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How to get the MD5 hash of a file in C++? [closed]

Posted by: admin November 29, 2017 Leave a comment

Questions:

I’ve the file path. How can I get the MD5 hash of it?

Answers:

Here’s a straight forward implementation of the md5sum command that computes and displays the MD5 of the file specified on the command-line. It needs to be linked against the OpenSSL library (gcc md5.c -o md5 -lssl) to work. It’s pure C, but you should be able to adapt it to your C++ application easily enough.

#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <openssl/md5.h>

unsigned char result[MD5_DIGEST_LENGTH];

// Print the MD5 sum as hex-digits.
void print_md5_sum(unsigned char* md) {
    int i;
    for(i=0; i <MD5_DIGEST_LENGTH; i++) {
            printf("%02x",md[i]);
    }
}

// Get the size of the file by its file descriptor
unsigned long get_size_by_fd(int fd) {
    struct stat statbuf;
    if(fstat(fd, &statbuf) < 0) exit(-1);
    return statbuf.st_size;
}

int main(int argc, char *argv[]) {
    int file_descript;
    unsigned long file_size;
    char* file_buffer;

    if(argc != 2) { 
            printf("Must specify the file\n");
            exit(-1);
    }
    printf("using file:\t%s\n", argv[1]);

    file_descript = open(argv[1], O_RDONLY);
    if(file_descript < 0) exit(-1);

    file_size = get_size_by_fd(file_descript);
    printf("file size:\t%lu\n", file_size);

    file_buffer = mmap(0, file_size, PROT_READ, MAP_SHARED, file_descript, 0);
    MD5((unsigned char*) file_buffer, file_size, result);
    munmap(file_buffer, file_size); 

    print_md5_sum(result);
    printf("  %s\n", argv[1]);

    return 0;
}

Questions:
Answers:

You can implement the MD5 algorithm yourself (examples are all over the web), or you can link against the OpenSSL libs and use OpenSSL’s digest functions.
here’s an example to get the MD5 of a byte array:

#include <openssl/md5.h>
QByteArray AESWrapper::md5 ( const QByteArray& data) {
    unsigned char * tmp_hash;
    tmp_hash = MD5((const unsigned char*)data.constData(), data.length(), NULL);
    return QByteArray((const char*)tmp_hash, MD5_DIGEST_LENGTH);
}

Questions:
Answers:
QFile file("bigimage.jpg");

if (file.open(QIODevice::ReadOnly))
{
    QByteArray fileData = file.readAll();

    QByteArray hashData = QCryptographicHash::hash(fileData,QCryptographicHash::Md5); // or QCryptographicHash::Sha1
    qDebug() << hashData.toHex();  // 0e0c2180dfd784dd84423b00af86e2fc

}

Questions:
Answers:

For anyone redirected from “https://stackoverflow.com/questions/4393017/md5-implementation-in-c” because it’s been incorrectly labelled a duplicate.

The example located here works:

http://www.zedwood.com/article/cpp-md5-function

If you are compiling in VC++2010 then you will need to change his main.cpp to this:

#include <iostream> //for std::cout
#include <string.h> //for std::string
#include "MD5.h"

using std::cout; using std::endl;

int main(int argc, char *argv[])
{
    std::string Temp =  md5("The quick brown fox jumps over the lazy dog");
    cout << Temp.c_str() << endl;

    return 0;
}

You will have to change the MD5 class slightly if you are to read in a char * array instead of a string to answer the question on this page here.

EDIT:

Apparently modifying the MD5 library isn’t clear, well a Full VC++2010 solution is here for your convenience to include char *’s:

https://github.com/alm4096/MD5-Hash-Example-VS

A bit of an explanation is here:

#include <iostream> //for std::cout
#include <string.h> //for std::string
#include <fstream>
#include "MD5.h"

using std::cout; using std::endl;

int main(int argc, char *argv[])
{
    //Start opening your file
    ifstream inBigArrayfile;
    inBigArrayfile.open ("Data.dat", std::ios::binary | std::ios::in);

    //Find length of file
    inBigArrayfile.seekg (0, std::ios::end);
    long Length = inBigArrayfile.tellg();
    inBigArrayfile.seekg (0, std::ios::beg);    

    //read in the data from your file
    char * InFileData = new char[Length];
    inBigArrayfile.read(InFileData,Length);

    //Calculate MD5 hash
    std::string Temp =  md5(InFileData,Length);
    cout << Temp.c_str() << endl;

    //Clean up
    delete [] InFileData;

    return 0;
}

I have simply added the following into the MD5 library:

MD5.cpp:

MD5::MD5(char * Input, long length)
{
  init();
  update(Input, length);
  finalize();
}

MD5.h:

std::string md5(char * Input, long length);

Questions:
Answers:

I use this file http://people.csail.mit.edu/rivest/Md5.c

Questions:
Answers:

I have used Botan to perform this operation and others before. AraK has pointed out Crypto++. I guess both libraries are perfectly valid. Now it is up to you :-).

