//! Key derivation functions

//!

//! Tor has three relevant key derivation functions that we use for

//! deriving keys used for relay encryption.

//!

//! The *KDF-TOR* KDF (implemented by `LegacyKdf`) is used with the old

//! TAP handshake.  It is ugly, it is based on SHA-1, and it should be

//! avoided for new uses.

//!

//! The *HKDF-SHA256* KDF (implemented by `Ntor1Kdf`) is used with the

//! Ntor handshake.  It is based on RFC5869 and SHA256.

//!

//! The *SHAKE* KDF (implemented by `ShakeKdf` is used with v3 onion

//! services, and is likely to be used by other places in the future.

//! It is based on SHAKE-256.

use crate::{Error, Result, SecretBytes};

use digest::{ExtendableOutput, Update, XofReader};

use tor_llcrypto::d::{Sha1, Sha256, Shake256};

use zeroize::Zeroizing;

/// A trait for a key derivation function.

pub(crate) trait Kdf {

    /// Derive `n_bytes` of key data from some secret `seed`.

    fn derive(&self, seed: &[u8], n_bytes: usize) -> Result<SecretBytes>;

}

/// A legacy KDF, for use with TAP.

///

/// This KDF is based on SHA1.  Don't use this for anything new.

pub(crate) struct LegacyKdf {

    /// Starting index value for the TAP kdf.  should always be 1.

    idx: u8,

}

/// A parameterized KDF, for use with ntor.

///

/// This KDF is based on HKDF-SHA256.

pub(crate) struct Ntor1Kdf<'a, 'b> {

    /// A constant for parameterizing the kdf, during the key extraction

    /// phase.

    t_key: &'a [u8],

    /// Another constant for parameterizing the kdf, during the key

    /// expansion phase.

    m_expand: &'b [u8],

}

/// A modern KDF, for use with v3 onion services.

///

/// This KDF is based on SHAKE256

pub(crate) struct ShakeKdf();

impl LegacyKdf {

    /// Instantiate a LegacyKdf.

}

impl Kdf for LegacyKdf {

}

impl<'a, 'b> Ntor1Kdf<'a, 'b> {

    /// Instantiate an Ntor1Kdf, with given values for t_key and m_expand.

}

impl Kdf for Ntor1Kdf<'_, '_> {

}

impl ShakeKdf {

    /// Instantiate a ShakeKdf.

}

impl Kdf for ShakeKdf {

}

#[cfg(test)]

mod test {

    #![allow(clippy::unwrap_used)]

    use super::*;

    use hex_literal::hex;

    #[test]

    fn clearbox_tap_kdf() {

        // Calculate an instance of the TAP KDF, based on its spec in

        // tor-spec.txt.

        use digest::Digest;

        let input = b"here is an example key seed that we will expand";

        let result = LegacyKdf::new(6).derive(input, 99).unwrap();

        let mut expect_result = Vec::new();

        let mut k0: Vec<u8> = Vec::new();

        k0.extend(&input[..]);

        for x in 6..11 {

            k0.push(x);

            expect_result.extend(Sha1::digest(&k0));

            k0.pop();

        }

        expect_result.truncate(99);

        assert_eq!(&result[..], &expect_result[..]);

    }

    #[test]

    fn testvec_tap_kdf() {

        // Taken from test_crypto.c in Tor, generated by a python script.

        fn expand(b: &[u8]) -> SecretBytes {

            LegacyKdf::new(0).derive(b, 100).unwrap()

        }

        let expect = hex!(

            "5ba93c9db0cff93f52b521d7420e43f6eda2784fbf8b4530d8

             d246dd74ac53a13471bba17941dff7c4ea21bb365bbeeaf5f2

             c654883e56d11e43c44e9842926af7ca0a8cca12604f945414

             f07b01e13da42c6cf1de3abfdea9b95f34687cbbe92b9a7383"

