Analysis of tars source code 1

base64 Is a reversible transformation algorithm , Not encryption , Nor is it a hash , Let's take a look at the implementation . I have written before ：

#include "util/tc_base64.h"

namespace tars
{
// Base64 Encoding table ： Fetch the input data stream every time 6 bit, Use this 6 bit Value (0-63) Look up the table as an index , Output corresponding characters . such , Every time 3 Bytes will be encoded as 4 Characters (3×8 → 4×6); discontent 4 A character with '=' fill .
const char  TC_Base64::EnBase64Tab[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    
// Base64 Decoding table ： take 64 The value of printable characters is used as the index , The value obtained by looking up the table （ The scope is 0-63） Connect them in turn to get the decoding result .
//  Decoding table size by 256, Illegal characters will be decoded into ASCII 0
const char  TC_Base64::DeBase64Tab[] =
{
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 
    62,        // '+'
    0, 0, 0,
    63,        // '/'
    52, 53, 54, 55, 56, 57, 58, 59, 60, 61,        // '0'-'9'
    0, 0, 0, 0, 0, 0, 0,
    0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
    13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,        // 'A'-'Z'
    0, 0, 0, 0, 0, 0,
    26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38,
    39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51,        // 'a'-'z'
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
};

string TC_Base64::encode(const string &data, bool bChangeLine/* = false*/)
{
    if(data.empty())
        return "";    
    // Let the original string length be a, The results include carriage return, line feed and '/0', The final length is (a/3+1)*4+(a/3+1)*4*2/76+1, about 1.369*a+6
    char *pDst = NULL;
    int iBufSize = (int)(data.size()*1.4) + 6;
    pDst = new char[iBufSize];
    if(pDst == NULL)
        return "";   
    int iDstLen = encode((unsigned char*)data.c_str(), data.size(), pDst, bChangeLine);
    string ret(pDst,iDstLen);
    delete [] pDst;
    return ret;
}

string TC_Base64::decode(const string &data)
{
    if(data.empty())
        return "";
    unsigned char *pDst = NULL;  
    pDst = new unsigned char[data.size()];
    if(pDst == NULL)
        return "";
    int iDstLen = decode(data.c_str(), data.size(), pDst);
    string ret((char*)pDst,iDstLen);
    delete [] pDst;
    return ret;
}

int TC_Base64::encode(const unsigned char* pSrc, int nSrcLen, char* pDst, bool bChangeLine/* = false*/)
{
    unsigned char c1, c2, c3;   
    int nDstLen = 0;             
    int nLineLen = 0;         
    int nDiv = nSrcLen / 3;      
    int nMod = nSrcLen % 3;    
    //  Every time 3 Bytes , Code as 4 Characters 
    for (int i = 0; i < nDiv; i ++)
    {
        c1 = *pSrc++;
        c2 = *pSrc++;
        c3 = *pSrc++;
 
        *pDst++ = EnBase64Tab[c1 >> 2];
        *pDst++ = EnBase64Tab[((c1 << 4) | (c2 >> 4)) & 0x3f];
        *pDst++ = EnBase64Tab[((c2 << 2) | (c3 >> 6)) & 0x3f];
        *pDst++ = EnBase64Tab[c3 & 0x3f];
        nLineLen += 4;
        nDstLen += 4;
        //  relevant RFC More than per line in 76 You need to add carriage return line feed 
        if (bChangeLine && nLineLen > 72)
        {
            *pDst++ = '\r';
            *pDst++ = '\n';
            nLineLen = 0;
            nDstLen += 2;
        }
    }
    //  Encode the remaining bytes 
    if (nMod == 1)
    {
        c1 = *pSrc++;
        *pDst++ = EnBase64Tab[(c1 & 0xfc) >> 2];
        *pDst++ = EnBase64Tab[((c1 & 0x03) << 4)];
        *pDst++ = '=';
        *pDst++ = '=';
        nLineLen += 4;
        nDstLen += 4;
    }
    else if (nMod == 2)
    {
        c1 = *pSrc++;
        c2 = *pSrc++;
        *pDst++ = EnBase64Tab[(c1 & 0xfc) >> 2];
        *pDst++ = EnBase64Tab[((c1 & 0x03) << 4) | ((c2 & 0xf0) >> 4)];
        *pDst++ = EnBase64Tab[((c2 & 0x0f) << 2)];
        *pDst++ = '=';
        nDstLen += 4;
    }
    //  Output with a terminator 
    *pDst = '\0';
 
    return nDstLen;
}

 
int  TC_Base64::decode(const char* pSrc, int nSrcLen, unsigned char* pDst)
{
    int nDstLen;            //  Output character count  
    int nValue;             //  Integer used for decoding 
    int i; 
    i = 0;
    nDstLen = 0;
 
    //  take 4 Characters , Decode to a long integer , Then through the shift, we get 3 Bytes 
    while (i < nSrcLen)
    {    
        //  Skip carriage return and line feed     
        if (*pSrc != '\r' && *pSrc!='\n')
        {
            if(i + 4 > nSrcLen)                             // The string to be decoded is illegal , Stop decoding immediately and return 
                break;           

            nValue = DeBase64Tab[int(*pSrc++)] << 18;         
            nValue += DeBase64Tab[int(*pSrc++)] << 12;
            *pDst++ = (nValue & 0x00ff0000) >> 16;
            nDstLen++; 
            if (*pSrc != '=')
            {                
                nValue += DeBase64Tab[int(*pSrc++)] << 6;
                *pDst++ = (nValue & 0x0000ff00) >> 8;
                nDstLen++; 
                if (*pSrc != '=')
                {                    
                    nValue += DeBase64Tab[int(*pSrc++)];
                    *pDst++ =nValue & 0x000000ff;
                    nDstLen++;
                }
            }
 
            i += 4;
        }
        else       
        {
            pSrc++;
            i++;
        }
     }
    //  Output with a terminator 
    *pDst = '\0'; 
    return nDstLen;
}

}