How to convert a number (bigger than 8 bytes) from a char array to its ASCII representation?

Question

I am on a Windows-XP platform, using C++98. Here is my problem:

1. I receive a number encoded in base64 in a char array. The number is 16 bytes long
2. After decoding the number to base10, I end up with a number of 12 bytes long
3. Those 12 bytes (still in a char array) should be converted to a ASCII representation (requirement).

An example:

• Reception of data : "J+QbMuDxYkxxtSRA" (in a char array : 16 bytes)
• Decoding from base64: 0x27 0xE4 0x1B 0x32 0xE0 0xF1 0x62 0x4C 0x71 0xB5 0x24 0x40 (hex in a char array: 12 bytes)
• Conversion from decimal to ASCII : "12345678912345678912345678912" (in a char array: 29 bytes)

I am currently stuck at the last step, I have no idea how to do it. When I searched, the only thing I found is how to convert from int to string (so, for an int containing the value 10, a sprintf("%d", buffer) would do the trick I guess. Since my number is greater than 8 bytes/64 bits (12 bytes) and is not stored in a int/unsigned long long, I am pretty sure I need to do something else to convert my number.

Any help would be greatly appreciated

EDIT for a code example (link to the base64 library : http://libb64.sourceforge.net/

#include <iostream>
#include <cstring>
#include "b64/decode.h"

class Tokenizer{
public:
    Tokenizer();
    void decodeFrame(char *buffer, int bufferSize);
    void retrieveFrame(char *buffer);
private:
    char m_frame[255];
};

using namespace std;
int main(int argc, char *argv[])
{
    Tokenizer token;

    char base64Message[] = "J+QbMuDxYkxxtSRA";  // base64 message of 16 (revelant) bytes
    char asciiMessage[30] = {0};                // final message, a number in ascii representation of 29 bytes

    cout << "begin, message = " << base64Message << endl;

    token.decodeFrame(base64Message, 16);              // decode from base64 and convert to ASCII
    token.retrieveFrame(asciiMessage);          // retrieve the ASCII message

    cout << "final message : " << asciiMessage << endl;                       // the message should be "12345678912345678912345678912"

    return 0;
}
Tokenizer::Tokenizer()
{
}
void Tokenizer::decodeFrame(char *buffer, int bufferSize)
{
    memset(m_frame, 0x00, 255);
    char decodedFrame[255] = {0};  // there is maximum 255 byte in ENCODED (base64). When decoded, the size will automatically be lower
    int decodedSize = 0;
    base64::decoder base64Decoder;  // base64 to base10 decoder, found there: http://libb64.sourceforge.net/

    decodedSize = base64Decoder.decode(buffer, bufferSize, decodedFrame); // int decode(const char* code_in, const int length_in, char* plaintext_out)

    // the frame now must be decoded to produce ASCII, I have no idea how to do it
    for(int i = 0; i < 30; i++){    m_frame[i] = decodedFrame[i]; }
}
void Tokenizer::retrieveFrame(char *buffer)
{
    if(buffer != NULL){
        for(int i = 0; m_frame[i] != 0; i++){
            buffer[i] = m_frame[i];
        }
    }
}

Answer 1

Building on an earlier answer you can easily build an arbitrary length integer from a bunch of bytes.

const char *example = "\x27\xE4\x1B\x32\xE0\xF1\x62\x4C\x71\xB5\x24\x40";
Bignum bn = 0;
for (int i = 0; i < 12; ++i)
{
    bn *= 256;
    bn += example[i] & 0xff;
}
std::cout << bn << std::endl;

See the whole code here: http://coliru.stacked-crooked.com/a/d1d9f39a6d575686

Answer 2

@Farouk : your error in your reasonment is due to at the convertion base64 string to decimal (base10) directly. Indeed this convertion change the original number, you have jumping a step in your convertion for more details follow below : example : receive base64 : J+QbMuDxYkxxtSRA

**your steps :**
**base64 -> dec**  : 39 18162 3170 76 113 22784
**dec -> hex**     : 153D8AE2CE845159A0

**logical steps:**
**base64 -> hex**  : 27e41b32e0f1624c71b52440
**hex -> deci**    : 12345678912345678912345678912
**deci -> hex**    : 27E41B32E0F1624C71B52440
**hex -> base64**  : J+QbMuDxYkxxtSRA

For the solution c++ to convert base64tohex i thinks you can find a librairy on the web if not i can code for you this convertion later.