reverse逆向算法之base64和RC4

最新推荐文章于 2025-05-27 18:22:50 发布

re3sry

最新推荐文章于 2025-05-27 18:22:50 发布

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文章标签：算法 ctf reverse

本文链接：https://blog.youkuaiyun.com/yhfgs/article/details/120729703

本文介绍了Base64编码的基本原理和C语言实现，包括将字符转换为六位二进制并查表的过程。接着详细阐述了RC4加密算法的步骤，包括初始化状态向量S和临时向量T，以及生成密钥流和加密过程，并给出了完整的C语言代码实现。通过对这两个经典加密算法的理解和实践，有助于深入掌握信息安全基础知识。

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一.base64

二.RC4加密

一.base64

1.简述：

base64是逆向很常见，也很简单的算法，基本上是新手CTF必备。

2.主要原理：

将三个八位的字符转化位四个六位的二进制数（不足八位，高位补0），在通过查表来转化为字符。

（众所周知，一个字符通常可以由八个二进制数来表示，例如：a 他的ascii码为0x61，转化为二进制数为：0110 0001）

例如，将"abc'进行base64加密.

将abc转化为二进制数为01100001 01100010 01100011

转化为四个六位的数，011000(0x18) 010110(0x16) 001001(0x9) 100011(0x23)（高位补零，凑足八位）

查表替换:YWJj

检索表：

3.代码实现。

 
const char * const base64char = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
 
int base64_decode( const char * base64, unsigned char * bindata )
{
    int i, j;
    unsigned char k;
    unsigned char temp[4];
 
    for ( i = 0, j = 0; base64[i] != '\0' ; i += 4 )
    {
        memset( temp, 0xFF, sizeof(temp) );
        for ( k = 0 ; k < 64 ; k ++ )
        {
            if ( base64char[k] == base64[i] )
                temp[0] = k;
        }
        for ( k = 0 ; k < 64 ; k ++ )
        {
            if ( base64char[k] == base64[i + 1] )
                temp[1] = k;
        }
        for ( k = 0 ; k < 64 ; k ++ )
        {
            if ( base64char[k] == base64[i + 2] )
                temp[2] = k;
        }
        for ( k = 0 ; k < 64 ; k ++ )
        {
            if ( base64char[k] == base64[i + 3] )
                temp[3] = k;
        }
 
        bindata[j++] = ((unsigned char)(((unsigned char)(temp[0] << 2)) & 0xFC)) |
                       ((unsigned char)((unsigned char)(temp[1] >> 4) & 0x03));
        if ( base64[i + 2] == '=' )
            break;
 
        bindata[j++] = ((unsigned char)(((unsigned char)(temp[1] << 4)) & 0xF0)) |
                       ((unsigned char)((unsigned char)(temp[2] >> 2) & 0x0F));
        if ( base64[i + 3] == '=' )
            break;
 
        bindata[j++] = ((unsigned char)(((unsigned char)(temp[2] << 6)) & 0xF0)) |
                       ((unsigned char)(temp[3] & 0x3F));
    }
    return j;
}
 
char * base64_encode( const unsigned char * bindata, char * base64, int binlength )
{
    int i, j;
    unsigned char current;
 
    for ( i = 0, j = 0 ; i < binlength ; i += 3 )
    {
        current = (bindata[i] >> 2) ;
        current &= (unsigned char)0x3F;
        base64[j++] = base64char[(int)current];
 
        current = ( (unsigned char)(bindata[i] << 4 ) ) & ( (unsigned char)0x30 ) ;
        if ( i + 1 >= binlength )
        {
            base64[j++] = base64char[(int)current];
            base64[j++] = '=';
            base64[j++] = '=';
            break;
        }
        current |= ( (unsigned char)(bindata[i + 1] >> 4) ) & ( (unsigned char) 0x0F );
        base64[j++] = base64char[(int)current];
 
