AES加密CBC模式的c语言实现

手写了一遍AES加密，虽然是参考网友的（基本一毛一样），但还是在写的过程中有所收获。不过还是没弄明白s盒的计算方法，即在有限域上求乘法逆，那个扩展欧几里得实在是看不明白。能力太差，以下实现只是能做学习AES原理用，解题或者平时计算还是用高级语言中对应的算法库。

参考

• AES算法描述及C语言实现
• 伽罗华域运算及C语言实现
• 手动推导计算AES中的s盒的输出
• AES128加密-S盒和逆S盒构造推导及代码实现
• (看雪)AES中S盒的生成原理与变化
• 扩展欧几里得算法详解
• 浅析uint8_t / uint16_t / uint32_t /uint64_t
• do{…}while(0)的妙用

原理

AES这种对称秘钥的分组密码，算法本身基本可以分成两部分：迭代结构和轮函数。AES采用SPN的迭代结构，简单漂亮。

轮函数主要作用代换置换，目的是混淆和扩散：

• 混淆的主要做法就是利用所谓的s盒，非线性代换原来字节
• 扩散可以称之为是p置换（p盒），就是类似拧魔方一样的操作，目的是让所有比特位互相影响。

• AK：加轮秘钥（AddRoundKey)
• SB：S盒代换 (SubBytes)
• SR：行移位 (ShiftRows)
• MC：列混合 (MixColumns)

这两步都需要是可逆的，代换还能倒着换回来，魔方还能倒着拧回来。在数学上分别是有限域上求乘法逆，矩阵在有限域上求逆

收获

知道了为啥加密算法要有这么多参数，在c代码中有：密文，明文，秘钥，初始向量，长度，之所以有长度的一个参数是因为密文是字节流，无法通过strlen这种函数获取长度，而传递给加密和解密函数的明文或者密文通常是一个指向一块内存的指针，所以还是需要指明明密文长度。正因为加密算法的参数比较复杂，所以常见的加密算法的用法并不是仅仅用一个函数然后跟一堆参数，而是用一些参数初始化一个对象，然后在调用这个对象加密和解密的方法，例如在python中：

# sudo pip install pycrypto
from Crypto.Cipher import AES

key = '0'*16
iv  = '0'*16
mode = AES.MODE_CBC

cryptos = AES.new(key, mode, iv)
encrypt_text = cryptos.encrypt('A'*16+'B'*16+'C'*16)

cryptos = AES.new(key, mode, iv)
decrypt_text = cryptos.decrypt(encrypt_text)

print encrypt_text.encode("hex")
print decrypt_text

在c语言中安排这种明密文存储，就有点复杂。因为是大小不固定的字节流，如果在函数中使用malloc，并希望新的地址可以在函数参数中返回，在函数的形参中，则是有两个*，表示传的变量进来的是个地址，而且这个变量还是个地址：

void genBlock(uint8_t ** block){
    *block = malloc(1024);
}

int main(){
    uint8_t * block = NULL;
    genBlock(&block);
    free(block);
    return 0;
}

这看起来就是比较危险的把栈上变量的地址传出去了，在逆向西门子算法的时候常见，当时不明白为什么会这么写。现在想想，就是这种加密解密，肯定会涉及到大量的字节数组的运算。那这些字节数组可以放在data段，栈段，堆段。虽然如果计算的字节数组大小固定时，可以在程序中写死数组的大小，甚至放在全局变量里即把数组放在data段，但是在进行加密解密的过程中一定涉及到对于这些运算的中间状态和中间结果的存储，那么还是要有堆或者栈来存储。而且加密与解密数组时，绝对不是一整个函数就搞定的，一定会涉及到多个函数对一个内存区域的操作，这时就会出现函数把自己栈上变量的地址传给自己调用的函数。

另外的一些收获：

1. 密文是存在不可打印字符的字节流，所以为了便于观察一般需要再次编码输出，一般输出为base64编或者hex编码
2. uint8_t,uint16_t等，是在stdint.h中的定义，防止在不同字长的机器上变量的位数不同
3. 一条宏定义涉及多个语句，则可以用do{ }while(0)包括，防止宏定义解释出错

实现

#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>


#define BLOCKSIZE 16

typedef struct{
    uint32_t eK[44],dK[44];
    int rounds;
} AESKEY;

#define ROF32(x,n) ((x << n) | (x >> (32-n)))

#define ROR32(x,n) ((x >> n) | (x << (32-n)))

#define BYTE(x,n) (((x)>>((n)*8)) & 0xff)

