KeeLoq算法的核心思想就是用8byte密钥加密4byte明文，从而得到4byte密文或者用8byte密钥解密4byte密文，还原出原4byte明文。KeeLoq算法演算过程需要定义一个数据寄存器，用于存放4byte明文y31_{y0或者4byte密文y31}y0，和一个密钥寄存器，用于存放8byte密钥k63~k0。其加密特点是运算速度快，加密性高，线性等。

#include<stdio.h>

unsigned char NLF[2][2][2][2][2];

void init()
{
    NLF[0][0][0][0][0]=0;
    NLF[0][0][0][0][1]=1;
    NLF[0][0][0][1][0]=1;
    NLF[0][0][0][1][1]=1;
    NLF[0][0][1][0][0]=0;
    NLF[0][0][1][0][1]=1;
    NLF[0][0][1][1][0]=0;
    NLF[0][0][1][1][1]=0;
    
    NLF[0][1][0][0][0]=0;
    NLF[0][1][0][0][1]=0;
    NLF[0][1][0][1][0]=1;
    NLF[0][1][0][1][1]=0;
    NLF[0][1][1][0][0]=1;
    NLF[0][1][1][0][1]=1;
    NLF[0][1][1][1][0]=1;
    NLF[0][1][1][1][1]=0;
    
    NLF[1][0][0][0][0]=0;
    NLF[1][0][0][0][1]=0;
    NLF[1][0][0][1][0]=1;
    NLF[1][0][0][1][1]=1;
    NLF[1][0][1][0][0]=1;
    NLF[1][0][1][0][1]=0;
    NLF[1][0][1][1][0]=1;
    NLF[1][0][1][1][1]=0;
    
    NLF[1][1][0][0][0]=0;
    NLF[1][1][0][0][1]=1;
    NLF[1][1][0][1][0]=0;
    NLF[1][1][0][1][1]=1;
    NLF[1][1][1][0][0]=1;
    NLF[1][1][1][0][1]=1;
    NLF[1][1][1][1][0]=0;
    NLF[1][1][1][1][1]=0;
}

//获取source第n个位数
unsigned char getBit(unsigned char source[],int n){
    unsigned char temp0=(unsigned char)1<<(n%8);
    unsigned char temp1=source[n/8]&temp0;
    if(temp1!=0)
    {
        return 1;
    }
    return 0;
}

//source带进位右移
unsigned char * RRC(unsigned char source[],char c,char n)
{
    int i=0;
    unsigned char temp;
    for(i=n-1;i>=0;i--)
    {
        temp=source[i];
        if(c!=0){
            source[i]=(source[i]>>1)|0x80;
        }else{
            source[i]=(source[i]>>1)&0x7f;
        }
        
        if((temp&0x01) != 0){
            c=1;
        }else{
            c=0;
        }
    }
    return source;
}

//source带进位左移
unsigned char * RLC(unsigned char source[],char c,char n)
{
    int i=0;
    unsigned char temp;
    for(i=0;i<n;i++)
    {
        temp=source[i];
        if(c!=0){
            source[i]=(source[i]<<1)|0x01;
        }else{
            source[i]=(source[i]<<1)&0xfe;
        }
        
        if((temp&0x80)!=0){
            c=1;
        }else{
            c=0;
        }
    }
    return source;
}

//加密
unsigned char * CRYPT(unsigned char *source,unsigned char key[])
{
    int i=0;
    unsigned char c=0;
    unsigned char nlf,y16,y0,k,result;
    init();
    for (i = 0; i < 528; i++)
    {
        nlf=NLF[getBit(source, 31)][getBit(source, 26)][getBit(source, 20)][getBit(source, 9)][getBit(source, 1)];
        y16=getBit(source, 16);
        y0=getBit(source, 0);
        k=getBit(key, i%64);
        result=nlf^y16^y0^k;
        if (result!=0) {
            c=1;
        }else {
            c=0;
        }
        source=RRC(source,c,4);
    }
    return source;
}

//解密
unsigned char *  DECRYPT(unsigned char *source,unsigned char key[])
{
    int i=0;
    unsigned char c=0;
    unsigned char nlf,y15,y31,k,result;
    init();
    for (i = 528; i >0; i--)
    {
        nlf=NLF[getBit(source, 30)][getBit(source, 25)][getBit(source, 19)][getBit(source, 8)][getBit(source, 0)];
        y15=getBit(source, 15);
        y31=getBit(source, 31);
        k=getBit(key, (i-1)%64);
        result=nlf^y15^y31^k;
        if ( result != 0)      c=1;
        else                 c=0;
        
        source=RLC(source,c,4);
    }
    return source;
}


int main()
{
    
    unsigned char key[]={0X09,0X87,0X65,0X43,0X21,0XAB,0XCD,0XEF};
    unsigned int data = 1520149488;
    unsigned char source[4];
    source[0]=(unsigned char)(data);
    source[1]=(unsigned char)(data>>8);
    source[2]=(unsigned char)(data>>16);
    source[3]=(unsigned char)(data>>24);
    printf("加密前数据:%d\n",data);
    unsigned char *crypt = CRYPT(source,key);
    unsigned int decrypt = (crypt[3]<<24)+(crypt[2]<<16)+(crypt[1]<<8)+(crypt[0]);
    printf("加密后数据:%d\n",decrypt);
    crypt = DECRYPT(crypt, key);
    printf("解密后数据:%d\n",(crypt[3]<<24)+(crypt[2]<<16)+(crypt[1]<<8)+(crypt[0]));
    return 0;
    
}

控制台输出

屏幕快照 2018-04-04 10.54.16.png

附：
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