//! Hash-based Key Derivation Function
//! We just repackage other crates

use ::hkdf::Hkdf;
use ::sha3::Sha3_256;
use ::zeroize::Zeroize;

use crate::enc::sym::Secret;

// Hack & tricks:
// HKDF are pretty important, but this lib don't zero out the data.
// we can't use #[derive(Zeroing)] either.
// So we craete a union with a Zeroing object, and drop both manually.

#[derive(Zeroize)]
#[zeroize(drop)]
struct Zeroable([u8; ::core::mem::size_of::<Hkdf<Sha3_256>>()]);

union HkdfInner {
    hkdf: ::core::mem::ManuallyDrop<Hkdf<Sha3_256>>,
    zeroable: ::core::mem::ManuallyDrop<Zeroable>,
}

impl Drop for HkdfInner {
    fn drop(&mut self) {
        #[allow(unsafe_code)]
        unsafe {
            drop(&mut self.hkdf);
            drop(&mut self.zeroable);
        }
    }
}

/// Sha3 based HKDF
#[allow(missing_debug_implementations)]
pub struct HkdfSha3 {
    inner: HkdfInner,
}

impl HkdfSha3 {
    /// Instantiate a new HKDF with Sha3-256
    pub fn new(salt: &[u8], key: Secret) -> Self {
        let hkdf = Hkdf::<Sha3_256>::new(Some(salt), key.as_ref());
        #[allow(unsafe_code)]
        unsafe {
            Self {
                inner: HkdfInner {
                    hkdf: ::core::mem::ManuallyDrop::new(hkdf),
                },
            }
        }
    }
    /// Get a secret generated from the key and a given context
    pub fn get_secret(&self, context: &[u8]) -> Secret {
        let mut out: [u8; 32] = [0; 32];
        #[allow(unsafe_code)]
        unsafe {
            self.inner.hkdf.expand(context, &mut out);
        }
        out.into()
    }
}