CODEWITHSUNDEEP
c++iosha256
Bobby Strickland
Bobby Strickland

Reputation: 175

How to SHA256 hash a text file in chunks with openssl/sha.h

I'm trying to hash a file on my system and the hash in my C++ code is the correct length but it's a different hash that what I get when I $ echo -n file.txt | sha256sum

I've tried to implement a mixture of what I've seen so far on stackoverflow and finally got something to almost work.

void sha256_file(const std::string &fn)
{
     FILE *file;

     unsigned char buf[8192];
     unsigned char output[SHA256_DIGEST_LENGTH];
     size_t len;

     SHA256_CTX sha256;

     file = fopen(fn.c_str(), "rb");

     if(file == NULL)
           // do whatever
     else
     {
          SHA256_Init(&sha256);
          while((len = fread(buf, 1, sizeof buf, file)) != 0)
               SHA256_Update(&sha256, buf, len);
          fclose(file);
          SHA256_Final(output, &sha256);

          for(int i = 0; i < SHA256_DIGEST_LENGTH; i++)
                printf("%02x", output[1]);
          printf("\n");
     }

Please excuse me I'm trying to learn how to use this with the little documentation and most people are just trying to hash short strings.

$ echo -n file.txt | sha256sum is what I'm using to check the hash but the outputs are different. I'd copy paste but it's on another system.

Upvotes: 0

Views: 1410

Answers (1)

jcarpenter2
jcarpenter2

Reputation: 5478

Easy, you did [1] instead of [i] in your print loop.

      for(int i = 0; i < SHA256_DIGEST_LENGTH; i++)
            printf("%02x", output[1]);

should be

      for(int i = 0; i < SHA256_DIGEST_LENGTH; i++)
            printf("%02x", output[i]);

Silly Latin alphabet!

Upvotes: 1

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