#define MIX(x)  (((S[BYTE((x),2)] << 24) & 0xff000000) ^ \
                 ((S[BYTE((x),1)] << 16) & 0xff0000) ^ \
                 ((S[BYTE((x),0)] << 8) & 0xff00) ^ \
                 ((S[BYTE((x),3)]) & 0xff))
                

#define LOAD32(x,y) do{ (x) = \
    ((uint32_t)((y)[0]&0xff)<<24) |\
    ((uint32_t)((y)[1]&0xff)<<16) |\
    ((uint32_t)((y)[2]&0xff)<<8 ) |\
    ((uint32_t)((y)[3]&0xff));\
}while(0)

#define  STORE32(x,y) do{ \
    (y)[0] = (uint8_t)(((x)>>24) & 0xff) ; \
    (y)[1] = (uint8_t)(((x)>>16) & 0xff) ; \
    (y)[2] = (uint8_t)(((x)>>8) & 0xff) ; \
    (y)[3] = (uint8_t)((x) & 0xff) ; \
}while(0)

 
unsigned char S[256] = {
    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};

unsigned char inv_S[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};

uint8_t M[4][4] = {
    {0x02, 0x03, 0x01, 0x01},
    {0x01, 0x02, 0x03, 0x01},
    {0x01, 0x01, 0x02, 0x03},
    {0x03, 0x01, 0x01, 0x02}
};

uint8_t inv_M[4][4] = {
    {0x0E, 0x0B, 0x0D, 0x09},
    {0x09, 0x0E, 0x0B, 0x0D},
    {0x0D, 0x09, 0x0E, 0x0B},
    {0x0B, 0x0D, 0x09, 0x0E}
};


static const uint32_t rcon[10] = {
    0x01000000UL, 0x02000000UL, 0x04000000UL, 0x08000000UL, 0x10000000UL,
    0x20000000UL, 0x40000000UL, 0x80000000UL, 0x1B000000UL, 0x36000000UL
};


int KeyExpansion(uint8_t * key,AESKEY * aeskey){
    uint32_t * w = aeskey ->eK;
    uint32_t * v = aeskey ->dK;
    for(int i=0 ; i<4 ; i++ ){
        LOAD32(w[i], key+i*4);
    }
    for(int i=0;i<10;i++){
        w[4]=w[0]^MIX(w[3])^rcon[i];
        w[5]=w[1]^w[4];
        w[6]=w[2]^w[5];
        w[7]=w[3]^w[6];
        w+=4;
    }
    w = (aeskey -> eK) + 40;
    for(int j=0 ;j<11;j++){
        for(int i = 0 ; i<4;i++){
            v[i]=w[i];
        }
        v+=4;
        w-=4;
    }
    return 0;
}

void loadStateArray(uint8_t (*state)[4],uint8_t *in){
    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++){
            state[j][i]=*in++;
        }
    }
}

void storeStateArray(uint8_t (*state)[4],uint8_t *out){
    for(int i=0;i<4;i++){
        for(int j=0;j<4;j++){
            *out++=state[j][i];
        }
    }
}

void shiftRows(uint8_t (*state)[4]){
    uint32_t temp[4]={0};
    for(int i=0;i<4;i++){
        LOAD32(temp[i], state[i]);
        temp[i]=ROF32(temp[i], i*8);
        STORE32(temp[i], state[i]);
    }
}

void invShiftRows(uint8_t (*state)[4]) {
    uint32_t temp[4] = {0};
    for (int i = 0; i < 4; i++) {
        LOAD32(temp[i], state[i]);
        temp[i] = ROR32(temp[i], i*8);
        STORE32(temp[i], state[i]);
    }
}

uint8_t GMul(uint8_t u, uint8_t v) {
    uint8_t p = 0;
    for (int i = 0; i < 8; ++i) {
        if (u & 0x01) {    //
            p ^= v;
        }
        int flag = (v & 0x80);
        v <<= 1;
        if (flag) {
            v ^= 0x1B;
        }
        u >>= 1;
    }
    return p;
}

void mixColumns(uint8_t (*state)[4]){
    uint8_t tmp[4][4];
    
    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j){
            tmp[i][j] = state[i][j];
        }
    }

    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j) {
            state[i][j] = GMul(M[i][0], tmp[0][j]) ^
                          GMul(M[i][1], tmp[1][j]) ^
                          GMul(M[i][2], tmp[2][j]) ^
                          GMul(M[i][3], tmp[3][j]) ;
        }
    }
    
}

void invMixColumns(uint8_t (*state)[4]){
    uint8_t tmp[4][4];
    
