

//Recieve the Key from the user

printf(“Enter the Key in hexadecimal: “);

for(i=0;i<Nk*4;i++)

{

scanf(“%x”,&Key[i]);

}

 

printf(“Enter the CipherText in hexadecimal: “);

for(i=0;i<Nb*4;i++)

{

scanf(“%x”,&in[i]);

}

*/

//             ********************************************************

 

 

//The Key-Expansion routine must be called before the decryption routine.

KeyExpansion();

 

// The next function call decrypts the CipherText with the Key using AES algorithm.

InvCipher();

 

// Output the decrypted text.

printf(“\nText after decryption:\n”);

for(i=0;i<Nb*4;i++)

{

printf(“%02x “,out[i]);

}

printf(“\n\n”);

 

}

 

 

 

— End of Source code —

 

License: No license. You are free to use this as you like.

Anyway if you use this source code, it would be nice if you could link back to us or give credit to the author.

Lot of work has gone into the development of this work. Anyway, thank you for visiting.

Author: Niyaz PK

 

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