How to store SHA1 output to a file using C

Question

I have computed the SHA1 for a piece of data and I need to store it into a file to be read by another person later. The SHA1 gives the output as 190 bit's, but I can't store as bits into a file, fputc() function does it characters which means bytes.

So I want to either store it as bytes into the file or as its hexadecimal representation (preferably). In C, so far I can only get it to print the hash to the terminal as the hexadecimal representation (using a way I found in another question posted in this site), but I can't figure out how to store it to a file properly.

Please help!

Answer 1

If you want to write the SHA1 digest as binary digest in the file, you can use:

fwrite ( &sha1_digest, sizeof(sha1_digest), 1, stream );

Where stream is a FILE* opened in binary mode.

If you want to save it as hexadecimal string, you can do a loop :

char *p=(void*)&sha1_digest; 
int i=sizeof(sha1_digest); 
while (i--) 
    fprintf (stream, "%02x",*p++); 

where tream is a FILE* opened in text or binary mode

I assumed that sha1_digest was a structure. If it's an array, you don't need the & and if the array is not fixed size, then you must provide the size of the digest (because in this case, sizeof() would return the sizeof a pointer isntead of the array.)

You are certainly aware, but SHA1 is no longer considered as a safe hash, if your use is somewhat related to security.