hkdf/

lib.rs

//! This crate implements the [RFC5869] hash based key derivation function using
//! [`bitcoin_hashes`].
//!
//! [RFC5869]: https://www.rfc-editor.org/rfc/rfc5869
//! [`bitcoin_hashes`]: https://docs.rs/bitcoin_hashes/latest/bitcoin_hashes/

use std::cmp::min;

pub use bitcoin_hashes;
pub use bitcoin_hashes::Hash as BitcoinHash;
use bitcoin_hashes::{HashEngine, Hmac, HmacEngine};

pub mod hashes {
    pub use bitcoin_hashes::hash160::Hash as Hash160;
    pub use bitcoin_hashes::ripemd160::Hash as Ripemd160;
    pub use bitcoin_hashes::sha1::Hash as Sha1;
    pub use bitcoin_hashes::sha256::Hash as Sha256;
    pub use bitcoin_hashes::sha256d::Hash as Sha256d;
    pub use bitcoin_hashes::sha512::Hash as Sha512;
    pub use bitcoin_hashes::siphash24::Hash as Siphash24;
}

/// Implements the [RFC5869] hash based key derivation function using the hash
/// function `H`.
///
/// [RFC5869]: https://www.rfc-editor.org/rfc/rfc5869
#[derive(Clone)]
pub struct Hkdf<H: BitcoinHash> {
    prk: Hmac<H>,
}

impl<H: BitcoinHash> Hkdf<H> {
    /// Run HKDF-extract and keep the resulting pseudo random key as internal
    /// state
    ///
    /// ## Inputs
    /// * `ikm`: Input keying material, secret key material our keys will be
    ///   derived from
    /// * `salt`: Optional salt value, if not required set to `&[0; H::LEN]`. As
    ///   noted in the RFC the salt value can also be a secret.
    pub fn new(ikm: &[u8], salt: Option<&[u8]>) -> Self {
        let mut engine = HmacEngine::new(salt.unwrap_or(&vec![0x00; H::LEN]));
        engine.input(ikm);

        Hkdf {
            prk: Hmac::from_engine(engine),
        }
    }

    /// Construct the HKDF from a pseudo random key that has the correct
    /// distribution and length already (e.g. because it's the output of a
    /// previous HKDF round), skipping the HKDF-extract step. **If in doubt,
    /// please use `Hkdf::new` instead!**
    ///
    /// See also [`Hkdf::derive_hmac`].
    pub fn from_prk(prk: Hmac<H>) -> Self {
        Hkdf { prk }
    }

    /// Run HKDF-expand to generate new key material
    ///
    /// ## Inputs
    /// * `info`: Defines which key to derive. Different values lead to
    ///   different keys.
    /// * `LEN`: Defines the length of the key material to generate in octets.
    ///   Note that `LEN <= H::LEN * 255` has to be true.
    ///
    /// ## Panics
    /// If `LEN > H::LEN * 255`.
    pub fn derive<const LEN: usize>(&self, info: &[u8]) -> [u8; LEN] {
        // TODO: make const once rust allows
        let iterations = if LEN % H::LEN == 0 {
            LEN / H::LEN
        } else {
            LEN / H::LEN + 1
        };

        // Make sure we can cast iteration numbers to u8 later
        assert!(
            iterations <= 255,
            "RFC5869 only supports output length of up to 255*HashLength"
        );

        let mut output = [0u8; LEN];
        for iteration in 0..iterations {
            let current_slice = (H::LEN * iteration)..min(H::LEN * (iteration + 1), LEN);
            let last_slice = if iteration == 0 {
                0..0
            } else {
                (H::LEN * (iteration - 1))..(H::LEN * iteration)
            };