        current = ( (unsigned char)(bindata[i + 1] << 2) ) & ( (unsigned char)0x3C ) ;
        if ( i + 2 >= binlength )
        {
            base64[j++] = base64char[(int)current];
            base64[j++] = '=';
            break;
        }
        current |= ( (unsigned char)(bindata[i + 2] >> 6) ) & ( (unsigned char) 0x03 );
        base64[j++] = base64char[(int)current];
 
        current = ( (unsigned char)bindata[i + 2] ) & ( (unsigned char)0x3F ) ;
        base64[j++] = base64char[(int)current];
    }
    base64[j] = '\0';
    return base64;
}

二.RC4加密

1.原理：

（1）初始化S（状态向量）和T（临时向量）：

T的初始化是根据密钥生成的，例如：密钥是12345，则按顺序1，2，3，4，5，1，2，3，4，5.。。。。轮转键入，直到生成256位。


        int i = 0;

        for(i=0; i<256; i++)
        {
                S[i] = i;
                T[i] = K[i%len];
        }

（2）对S进行置换

unsigned char tmp = 0x00;
        int i = 0;
        int j = 0;

        for(i=0; i<256; i++)
        {
                j = (j + S[i] + T[i]) % 256;
                tmp = S[j];
                S[j] = S[i];
                S[i] = tmp;
        }

（3）生成密钥流及加密

   unsigned char tmp = 0x00;
        int i = 0;
        int j = 0;
        int r = 0;
        int t = 0;

        for(r=0; r<len; r++)
        {
                i = (i + 1) % 256;
                j = (j + S[i]) % 256;
                tmp = S[j];
                S[j] = S[i];
                S[i] = tmp;
                t = (S[i] + S[j]) % 256;
                K[r] = S[t];



                E[i] = (M[i] ^ K[i])//M[]:明文 E[]:密文
        }

2.代码实现（总）：

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

int initST(unsigned char *S, unsigned char *T, unsigned char *K, int len)
{
        int i = 0;

        for(i=0; i<256; i++)
        {
                S[i] = i;
                T[i] = K[i%len];
        }

        return 0;
}

int initS(unsigned char *S, unsigned char *T)
{
        unsigned char tmp = 0x00;
        int i = 0;
        int j = 0;

        for(i=0; i<256; i++)
        {
                j = (j + S[i] + T[i]) % 256;
                tmp = S[j];
                S[j] = S[i];
                S[i] = tmp;
        }

        return 0;
}

int initK(unsigned char *S, unsigned char *K, int len)
{
        unsigned char tmp = 0x00;
        int i = 0;
        int j = 0;
        int r = 0;
        int t = 0;

        for(r=0; r<len; r++)
        {
                i = (i + 1) % 256;
                j = (j + S[i]) % 256;
                tmp = S[j];
                S[j] = S[i];
                S[i] = tmp;
                t = (S[i] + S[j]) % 256;
                K[r] = S[t];
        }

        return 0;
}

int rc4_enc(unsigned char *K, unsigned char *M, unsigned char *E, int len)
{
        int i = 0;

        for(i=0; i<len; i++)
        {
                E[i] = (M[i] ^ K[i]);
        }

        return 0;
}

int main()
{
        unsigned char S[256];
        unsigned char T[256];
        unsigned char K[256];
        unsigned char M[256];
        unsigned char E[256];
        unsigned char C[256];

        memset(S, 0x00, sizeof(S));
        memset(T, 0x00, sizeof(T));
        memset(K, 0x00, sizeof(K));
        memset(E, 0x00, sizeof(E));
        memset(C, 0x00, sizeof(C));

        strcpy(K, "123");
        strcpy(M, "你好");

        initST(S, T, K, 4);
        initS(S, T);
        initK(S, K, 256);

        rc4_enc(K, M, E, 256);
        rc4_enc(K, E, C, 256);

        printf("%s", E);

        return 0;
}