How to use SHA1 hashing in C programming

Question

I am trying to write a C program that proves SHA1 is nearly collision free, but I cannot figure out how to actually create the hash for my input values. I just need to create the hash, and store the hex value into an array. After some Google searches, I've found OpenSSL documentation directing me to use this:

 #include <openssl/sha.h>   unsigned char *SHA1(const unsigned char *d, unsigned long n,                   unsigned char *md);   int SHA1_Init(SHA_CTX *c);  int SHA1_Update(SHA_CTX *c, const void *data,                   unsigned long len);  int SHA1_Final(unsigned char *md, SHA_CTX *c); 

I believe I should be using either unsigned char *SHA1 or SHA1_Init, but I am not sure what the arguments would be, given x is my input to be hashed. Would someone please clear this up for me? Thanks.

Answer 1

If you have all of your data at once, just use the SHA1 function:

// The data to be hashed char data[] = "Hello, world!"; size_t length = strlen(data);  unsigned char hash[SHA_DIGEST_LENGTH]; SHA1(data, length, hash); // hash now contains the 20-byte SHA-1 hash 

If, on the other hand, you only get your data one piece at a time and you want to compute the hash as you receive that data, then use the other functions:

// Error checking omitted for expository purposes  // Object to hold the current state of the hash SHA_CTX ctx; SHA1_Init(&ctx);  // Hash each piece of data as it comes in: SHA1_Update(&ctx, "Hello, ", 7); ... SHA1_Update(&ctx, "world!", 6); // etc. ... // When you're done with the data, finalize it: unsigned char hash[SHA_DIGEST_LENGTH]; SHA1_Final(hash, &ctx);