Cogs and Levers A blog full of technical stuff

Hashing with OpenSSL

20 Feb 2017

Today’s post will be a quick tip on generating a hash using OpenSSL.

Setup your makefile

We need to reference libssl and libcrypto in our Makefile:

$(CC) $^ -o $(TARGET) -lssl -lcrypto

The code

A simple main function that will hash a simple message:

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

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

   SHA256_CTX ctx;
   unsigned char digest[32];
   char *msg = "hello";

   SHA256_Init(&ctx);
   SHA256_Update(&ctx, msg, strlen(msg));
   SHA256_Final(digest, &ctx);

   int i = 0;

   for (i = 0; i < 32; i ++) {
      printf("%x", digest[i]);
   }

   return 0;
}

Testing

Running this application just generates a hash of the word “hello”:

$ ./test
2cf24dba5fb0a3e26e83b2ac5b9e29e1b161e5c1fa7425e7343362938b9824%

We can verify our result using sha256sum:

$ echo -n hello | sha256sum 
2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824  -

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