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

/*
  Append a new messages onto the end of a MAC
  Author: Tim Pierson, Dartmouth CS55, Winter 2021

  based example from Du: https://github.com/kevin-w-du/BookCode/blob/master/One_Way_Hash/sha256_padding.c

  compile:
  gcc sha256_padding.c -o sha256_padding -lcrypto

  run:
  ./sha256_padding
  
  will output the SHA-256 hash of an original message with an additional message appended.
*/


int main(int argc, const char *argv[])
{
  SHA256_CTX c;
  unsigned char buffer[SHA256_DIGEST_LENGTH];
  int i;
  char *orig_msg = "Launch a missile at Target A";
  char *key = "theKey";
  char *append_msg = " and at Cambridge";

  SHA256_Init(&c);
  SHA256_Update(&c,
    "theKey:Launch a missile at Target A"     
    "\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"
    "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x18"
    " and at Cambridge",      
    64+17);
  SHA256_Final(buffer, &c);

  printf("New MAC\n");
  for (i = 0; i < 32; i++) {
    printf("%02x", buffer[i]);
  }
  printf("\n");
  return 0;
}