//! HMAC Key Derivation Function (HKDF)
//!
//! This module implements the HMAC-based Extract-and-Expand Key
//! Derivation Function as specified by [1]
//!
//! # Examples
//!
//! ```
//! use cryptoxide::{sha2::Sha256, hkdf::{hkdf_extract, hkdf_expand}};
//!
//! let salt = b"salt";
//! let input = b"input";
//! let mut prk = [0u8; 32];
//! hkdf_extract(Sha256::new(), salt, input, &mut prk);
//! ```
//!
//! [1]: <https://tools.ietf.org/html/rfc5869>

use alloc::vec::Vec;
use core::iter::repeat;

use crate::cryptoutil::copy_memory;
use crate::digest::Digest;
use crate::hmac::Hmac;
use crate::mac::Mac;

/// Execute the HKDF-Extract function.  Applications MUST NOT use this for
/// password hashing.
///
/// # Arguments
/// * digest - The digest function to use.
/// * salt - The optional salt value (a non-secret random value) to use.
/// * ikm - The input keying material to use.
/// * prk - The output buffer to fill with a `digest.output_bytes()` length
///         pseudo random key.
pub fn hkdf_extract<D: Digest>(mut digest: D, salt: &[u8], ikm: &[u8], prk: &mut [u8]) {
    assert!(prk.len() == digest.output_bytes());
    digest.reset();

    let mut mac = Hmac::new(digest, salt);
    mac.input(ikm);
    mac.raw_result(prk);
    mac.reset();
}

/// Execute the HKDF-Expand function.  Applications MUST NOT use this for
/// password hashing.
///
/// # Arguments
/// * digest - The digest function to use.
/// * prk - The pseudorandom key of at least `digest.output_bytes()` octets.
/// * info - The optional context and application specific information to use.
/// * okm - The output buffer to fill with the derived key value.
pub fn hkdf_expand<D: Digest>(mut digest: D, prk: &[u8], info: &[u8], okm: &mut [u8]) {
    digest.reset();

    let mut mac = Hmac::new(digest, prk);
    let os = mac.output_bytes();
    let mut t: Vec<u8> = repeat(0).take(os).collect();
    let mut n: u8 = 0;

    for chunk in okm.chunks_mut(os) {
        // The block index starts at 1. So, this is supposed to run on the first execution.
        n = n.checked_add(1).expect("HKDF size limit exceeded.");

        if n != 1 {
            mac.input(&t[..]);
        }
        let nbuf = [n];
        mac.input(info);
        mac.input(&nbuf);
        mac.raw_result(&mut t);
        mac.reset();
        let chunk_len = chunk.len();
        copy_memory(&t[..chunk_len], chunk);
    }
}

#[cfg(test)]
mod test {
    use std::iter::repeat;
    use std::vec::Vec;

    use crate::digest::Digest;
    use crate::hkdf::{hkdf_expand, hkdf_extract};
    use crate::sha2::Sha256;

    struct TestVector<D: Digest> {
        digest: D,
        ikm: Vec<u8>,
        salt: Vec<u8>,
        info: Vec<u8>,
        l: usize,

