C Hashing Files With OpenSSL

2020-07-23

3 minute read

C , Programming

Hashing files with C is actually quite easily accomplished using existing OpenSSL library headers. OpenSSL is quite commonly used for many applications such as for TLS/SSL based web servers or even basic cryptographic tasks such as file hashing and encryption.

Requirements:

OpenSSL headers

Make sure to compile the following programs using the -lcrypto flag.

gcc -o main main.c -lcrypto

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/* String hashing example */
#include <stdio.h>
#include <string.h>
#include <openssl/evp.h>	/* OpenSSL EVP headers for hashing */

#define BUFF_SIZE 4096	/* buffer size */

int main()
{
	size_t          i;					        /* loop variables */
	char            buff[BUFF_SIZE];	        /* buffer for hash */
	unsigned int    md_len;				        /* stores hash length */
	EVP_MD_CTX      *mdctx;				        /* stores hash context */
	unsigned char md_value[EVP_MAX_MD_SIZE];    /* stores hash */
	char *input = "test hash string";		    /* input */

	mdctx = EVP_MD_CTX_new();	/* Initialize new ctx */
	const EVP_MD *EVP_md5();    /* use EVP md5 function */

	EVP_DigestInit_ex(mdctx, EVP_md5(), NULL);	    /* Initializes digest type */
	EVP_DigestUpdate(mdctx, input, strlen(input));	/* Add input to hash context */
	EVP_DigestFinal_ex(mdctx, md_value, &md_len);	/* Finalize hash context */
	
	for (i = 0; i < md_len; i++)    /* loops through hash length */
	{
        printf("%02x", md_value[i]);    /* prints 2 hex-values of hash per loop */
	}
	printf("  %s\n", input);	/* prints out input after hash */

	return 0;	/* returns 0 */
}

This program example takes in a simple string and hashes it and then displays the hash encoded as hex next to input string.

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/* File hashing example */
#include <stdio.h>
#include <stdlib.h>
#include <openssl/evp.h>        /* OpenSSL EVP headers for hashing */

#define BUFF_SIZE 4096  /* buffer size */

int main()
{
    size_t          i, n;                       /* loop variables */
    FILE            *fp;					    /* file pointer */
    char            buff[BUFF_SIZE];            /* buffer for hash */
    unsigned int    md_len;                     /* stores hash length */
    EVP_MD_CTX      *mdctx;                     /* stores hash context */
    unsigned char   md_value[EVP_MAX_MD_SIZE];  /* stores hash */
    char            *input = "/bin/echo";       /* input file */

    fp = fopen(input, "rb");	/* reads in file */
    if (fp == NULL)
    {
    	printf("failed to open file %s", input);
	    exit(1);
    }

    mdctx = EVP_MD_CTX_new();   /* Initialize new ctx */
    const EVP_MD *EVP_md5();    /* use EVP md5 function */

    EVP_DigestInit_ex(mdctx, EVP_md5(), NULL);  /* Initializes digest type */

    while ((n = fread(buff, 1, sizeof(buff), fp)))  /* reads in values from buffer containing file pointer */
    {
            EVP_DigestUpdate(mdctx, buff, n);   /* Add buffer to hash context */
    }

    EVP_DigestFinal_ex(mdctx, md_value, &md_len);   /* Finalize hash context */

    for (i = 0; i < md_len; i++)    /* loops through hash length */
    {
            printf("%02x", md_value[i]);    /* prints 2 hex-values of hash per loop */
    }
    printf("  %s\n", input);    /* prints out input after hash */

    return 0;       /* returns 0 */
}

The program above takes in a file path containing a file and then diplays the hash encoded as a hex string next to the input file.

C Programming

References:

OpenSSL