            // TODO: re-use midstate
            let mut engine = HmacEngine::<H>::new(&self.prk[..]);
            engine.input(&output[last_slice]);
            engine.input(info);
            engine.input(&[(iteration + 1) as u8]);
            let output_bytes = Hmac::from_engine(engine);

            let bytes_to_copy = current_slice.end - current_slice.start;
            output[current_slice].copy_from_slice(&output_bytes[0..bytes_to_copy]);
        }

        output
    }

    /// Run HKDF-expand to generate new key material with `L = H::LEN`
    ///
    /// See [`Hkdf::derive`] for more information.
    pub fn derive_hmac(&self, info: &[u8]) -> Hmac<H> {
        let mut engine = HmacEngine::<H>::new(&self.prk[..]);
        engine.input(info);
        engine.input(&[1u8]);
        Hmac::from_engine(engine)
    }
}

#[cfg(test)]
mod tests {
    use bitcoin_hashes::Hash as BitcoinHash;

    use crate::Hkdf;

    #[test]
    #[should_panic(expected = "RFC5869 only supports output length of up to 255*HashLength")]
    fn test_too_long_output_key() {
        let hkdf = Hkdf::<crate::hashes::Sha512>::new("foo".as_bytes(), None);
        hkdf.derive::<16321>(&[]);
    }

    #[test]
    fn test_long_output_key_ok() {
        let hkdf = Hkdf::<crate::hashes::Sha512>::new("foo".as_bytes(), None);
        hkdf.derive::<16320>(&[]);
    }

    #[test]
    fn rfc5896_test_vector_1() {
        let input_key = [
            0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
            0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
        ];
        let salt = Some(
            &[
                0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c,
            ][..],
        );
        let info = &[0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9];
        let hkdf = Hkdf::<crate::hashes::Sha256>::new(&input_key, salt);
        let output_key = hkdf.derive::<42>(info);
        assert_eq!(
            &hkdf.prk[..],
            &[
                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,
            ]
        );
        assert_eq!(
            output_key,
            [
                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,
            ]
        );

        // Addition to test derive_hkdf
        let output_key_hkdf = hkdf.derive_hmac(info);
        assert_eq!(
            output_key[0..crate::hashes::Sha256::LEN],
            output_key_hkdf[..]
        )
    }

    #[test]
    fn rfc5896_test_vector_2() {
        let input_key = [
            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d,
            0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b,
            0x1c, 0x1d, 0x1e, 0x1f, 0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29,
            0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
            0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45,
            0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f,
        ];
        let salt = Some(
            &[
                0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d,
                0x6e, 0x6f, 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7a, 0x7b,
                0x7c, 0x7d, 0x7e, 0x7f, 0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89,
                0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f, 0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97,
                0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f, 0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5,
                0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf,
            ][..],
        );
        let info = [
            0xb0, 0xb1, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xbb, 0xbc, 0xbd,
            0xbe, 0xbf, 0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xcb,
            0xcc, 0xcd, 0xce, 0xcf, 0xd0, 0xd1, 0xd2, 0xd3, 0xd4, 0xd5, 0xd6, 0xd7, 0xd8, 0xd9,
            0xda, 0xdb, 0xdc, 0xdd, 0xde, 0xdf, 0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
            0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef, 0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5,
            0xf6, 0xf7, 0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff,
        ];
        let hkdf = Hkdf::<crate::hashes::Sha256>::new(&input_key, salt);
        let output_key = hkdf.derive(&info);
        assert_eq!(
            &hkdf.prk[..],
            &[
                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,
            ]
        );
        assert_eq!(
            output_key,
            [
                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,
            ]
        );
    }

    #[test]
    fn rfc5896_test_vector_3() {
        let input_key = [
            0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
            0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
        ];
        let salt = Some(&[][..]);
        let info = [];
        let hkdf = Hkdf::<crate::hashes::Sha256>::new(&input_key, salt);
        let output_key = hkdf.derive(&info);
        assert_eq!(
            &hkdf.prk[..],
            &[
                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,
            ]
        );
        assert_eq!(
            output_key,
            [
                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,
            ]
        );
    }

