xuenixiang_2020_re_Reverse8
# babybaseX Writeup
> 主要考察base family中的base58编码
但是给的表是打乱的具有迷惑性的`vrYenHCzNgu7FRTDbLiqtBpQZoUS3f5dKWsaM8Gm1EyVJkjw4cA6X92Pxh0OLl+/ `，在base58后加入了base64的`0OLl+/`部分，但其实并不会用到这六个字符同时在题目中也未明确出现58字样，而是通过`int base = *(pbegin+1)-*pbegin+25; `隐式给出了base，所以在写base解码的时候不能直接写出base58，而是需要测试base的值，最终得到与加密后结果`gmJNxnNCPChRefqDYSU1KZ`相同的输入，即为flag。
base58解码的writeup如下：
```shell
#include <iostream>
#include <vector>
#include <assert.h>
#include <string.h>
#include <iomanip> // std::setw
static const char* pszBase58 = "vrYenHCzNgu7FRTDbLiqtBpQZoUS3f5dKWsaM8Gm1EyVJkjw4cA6X92Pxh0OLl+/";
bool DecodeBase58(const char* psz, std::vector<unsigned char>& vch)
{
// Skip leading spaces.
while (*psz && isspace(*psz))
psz++;
// Skip and count leading '1's.
int zeroes = 0;
int length = 0;
while (*psz == '1') {
zeroes++;
psz++;
}
// Allocate enough space in big-endian base256 representation.
int size = strlen(psz) * 733 /1000 + 1; // log(58) / log(256), rounded up.
std::vector<unsigned char> b256(size);
// Process the characters.
while (*psz && !isspace(*psz)) {
// Decode base58 character
const char* ch = strchr(pszBase58, *psz);
if (ch == nullptr)
return false;
// Apply "b256 = b256 * 58 + ch".
int carry = ch - pszBase58;
int i = 0;
for (std::vector<unsigned char>::reverse_iterator it = b256.rbegin(); (carry != 0 || i < length) && (it != b256.rend()); ++it, ++i) {
carry += 58 * (*it);
*it = carry % 256;
carry /= 256;
}
assert(carry == 0);
length = i;
psz++;
}
// Skip trailing spaces.
while (isspace(*psz))
psz++;
if (*psz != 0)
return false;
// Skip leading zeroes in b256.
std::vector<unsigned char>::iterator it = b256.begin() + (size - length);
while (it != b256.end() && *it == 0)
it++;
vch.reserve(zeroes + (b256.end() - it));
vch.assign(zeroes, 0x00);
while (it != b256.end())
vch.push_back(*(it++));
return true;
}
bool DecodeBase58(const std::string& str, std::vector<unsigned char>& vchRet)
{
return DecodeBase58(str.c_str(), vchRet);
}
int main(int argc, char* argv[]){
std::string encode="gmJNxnNCPChRefqDYSU1KZ";
std::vector<unsigned char> decode;
DecodeBase58(encode, decode);
std::cout<<"Decode: ";
for(std::vector<unsigned char>::iterator iter = decode.begin(); iter != decode.end(); ++iter)
{
std::cout<<*iter;
}
std::cout<<std::endl;
return 0;
}
```
`flag:flag{NowYouKnowBa5358}`
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