        prk: Vec<u8>,
        okm: Vec<u8>,
    }

    #[test]
    fn test_hkdf_rfc5869_sha256_vectors() {
        let test_vectors = vec![
            TestVector {
                digest: Sha256::new(),
                ikm: repeat(0x0b).take(22).collect(),
                salt: (0x00..=0x0c).collect(),
                info: (0xf0..=0xf9).collect(),
                l: 42,
                prk: vec![
                    0x07, 0x77, 0x09, 0x36, 0x2c, 0x2e, 0x32, 0xdf, 0x0d, 0xdc, 0x3f, 0x0d, 0xc4,
                    0x7b, 0xba, 0x63, 0x90, 0xb6, 0xc7, 0x3b, 0xb5, 0x0f, 0x9c, 0x31, 0x22, 0xec,
                    0x84, 0x4a, 0xd7, 0xc2, 0xb3, 0xe5,
                ],
                okm: vec![
                    0x3c, 0xb2, 0x5f, 0x25, 0xfa, 0xac, 0xd5, 0x7a, 0x90, 0x43, 0x4f, 0x64, 0xd0,
                    0x36, 0x2f, 0x2a, 0x2d, 0x2d, 0x0a, 0x90, 0xcf, 0x1a, 0x5a, 0x4c, 0x5d, 0xb0,
                    0x2d, 0x56, 0xec, 0xc4, 0xc5, 0xbf, 0x34, 0x00, 0x72, 0x08, 0xd5, 0xb8, 0x87,
                    0x18, 0x58, 0x65,
                ],
            },
            TestVector {
                digest: Sha256::new(),
                ikm: (0x00..=0x4f).collect(),
                salt: (0x60..=0xaf).collect(),
                info: (0xb0..=0xff).map(|x| x as u8).collect(),
                l: 82,
                prk: vec![
                    0x06, 0xa6, 0xb8, 0x8c, 0x58, 0x53, 0x36, 0x1a, 0x06, 0x10, 0x4c, 0x9c, 0xeb,
                    0x35, 0xb4, 0x5c, 0xef, 0x76, 0x00, 0x14, 0x90, 0x46, 0x71, 0x01, 0x4a, 0x19,
                    0x3f, 0x40, 0xc1, 0x5f, 0xc2, 0x44,
                ],
                okm: vec![
                    0xb1, 0x1e, 0x39, 0x8d, 0xc8, 0x03, 0x27, 0xa1, 0xc8, 0xe7, 0xf7, 0x8c, 0x59,
                    0x6a, 0x49, 0x34, 0x4f, 0x01, 0x2e, 0xda, 0x2d, 0x4e, 0xfa, 0xd8, 0xa0, 0x50,
                    0xcc, 0x4c, 0x19, 0xaf, 0xa9, 0x7c, 0x59, 0x04, 0x5a, 0x99, 0xca, 0xc7, 0x82,
                    0x72, 0x71, 0xcb, 0x41, 0xc6, 0x5e, 0x59, 0x0e, 0x09, 0xda, 0x32, 0x75, 0x60,
                    0x0c, 0x2f, 0x09, 0xb8, 0x36, 0x77, 0x93, 0xa9, 0xac, 0xa3, 0xdb, 0x71, 0xcc,
                    0x30, 0xc5, 0x81, 0x79, 0xec, 0x3e, 0x87, 0xc1, 0x4c, 0x01, 0xd5, 0xc1, 0xf3,
                    0x43, 0x4f, 0x1d, 0x87,
                ],
            },
            TestVector {
                digest: Sha256::new(),
                ikm: repeat(0x0b).take(22).collect(),
                salt: vec![],
                info: vec![],
                l: 42,
                prk: vec![
                    0x19, 0xef, 0x24, 0xa3, 0x2c, 0x71, 0x7b, 0x16, 0x7f, 0x33, 0xa9, 0x1d, 0x6f,
                    0x64, 0x8b, 0xdf, 0x96, 0x59, 0x67, 0x76, 0xaf, 0xdb, 0x63, 0x77, 0xac, 0x43,
                    0x4c, 0x1c, 0x29, 0x3c, 0xcb, 0x04,
                ],
                okm: vec![
                    0x8d, 0xa4, 0xe7, 0x75, 0xa5, 0x63, 0xc1, 0x8f, 0x71, 0x5f, 0x80, 0x2a, 0x06,
                    0x3c, 0x5a, 0x31, 0xb8, 0xa1, 0x1f, 0x5c, 0x5e, 0xe1, 0x87, 0x9e, 0xc3, 0x45,
                    0x4e, 0x5f, 0x3c, 0x73, 0x8d, 0x2d, 0x9d, 0x20, 0x13, 0x95, 0xfa, 0xa4, 0xb6,
                    0x1a, 0x96, 0xc8,
                ],
            },
        ];

        for t in test_vectors.iter() {
            let mut prk: Vec<u8> = repeat(0).take(t.prk.len()).collect();
            hkdf_extract(t.digest.clone(), &t.salt[..], &t.ikm[..], &mut prk);
            assert!(prk == t.prk);

            let mut okm: Vec<u8> = repeat(0).take(t.okm.len()).collect();
            assert!(okm.len() == t.l);
            hkdf_expand(t.digest.clone(), &prk[..], &t.info[..], &mut okm);
            assert!(okm == t.okm);
        }
    }
}