[gnutls-devel] GnuTLS | Support for ECH (encrypted client hello) (!1748)

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Daiki Ueno commented on a discussion on lib/nettle/hpke-hkdf.c: https://gitlab.com/gnutls/gnutls/-/merge_requests/1748#note_1421353259

> +			      const struct hpke_dhkem *dhkem,
> +			      struct ecc_scalar *sk, struct ecc_point *pk)
> +{
> +	int r = 1, counter = 0;
> +	uint8_t *dkp_prk = NULL, *bytes = NULL;
> +	mpz_t z, t;
> +
> +	mpz_init(z);
> +
> +	dkp_prk = _nettle_labeled_extract_kem(NULL, "dkp_prk", ikm, ikm_len,
> +					      dhkem);
> +	if (!dkp_prk) {
> +		r = 0;
> +		goto out;
> +	}
> +	while (ecc_scalar_set((struct hpke_dhkem *)dhkem, z)) {

The issue is that `struct dhkem *` and `ecc_scalar *` are completely different types and you can't safely typecast. Perhaps you could try adding this modification to the HPKE branch of nettle and run the tests, and I guess you will most likely see a segmentation fault.

If you don't want to touch `sk` in this loop, one option would be to create a temporary scalar variable:

```c
ecc_scalar tmp_sk;

ecc_scalar_init (&tmp_sk, dhkem->ecc);
while (!ecc_scalar_set (&tmp_sk, z))
  {
    ...
  }
ecc_scalar_clear (&tmp_sk);
```

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