CODEWITHSUNDEEP
cmd5cryptoapi
Natasha
Natasha

Reputation: 367

Computing hash with crypto API

I've been trying to compute md5 hash using crypto API but have faced a lot of issues. 

char * getHashedKey(char *keybuf) {
  char * output;
  struct scatterlist sg;
  struct crypto_hash *tfm;
  struct hash_desc desc;
  int i;

  printk("%s received keybuf %s %d\n", __func__, keybuf, strlen(keybuf));
  output = kmalloc(sizeof(*output) * 16, GFP_KERNEL);
  memset(output, 0x00, 16);

  // works if I overwrite value like this
  //keybuf = "abcdef012345";

  tfm = crypto_alloc_hash("md5", 0, CRYPTO_ALG_ASYNC);
  desc.tfm = tfm;
  desc.flags = 0;

  sg_init_one(&sg, keybuf, strlen(keybuf));
  // tried type casting too
  //sg_init_one(&sg, (u8 *) keybuf, strlen(keybuf));
  crypto_hash_init(&desc);
  crypto_hash_update(&desc, &sg, strlen(keybuf));
  crypto_hash_final(&desc, output);

  for(i = 0; i < 16; i++)
  {
      printk("%x", output[i]);
  }
  printk("\n");

  return output;
}

Here's what I tried: 1. If "output" is unsigned char, I get correct output but not when I'm using char.

  1. If I over write the value of keybuf, only then it works.

  2. I tried type casting keybuf as (u8*) as mentioned in some forums but that doesn't work too

The distorted output by using char is:

3ffffffe9ffffffb7ffffffb472ffffffe0ffffffed41225affffffebffffffdaffffffd3ffffffbaffffffabffffffde

Can someone help me with this?

Upvotes: 0

Views: 178

Answers (1)

user149341
user149341

Reputation:

You have two display issues.

First: Passing a char value -- implicitly a signed char -- as an argument to printf() causes it to be extended to a signed int. As a result, passing a value with the high bit set (e.g, 0xe9, the second byte in your sample output) will cause it to be sign-extended to 0xffffffe9 and printed as such.

To fix this, declare output as an array of unsigned char, or the equivalent type u8.

Second: You are printing each byte using the %x format string, with no padding specified. This will cause values between 0x0 and 0xf to be printed as a single character instead of two, resulting in ambiguous output.

To fix this, use the %02x format string.

Upvotes: 1

Related Questions