    #[test]
    fn rfc5896_test_vector_4() {
        let input_key = [
            0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
        ];
        let salt = Some(
            &[
                0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c,
            ][..],
        );
        let info = [0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9];
        let hkdf = Hkdf::<crate::hashes::Sha1>::new(&input_key, salt);
        let output_key = hkdf.derive(&info);
        assert_eq!(
            &hkdf.prk[..],
            &[
                0x9b, 0x6c, 0x18, 0xc4, 0x32, 0xa7, 0xbf, 0x8f, 0x0e, 0x71, 0xc8, 0xeb, 0x88, 0xf4,
                0xb3, 0x0b, 0xaa, 0x2b, 0xa2, 0x43,
            ]
        );
        assert_eq!(
            output_key,
            [
                0x08, 0x5a, 0x01, 0xea, 0x1b, 0x10, 0xf3, 0x69, 0x33, 0x06, 0x8b, 0x56, 0xef, 0xa5,
                0xad, 0x81, 0xa4, 0xf1, 0x4b, 0x82, 0x2f, 0x5b, 0x09, 0x15, 0x68, 0xa9, 0xcd, 0xd4,
                0xf1, 0x55, 0xfd, 0xa2, 0xc2, 0x2e, 0x42, 0x24, 0x78, 0xd3, 0x05, 0xf3, 0xf8, 0x96,
            ]
        );
    }

    #[test]
    fn rfc5896_test_vector_5() {
        let input_key = [
            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d,
            0x0e, 0x0f, 0x10, 0x11, 0x12, 0x13, 0x14, 0x15, 0x16, 0x17, 0x18, 0x19, 0x1a, 0x1b,
            0x1c, 0x1d, 0x1e, 0x1f, 0x20, 0x21, 0x22, 0x23, 0x24, 0x25, 0x26, 0x27, 0x28, 0x29,
            0x2a, 0x2b, 0x2c, 0x2d, 0x2e, 0x2f, 0x30, 0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37,
            0x38, 0x39, 0x3a, 0x3b, 0x3c, 0x3d, 0x3e, 0x3f, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45,
            0x46, 0x47, 0x48, 0x49, 0x4a, 0x4b, 0x4c, 0x4d, 0x4e, 0x4f,
        ];
        let salt = Some(
            &[
                0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68, 0x69, 0x6a, 0x6b, 0x6c, 0x6d,
                0x6e, 0x6f, 0x70, 0x71, 0x72, 0x73, 0x74, 0x75, 0x76, 0x77, 0x78, 0x79, 0x7a, 0x7b,
                0x7c, 0x7d, 0x7e, 0x7f, 0x80, 0x81, 0x82, 0x83, 0x84, 0x85, 0x86, 0x87, 0x88, 0x89,
                0x8a, 0x8b, 0x8c, 0x8d, 0x8e, 0x8f, 0x90, 0x91, 0x92, 0x93, 0x94, 0x95, 0x96, 0x97,
                0x98, 0x99, 0x9a, 0x9b, 0x9c, 0x9d, 0x9e, 0x9f, 0xa0, 0xa1, 0xa2, 0xa3, 0xa4, 0xa5,
                0xa6, 0xa7, 0xa8, 0xa9, 0xaa, 0xab, 0xac, 0xad, 0xae, 0xaf,
            ][..],
        );
        let info = [
            0xb0, 0xb1, 0xb2, 0xb3, 0xb4, 0xb5, 0xb6, 0xb7, 0xb8, 0xb9, 0xba, 0xbb, 0xbc, 0xbd,
            0xbe, 0xbf, 0xc0, 0xc1, 0xc2, 0xc3, 0xc4, 0xc5, 0xc6, 0xc7, 0xc8, 0xc9, 0xca, 0xcb,
            0xcc, 0xcd, 0xce, 0xcf, 0xd0, 0xd1, 0xd2, 0xd3, 0xd4, 0xd5, 0xd6, 0xd7, 0xd8, 0xd9,
            0xda, 0xdb, 0xdc, 0xdd, 0xde, 0xdf, 0xe0, 0xe1, 0xe2, 0xe3, 0xe4, 0xe5, 0xe6, 0xe7,
            0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef, 0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5,
            0xf6, 0xf7, 0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff,
        ];
        let hkdf = Hkdf::<crate::hashes::Sha1>::new(&input_key, salt);
        let output_key = hkdf.derive(&info);
        assert_eq!(
            &hkdf.prk[..],
            &[
                0x8a, 0xda, 0xe0, 0x9a, 0x2a, 0x30, 0x70, 0x59, 0x47, 0x8d, 0x30, 0x9b, 0x26, 0xc4,
                0x11, 0x5a, 0x22, 0x4c, 0xfa, 0xf6,
            ]
        );
        assert_eq!(
            output_key,
            [
                0x0b, 0xd7, 0x70, 0xa7, 0x4d, 0x11, 0x60, 0xf7, 0xc9, 0xf1, 0x2c, 0xd5, 0x91, 0x2a,
                0x06, 0xeb, 0xff, 0x6a, 0xdc, 0xae, 0x89, 0x9d, 0x92, 0x19, 0x1f, 0xe4, 0x30, 0x56,
                0x73, 0xba, 0x2f, 0xfe, 0x8f, 0xa3, 0xf1, 0xa4, 0xe5, 0xad, 0x79, 0xf3, 0xf3, 0x34,
                0xb3, 0xb2, 0x02, 0xb2, 0x17, 0x3c, 0x48, 0x6e, 0xa3, 0x7c, 0xe3, 0xd3, 0x97, 0xed,
                0x03, 0x4c, 0x7f, 0x9d, 0xfe, 0xb1, 0x5c, 0x5e, 0x92, 0x73, 0x36, 0xd0, 0x44, 0x1f,
                0x4c, 0x43, 0x00, 0xe2, 0xcf, 0xf0, 0xd0, 0x90, 0x0b, 0x52, 0xd3, 0xb4,
            ]
        );
    }