    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j){
            tmp[i][j] = state[i][j];
        }
    }

    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j) {
            state[i][j] = GMul(inv_M[i][0], tmp[0][j]) ^
                          GMul(inv_M[i][1], tmp[1][j]) ^
                          GMul(inv_M[i][2], tmp[2][j]) ^
                          GMul(inv_M[i][3], tmp[3][j]) ;
        }
    }
    
}

int addRoundKey(uint8_t (*state)[4], const uint32_t *key) {
    uint8_t k[4][4];
    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j) {
            k[j][i] = (uint8_t) BYTE(key[i], 3 - j); //按列异或秘钥
            state[j][i] ^= k[j][i];
        }
    }

    return 0;
}

int subBytes(uint8_t (*state)[4]) {
    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j) {
            state[i][j] = S[state[i][j]];
        }
    }
    return 0;
}

int invSubBytes(uint8_t (*state)[4]) {
    for (int i = 0; i < 4; ++i) {
        for (int j = 0; j < 4; ++j) {
            state[i][j] = inv_S[state[i][j]];
        }
    }
    return 0;
}


void aesEncryptBlock(uint8_t * mblock,uint8_t * cblock,uint8_t * key){
    AESKEY aeskey;
    uint8_t state[4][4]={0};
    KeyExpansion(key, &aeskey);
    loadStateArray(state, mblock);
    uint32_t * ekPointer = aeskey.eK;
    addRoundKey(state, ekPointer);
    for(int i=1;i<10;i++){
        ekPointer += 4;
        subBytes(state);
        shiftRows(state);
        mixColumns(state);
        addRoundKey(state, ekPointer);
    }
    ekPointer += 4;
    subBytes(state);
    shiftRows(state);
    addRoundKey(state, ekPointer);
    storeStateArray(state, cblock);
}

void aesDecryptBlock(uint8_t * mblock,uint8_t * cblock,uint8_t * key){
    AESKEY aeskey;
    uint8_t state[4][4]={0};
    KeyExpansion(key, &aeskey);
    loadStateArray(state, cblock);
    uint32_t * dkPointer = aeskey.dK;
    addRoundKey(state, dkPointer);
    
    for(int i=1;i<10;i++){
        dkPointer += 4;
        invShiftRows(state);
        invSubBytes(state);
        addRoundKey(state, dkPointer);
        invMixColumns(state);
        
    }
    dkPointer += 4;
    invSubBytes(state);
    invShiftRows(state);
    addRoundKey(state, dkPointer);
    storeStateArray(state, mblock);
}


int splitBlock(char * message,uint8_t ** blocks){
    int len = (int)strlen(message);
    int block_num = (len/16)+1;
    int mod = len % 16;
    *blocks = (uint8_t *)malloc(block_num*16);
    memcpy(*blocks, message, len);
    memset(*blocks+len,0,16-mod);
    return block_num;
}

uint8_t * aesEncryptCBC(uint8_t * blocks,uint8_t * key,int block_num,uint8_t * iv){
    uint8_t * tmp = iv;
    for(int i = 0;i<block_num;i++){
        for(int j = 0;j<16;j++){
            blocks[16*i+j] ^= tmp[j];
        }
        aesEncryptBlock(blocks+16*i, blocks+16*i, key);
        tmp = blocks+16*i;
    }
    return blocks;
}

void aesDecryptCBC(uint8_t * blocks,uint8_t * key,int block_num,uint8_t * iv){
    uint8_t * tmp = blocks+(16*(block_num-2));
    for(int i = block_num-1;i > -1;i--){
        aesDecryptBlock(blocks+16*i, blocks+16*i, key);
        for(int j = 0;j<16;j++){
            blocks[16*i+j] ^= tmp[j];
        }
        if(i==1){
            tmp = iv;
        }else{
            tmp -= 16;
        }
    }
}

int main(int argc, const char * argv[]) {
    char message[]="xuanxuanblingbling";
    uint8_t key[16]="1234567890123456";
    uint8_t iv[16]="1234567890123456";
    
    uint8_t * blocks = NULL;
    int block_num = splitBlock(message,&blocks);

    aesEncryptCBC(blocks,key,block_num,iv);

    printf("加密密文：");
    for(int i=0;i<block_num*16;i++){
       printf("\\x%02x",blocks[i]);
    }
    printf("\n");

    aesDecryptCBC(blocks,key,block_num,iv);

    printf("解密明文：%s\n",blocks);
    free(blocks);
    return 0;
    
}