How to create a md5 hash of a string in C

Question

I ve found some md5 code that consists of the following prototypes     I ve been trying to find out where I have to put the string I want to hash  what functions I need to call  and where to find the string once it has been hashed  I m confused with regards to what the uint32 buf 4  and uint32 bits 2  are in the struct   struct MD5Context       uint32 buf 4       uint32 bits 2       unsigned char in 64             Start MD5 accumulation   Set bit count to 0 and buffer to mysterious    initialization constants      void MD5Init struct MD5Context  context          Update context to reflect the concatenation of another buffer full    of bytes      void MD5Update struct MD5Context  context  unsigned char const  buf  unsigned len          Final wrapup - pad to 64-byte boundary with the bit pattern     1 0   64-bit count of bits processed  MSB-first      void MD5Final unsigned char digest 16   struct MD5Context  context          The core of the MD5 algorithm  this alters an existing MD5 hash to    reflect the addition of 16 longwords of new data   MD5Update blocks    the data and converts bytes into longwords for this routine      void MD5Transform uint32 buf 4   uint32 const in 16

User · Answer

All of the existing answers use the deprecated MD5Init(), MD5Update(), and MD5Final().

Instead, use EVP_DigestInit_ex(), EVP_DigestUpdate(), and EVP_DigestFinal_ex(), e.g.

// example.c
//
// gcc example.c -lssl -lcrypto -o example

#include <openssl/evp.h>
#include <stdio.h>
#include <string.h>

void bytes2md5(const char *data, int len, char *md5buf) {
  // Based on https://www.openssl.org/docs/manmaster/man3/EVP_DigestUpdate.html
  EVP_MD_CTX *mdctx = EVP_MD_CTX_new();
  const EVP_MD *md = EVP_md5();
  unsigned char md_value[EVP_MAX_MD_SIZE];
  unsigned int md_len, i;
  EVP_DigestInit_ex(mdctx, md, NULL);
  EVP_DigestUpdate(mdctx, data, len);
  EVP_DigestFinal_ex(mdctx, md_value, &md_len);
  EVP_MD_CTX_free(mdctx);
  for (i = 0; i < md_len; i++) {
    snprintf(&(md5buf[i * 2]), 16 * 2, "%02x", md_value[i]);
  }
}

int main(void) {
  const char *hello = "hello";
  char md5[33]; // 32 characters + null terminator
  bytes2md5(hello, strlen(hello), md5);
  printf("%s\n", md5);
}

User · Answer

To be honest  the comments accompanying the prototypes seem clear enough  Something like this should do the trick   void compute md5 char  str  unsigned char digest 16         MD5Context ctx      MD5Init  amp ctx       MD5Update  amp ctx  str  strlen str        MD5Final digest   amp ctx       where str is a C string you want the hash of  and digest is the resulting MD5 digest

User · Answer

It would appear that you should   Create a struct MD5context and pass it to MD5Init to get it into a proper starting condition Call MD5Update with the context and your data Call MD5Final to get the resulting hash   These three functions and the structure definition make a nice abstract interface to the hash algorithm  I m not sure why you were shown the core transform function in that header as you probably shouldn t interact with it directly   The author could have done a little more implementation hiding by making the structure an abstract type  but then you would have been forced to allocate the structure on the heap every time  as opposed to now where you can put it on the stack if you so desire

User · Answer

I don t know this particular library  but I ve used very similar calls  So this is my best guess   unsigned char digest 16   const char  string    Hello World   struct MD5Context context  MD5Init  amp context   MD5Update  amp context  string  strlen string    MD5Final digest   amp context     This will give you back an integer representation of the hash  You can then turn this into a hex representation if you want to pass it around as a string   char md5string 33   for int i   0  i  lt  16    i      sprintf  amp md5string i 2     02x    unsigned int digest i

User · Answer

As other answers have mentioned  the following calls will compute the hash   MD5Context md5  MD5Init  amp md5   MD5Update  amp md5  data  datalen   MD5Final digest   amp md5     The purpose of splitting it up into that many functions is to let you stream large datasets   For example  if you re hashing a 10GB file and it doesn t fit into ram  here s how you would go about doing it  You would read the file in smaller chunks and call MD5Update on them   MD5Context md5  MD5Init  amp md5    fread    Read a block into data      MD5Update  amp md5  data  datalen    fread    Read the next block into data      MD5Update  amp md5  data  datalen    fread    Read the next block into data      MD5Update  amp md5  data  datalen             Now finish to get the final hash value  MD5Final digest   amp md5

User · Answer

Here s a complete example    include  lt stdio h gt   include  lt stdlib h gt   include  lt string h gt   if defined   APPLE       define COMMON DIGEST FOR OPENSSL    include  lt CommonCrypto CommonDigest h gt     define SHA1 CC SHA1  else    include  lt openssl md5 h gt   endif  char  str2md5 const char  str  int length        int n      MD5 CTX c      unsigned char digest 16       char  out    char  malloc 33        MD5 Init  amp c        while  length  gt  0            if  length  gt  512                MD5 Update  amp c  str  512             else               MD5 Update  amp c  str  length                     length -  512          str    512             MD5 Final digest   amp c        for  n   0  n  lt  16    n            snprintf  amp  out n 2    16 2    02x    unsigned int digest n               return out         int main int argc  char   argv            char  output   str2md5  hello   strlen  hello             printf   s n   output           free output           return 0

[c] How to create a md5 hash of a string in C?

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