Questions:
Answers:

I needed to do this just now and required a cross-platform solution that was suitable for c++11, boost and openssl. I took D’Nabre’s solution as a starting point and boiled it down to the following:

#include <openssl/md5.h>
#include <iomanip>
#include <sstream>
#include <boost/iostreams/device/mapped_file.hpp>

const std::string md5_from_file(const std::string& path)
{
    unsigned char result[MD5_DIGEST_LENGTH];
    boost::iostreams::mapped_file_source src(path);
    MD5((unsigned char*)src.data(), src.size(), result);

    std::ostringstream sout;
    sout<<std::hex<<std::setfill('0');
    for(auto c: result) sout<<std::setw(2)<<(int)c;

    return sout.str();
}

A quick test executable demonstrates:

#include <iostream>

int main(int argc, char *argv[]) {
    if(argc != 2) {
        std::cerr<<"Must specify the file\n";
        exit(-1);
    }
    std::cout<<md5_from_file(argv[1])<<"  "<<argv[1]<<std::endl;
    return 0;
}

Some linking notes:
Linux: -lcrypto -lboost_iostreams
Windows: -DBOOST_ALL_DYN_LINK libeay32.lib ssleay32.lib

Questions:
Answers:

John Walker’s implementation comes with sources.

Questions:
Answers:

There is a pretty library at http://256stuff.com/sources/md5/, with example of use. This is the simplest library for MD5.

Questions:
Answers:

A rework of impementation by @D’Nabre for C++. Don’t forget to compile with -lcrypto at the end: gcc md5.c -o md5 -lcrypto.

#include <iostream>
#include <iomanip>
#include <fstream>
#include <string>

#include <openssl/md5.h>
using namespace std;

unsigned char result[MD5_DIGEST_LENGTH];

// function to print MD5 correctly
void printMD5(unsigned char* md, long size = MD5_DIGEST_LENGTH) {
    for (int i=0; i<size; i++) {
        cout<< hex << setw(2) << setfill('0') << (int) md[i];
    }
}

int main(int argc, char *argv[]) {

if(argc != 2) {
    cout << "Specify the file..." << endl;
    return 0;
}

ifstream::pos_type fileSize;
char * memBlock;

ifstream file (argv[1], ios::ate);

//check if opened
if (file.is_open() ) { cout<< "Using file\t"<< argv[1]<<endl; }
else {
    cout<< "Unnable to open\t"<< argv[1]<<endl;
    return 0;
}

//get file size & copy file to memory
//~ file.seekg(-1,ios::end); // exludes EOF
fileSize = file.tellg();
cout << "File size \t"<< fileSize << endl;
memBlock = new char[fileSize];
file.seekg(0,ios::beg);
file.read(memBlock, fileSize);
file.close();

//get md5 sum
MD5((unsigned char*) memBlock, fileSize, result);

//~ cout << "MD5_DIGEST_LENGTH = "<< MD5_DIGEST_LENGTH << endl;
printMD5(result);
cout<<endl;

return 0;
}

Questions:
Answers:

Using Crypto++, you could do the following:

#include <sha.h>
#include <iostream> 

SHA256 sha; 
while ( !f.eof() ) { 
   char buff[4096];
   int numchars = f.read(...); 
   sha.Update(buff, numchars); 
}
char hash[size]; 
sha.Final(hash); 
cout << hash <<endl; 

I have a need for something very similar, because I can’t read in multi-gigabyte files just to compute a hash. In theory I could memory map them, but I have to support 32bit platforms – that’s still problematic for large files.

Questions:
Answers:

md5.h also have MD5_* functions very useful for big file

#include <openssl/md5.h>
#include <fstream>
.......

std::ifstream file(filename, std::ifstream::binary);
MD5_CTX md5Context;
MD5_Init(&md5Context);
char buf[1024 * 16];
while (file.good()) {
    file.read(buf, sizeof(buf));
    MD5_Update(&md5Context, buf, file.gcount());
}
unsigned char result[MD5_DIGEST_LENGTH];
MD5_Final(result, &md5Context);

Very simple, isn`t it? Convertion to string also very simple:

#include <sstream>
#include <iomanip>
.......

std::stringstream md5string;
md5string << std::hex << std::uppercase << std::setfill('0');
for (const auto &byte: result)
    md5string << std::setw(2) << (int)byte;

return md5string.str();

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