        );

        assert_eq!(&expand(&b""[..])[..], &expect[..]);

        let expect = hex!(

            "776c6214fc647aaa5f683c737ee66ec44f03d0372e1cce6922

             7950f236ddf1e329a7ce7c227903303f525a8c6662426e8034

             870642a6dabbd41b5d97ec9bf2312ea729992f48f8ea2d0ba8

             3f45dfda1a80bdc8b80de01b23e3e0ffae099b3e4ccf28dc28"

        );

        assert_eq!(&expand(&b"Tor"[..])[..], &expect[..]);

        let brunner_quote = b"AN ALARMING ITEM TO FIND ON A MONTHLY AUTO-DEBIT NOTICE";

        let expect = hex!(

            "a340b5d126086c3ab29c2af4179196dbf95e1c72431419d331

             4844bf8f6afb6098db952b95581fb6c33625709d6f4400b8e7

             ace18a70579fad83c0982ef73f89395bcc39493ad53a685854

             daf2ba9b78733b805d9a6824c907ee1dba5ac27a1e466d4d10"

        );

        assert_eq!(&expand(&brunner_quote[..])[..], &expect[..]);

    }

    #[test]

    fn fail_tap_kdf() {

        let result = LegacyKdf::new(6).derive(&b"x"[..], 10000);

        assert!(result.is_err());

    }

    #[test]

    fn clearbox_ntor1_kdf() {

        // Calculate Ntor1Kdf, and make sure we get the same result by

        // following the calculation in the spec.

        let input = b"another example key seed that we will expand";

        let result = Ntor1Kdf::new(&b"key"[..], &b"expand"[..])

            .derive(input, 99)

            .unwrap();

        let kdf = hkdf::Hkdf::<Sha256>::new(Some(&b"key"[..]), &input[..]);

        let mut expect_result = vec![0_u8; 99];

        kdf.expand(&b"expand"[..], &mut expect_result[..]).unwrap();

        assert_eq!(&expect_result[..], &result[..]);

    }

    #[test]

    fn testvec_ntor1_kdf() {

        // From Tor's test_crypto.c; generated with ntor_ref.py

        fn expand(b: &[u8]) -> SecretBytes {

            let t_key = b"ntor-curve25519-sha256-1:key_extract";

            let m_expand = b"ntor-curve25519-sha256-1:key_expand";

            Ntor1Kdf::new(&t_key[..], &m_expand[..])

                .derive(b, 100)

                .unwrap()

        }

        let expect = hex!(

            "5521492a85139a8d9107a2d5c0d9c91610d0f95989975ebee6

             c02a4f8d622a6cfdf9b7c7edd3832e2760ded1eac309b76f8d

             66c4a3c4d6225429b3a016e3c3d45911152fc87bc2de9630c3

             961be9fdb9f93197ea8e5977180801926d3321fa21513e59ac"

        );

        assert_eq!(&expand(&b"Tor"[..])[..], &expect[..]);

        let brunner_quote = b"AN ALARMING ITEM TO FIND ON YOUR CREDIT-RATING STATEMENT";

        let expect = hex!(

            "a2aa9b50da7e481d30463adb8f233ff06e9571a0ca6ab6df0f

             b206fa34e5bc78d063fc291501beec53b36e5a0e434561200c

             5f8bd13e0f88b3459600b4dc21d69363e2895321c06184879d

             94b18f078411be70b767c7fc40679a9440a0c95ea83a23efbf"

        );

        assert_eq!(&expand(&brunner_quote[..])[..], &expect[..]);

    }

    #[test]

    fn testvec_shake_kdf() {

        // This is just one of the shake test vectors from tor-llcrypto

        let input = hex!(

            "76891a7bcc6c04490035b743152f64a8dd2ea18ab472b8d36ecf45

             858d0b0046"

        );

        let expected = hex!(

            "e8447df87d01beeb724c9a2a38ab00fcc24e9bd17860e673b02122

             2d621a7810e5d3"

        );

        let result = ShakeKdf::new().derive(&input[..], expected.len());

        assert_eq!(&result.unwrap()[..], &expected[..]);

    }

}