Getting hash of a binary file C

Question

I want to get hash of a binary file whose name I have. I have tried the following, but then realized that SHA1() is returning hash value for the string ( name o

Answer 1

You need to read the file chunk by chunk, and compute the digest chunk by chunk.

Read chunks up to e.g. 2048 bytes with fread. Use SHA1_Init in the beginning, SHA1_Update for each chunk, and SHA1_Final in the end.

You can use plain SHA1 function if you read the entire file in one gulp, but this is not recommended.

Another method is to memory-map the file (see mmap() or whatever the Windows equivalent is) and use plain SHA1. This method is very efficient but less portable than the other one.

Answer 2

I don't know how your SHA1() function works (is it from libssl?), but I assume that by

SHA1((unsigned char *)fileName, strlen(fileName),hash);

you are hashing file name, so /bin/ls string. You need to read file content byte by byte into a buffer and hash it.

Answer 3

Thanks to everyone's comments I solved the problem. I am posting the code here, so others might find it beneficial.

void getFileHash(char *fileName){

unsigned char result[2*SHA_DIGEST_LENGTH];
unsigned char hash[SHA_DIGEST_LENGTH];
int i;
FILE *f = fopen(fileName,"rb");
SHA_CTX mdContent;
int bytes;
unsigned char data[1024];

if(f == NULL){
    printf("%s couldn't open file\n",fileName);
    exit(1);
}

SHA1_Init(&mdContent);
while((bytes = fread(data, 1, 1024, f)) != 0){

    SHA1_Update(&mdContent, data, bytes);
}

SHA1_Final(hash,&mdContent);

for(i=0;i<SHA_DIGEST_LENGTH;i++){
    printf("%02x",hash[i]);
}
printf("\n");
/** if you want to see the plain text of the hash */
for(i=0; i < SHA_DIGEST_LENGTH;i++){
    sprintf((char *)&(result[i*2]), "%02x",hash[i]);
}

printf("%s\n",result);

fclose(f);
}

Answer 4

You need just give argument of file context to SHA1(). The variable fileName contains string "/bin/ls" and SHA1() function returns hash of that string. Here is simple example how to read file and get hash

   /* Used variables */
   FILE *fp;
   char string[2048];
   unsigned char hash[SHA_DIGEST_LENGTH];
   int t;

   /* Open file */
   fp=fopen("test","r");
   if (fp == NULL)
   {
      printf("Can not open file\n");
      exit(1);
   }

   /* Read file */
   fread (string,1,sizeof(string),fp);

   /* Get hash of file */
   SHA1((unsigned char *)string, strlen(string),hash);

I hope buffer size of string (2048) will be enough for the file context.