    #[test]
    fn rfc5896_test_vector_6() {
        let input_key = [
            0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
            0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b, 0x0b,
        ];
        let salt = Some(&[][..]);
        let info = [];
        let hkdf = Hkdf::<crate::hashes::Sha1>::new(&input_key, salt);
        let output_key = hkdf.derive(&info);
        assert_eq!(
            &hkdf.prk[..],
            &[
                0xda, 0x8c, 0x8a, 0x73, 0xc7, 0xfa, 0x77, 0x28, 0x8e, 0xc6, 0xf5, 0xe7, 0xc2, 0x97,
                0x78, 0x6a, 0xa0, 0xd3, 0x2d, 0x01,
            ]
        );
        assert_eq!(
            output_key,
            [
                0x0a, 0xc1, 0xaf, 0x70, 0x02, 0xb3, 0xd7, 0x61, 0xd1, 0xe5, 0x52, 0x98, 0xda, 0x9d,
                0x05, 0x06, 0xb9, 0xae, 0x52, 0x05, 0x72, 0x20, 0xa3, 0x06, 0xe0, 0x7b, 0x6b, 0x87,
                0xe8, 0xdf, 0x21, 0xd0, 0xea, 0x00, 0x03, 0x3d, 0xe0, 0x39, 0x84, 0xd3, 0x49, 0x18,
            ]
        );
    }

    #[test]
    fn rfc5896_test_vector_7() {
        let input_key = [
            0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c,
            0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c, 0x0c,
        ];
        let salt = None;
        let info = [];
        let hkdf = Hkdf::<crate::hashes::Sha1>::new(&input_key, salt);
        let output_key = hkdf.derive(&info);
        assert_eq!(
            &hkdf.prk[..],
            &[
                0x2a, 0xdc, 0xca, 0xda, 0x18, 0x77, 0x9e, 0x7c, 0x20, 0x77, 0xad, 0x2e, 0xb1, 0x9d,
                0x3f, 0x3e, 0x73, 0x13, 0x85, 0xdd,
            ]
        );
        assert_eq!(
            output_key,
            [
                0x2c, 0x91, 0x11, 0x72, 0x04, 0xd7, 0x45, 0xf3, 0x50, 0x0d, 0x63, 0x6a, 0x62, 0xf6,
                0x4f, 0x0a, 0xb3, 0xba, 0xe5, 0x48, 0xaa, 0x53, 0xd4, 0x23, 0xb0, 0xd1, 0xf2, 0x7e,
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            ]
        );
    }
}