Question

Below is my code for creating a hash in openssl.
I was wondering why is my output always partially diffrent.

int main()
{
    char alg[] = "MD5";
    char buf[EVP_MAX_KEY_LENGTH] = "5";
    unsigned char *testKey;
    unsigned int olen;  

    testKey = simple_digest(alg, buf, EVP_MAX_KEY_LENGTH,&olen);

    std::cout << "Printing key : >>";
    print_hex(testKey,EVP_MAX_KEY_LENGTH);
    std::cout << "<<" << std::endl;
}

unsigned char *simple_digest(char *alg, char *buf, unsigned int len,unsigned int *olen)
{
    const EVP_MD *m;
    EVP_MD_CTX ctx;
    unsigned char *ret;
    OpenSSL_add_all_digests();

    if (!(m = EVP_get_digestbyname(alg)))
        return NULL;
    if (!(ret = (unsigned char *)malloc(EVP_MAX_MD_SIZE)))
        return NULL;


    EVP_DigestInit(&ctx, m);
    EVP_DigestUpdate(&ctx, buf, len);
    EVP_DigestFinal(&ctx, ret, olen);

//  std::cout << "computed key" << ret << std::endl;

    return ret;
} 

void print_hex(unsigned char *bs, unsigned int n)
{
    int i;
    for (i = 0; i < n; i++)
        printf("%02x", bs[i]);
    printf("\n");
}

Output :

3afb8ebc9c93bf6d40285736f210b7856af8bab4d040ca090043ca09f840ca09
3afb8ebc9c93bf6d40285736f210b7856af8bab490a5f909c0a7f909b8a5f909

As you can see only the back few characters differ.

Thanks in advance! 😀

---

Update (correct working version) :

int main()
{
    char alg[] = "MD5";
    char buf[EVP_MAX_KEY_LENGTH] = "5";
    unsigned char *testKey;
    unsigned int olen;  

    testKey = simple_digest(alg, buf,strlen(buf),&olen);

    std::cout << "Printing key : >>";
    print_hex(testKey,olen);
    std::cout << "<<" << std::endl;

}

unsigned char *simple_digest(char *alg, char *buf, unsigned int len,unsigned int *olen)
{
    const EVP_MD *m;
    EVP_MD_CTX ctx;
    unsigned char *ret;
    OpenSSL_add_all_digests();

    if (!(m = EVP_get_digestbyname(alg)))
        return NULL;
    if (!(ret = (unsigned char *)malloc(EVP_MAX_KEY_LENGTH)))
        return NULL;

    EVP_DigestInit(&ctx, m);
    EVP_DigestUpdate(&ctx, buf, len);
    EVP_DigestFinal(&ctx, ret, olen);

    return ret;
} 

void print_hex(unsigned char *bs, unsigned int n)
{
    int i;
    for (i = 0; i < n; i++)
        printf("%02x", bs[i]);
}

output :

Printing key : >>e4da3b7fbbce2345d7772b0674a318d5<<

Answer 1

You are trying to hash a one-character string, “5”, but you are telling EVP_Digest_Update that your buffer length is EVP_MAX_KEY_LENGTH. You need to pass the actual length of your buffer, not the maximum length it could be.

For completeness, here is what your main function should look like:

int main()
{
    char alg[] = "MD5";
    char buf[EVP_MAX_KEY_LENGTH] = "5";
    unsigned char *testKey;
    unsigned int olen;  

    # Pass the true length of but
    testKey = simple_digest(alg, buf, strlen(buf), &olen);

    std::cout << "Printing key : >>";
    # Pass the true length of testKey
    print_hex(testKey,olen);
    std::cout << "<<" << std::endl;
}

One other caveat: you probably don’t want to use EVP_MAX_KEY_LENGTH for your buffer, as that would limit the size of the message you could hash.