ic_mac/

hmac.rs

//! FIPS 198-1 HMAC, generic over any digest.

use ic_core::traits::{Algorithm, Digest, Mac, SelfTest};
use ic_core::{ensure, Result, Zeroize};

/// The largest block size among supported digests.
///
/// SHA-512 uses 128 bytes; SHA3-224 has the widest rate at 144, so the padded
/// key buffers are sized for it.
const MAX_BLOCK_LEN: usize = 144;

/// HMAC over the digest `D`.
///
/// The key is processed per FIPS 198-1: hashed if longer than the block size,
/// zero-padded otherwise. The padded key, and the hash of a long key, are
/// zeroized before the constructor returns.
#[derive(Clone)]
pub struct Hmac<D: Digest> {
    inner: D,
    outer: D,
}

/// A digest that can name its HMAC instantiation in the ontology.
///
/// Rust cannot concatenate `&'static str` constants at compile time, so the
/// composed identifier (`"hmac-sha2-256"`) is declared explicitly per digest
/// rather than derived from [`Digest::ID`].
pub trait HmacDigest: Digest {
    /// Ontology identifier of the HMAC built on this digest.
    const HMAC_ID: &'static str;
    /// Display name of the HMAC built on this digest.
    const HMAC_NAME: &'static str;
}

impl<D: HmacDigest> Algorithm for Hmac<D> {
    const ID: &'static str = D::HMAC_ID;
    const NAME: &'static str = D::HMAC_NAME;
}

impl<D: Digest> Hmac<D> {
    /// Absorb the inner and outer pads into the two fresh states.
    ///
    /// One buffer, built as the inner pad directly -- the zero-padded key
    /// XOR 0x36 -- and turned into the outer pad in place, since
    /// `k ^ 0x5c == (k ^ 0x36) ^ (0x36 ^ 0x5c)`. The wipe is volatile and
    /// byte by byte, which is what makes it stick and also what makes it
    /// cost: it covers the one block this digest used, not two buffers sized
    /// for the widest digest there is. For SHA-256 that is 64 bytes where it
    /// was 288, on every HMAC key setup -- and HKDF sets up a key per call.
    ///
    /// The states are absorbed into where they will live rather than built as
    /// locals and moved into the result: for SHA-384 on Cortex-M4 the locals,
    /// the result and a third state for a long key had shared one 4.4 KiB frame.
    fn absorb_pads(&mut self, key: &[u8]) {
        let mut pad = [0x36u8; MAX_BLOCK_LEN];
        let block = &mut pad[..D::BLOCK_LEN];
        if key.len() > D::BLOCK_LEN {
            xor_hashed_key::<D>(key, block);
        } else {
            for (p, k) in block.iter_mut().zip(key) {
                *p ^= k;
            }
        }

        self.inner.update(block);
        for p in block.iter_mut() {
            *p ^= 0x36 ^ 0x5c;
        }
        self.outer.update(block);

        block.zeroize();
    }
}

// # Where the frames go
//
// A SHA-512 state carries its 640-byte message schedule, so SHA-384 and
// SHA-512 states are about 860 bytes and an HMAC over either is 1.7 KiB.
// Inlined into a caller -- HKDF, say -- key setup and finalization each
// brought their own copies of those states into the caller's frame beside the
// caller's: one HMAC-SHA384 tag reached 11 KiB of stack on Cortex-M4. For
// those digests, key setup and finalization stay out of line, so their
// frames are used one after another instead of all at once.
//
// Not for the small states. SHA-256 gained nothing from it on Cortex-M4, and
// the call cost a short HMAC-SHA256 about 8% on x86-64. The test is a
// constant for each digest, so whichever branch is not taken compiles away.

/// Whether `D`'s state is large enough to keep HMAC's work out of line.
const fn large_state<D>() -> bool {
    core::mem::size_of::<D>() > 512
}

/// `Hmac` keyed with `key`.
#[inline(always)]
fn keyed<D: Digest>(key: &[u8]) -> Hmac<D> {
    let mut mac = Hmac {
        inner: D::new(),
        outer: D::new(),
    };
    mac.absorb_pads(key);
    mac
}

#[inline(never)]
fn keyed_out_of_line<D: Digest>(key: &[u8]) -> Hmac<D> {
    keyed(key)
}

/// The tag: the outer hash over the inner one.
#[inline(always)]
fn tag<D: Digest>(mut mac: Hmac<D>) -> D::Output {
    let inner_digest = mac.inner.finalize();
    mac.outer.update(inner_digest.as_ref());
    mac.outer.finalize()
}

#[inline(never)]
fn tag_out_of_line<D: Digest>(mac: Hmac<D>) -> D::Output {
    tag(mac)
}

/// XOR `D(key)` into `block`, for a key longer than one block.
///
/// Out of line so that the third digest state it needs is on the stack only
/// while it runs, not for the whole of key setup.
#[inline(never)]
fn xor_hashed_key<D: Digest>(key: &[u8], block: &mut [u8]) {
    let mut hashed = D::digest(key);
    for (p, k) in block.iter_mut().zip(hashed.as_ref()) {
        *p ^= k;
    }
    // Key-equivalent: HMAC under the hash is HMAC under the key.
    hashed.as_mut().zeroize();
}

impl<D: HmacDigest> Mac for Hmac<D> {
    type Tag = D::Output;
    const TAG_LEN: usize = D::OUTPUT_LEN;

    fn new(key: &[u8]) -> Result<Self> {
        ensure!(
            D::BLOCK_LEN <= MAX_BLOCK_LEN,
            InvalidParameter,
            "digest block exceeds hmac buffer"
        );
        if large_state::<D>() {
            Ok(keyed_out_of_line(key))
        } else {
            Ok(keyed(key))
        }
    }

    fn update(&mut self, data: &[u8]) {
        self.inner.update(data);
    }

    fn finalize(self) -> Self::Tag {
        if large_state::<D>() {
            tag_out_of_line(self)
        } else {
            tag(self)
        }
    }
}

macro_rules! hmac_alias {
    (
        $name:ident, $digest:ty, $id:literal, $disp:literal, $taglen:literal,
        $kat_key:expr, $kat_msg:expr, $kat_tag:literal
    ) => {
        #[doc = concat!($disp, ".")]
        pub type $name = Hmac<$digest>;

        impl HmacDigest for $digest {
            const HMAC_ID: &'static str = $id;
            const HMAC_NAME: &'static str = $disp;
        }

        impl SelfTest for Hmac<$digest> {
            fn self_test() -> Result<()> {
                let mut key = [0u8; 20];
                ic_core::codec::hex_decode($kat_key.as_bytes(), &mut key)?;
                let tag = <Self as Mac>::mac(&key, $kat_msg)?;
                let mut want = [0u8; $taglen];
                ic_core::codec::hex_decode($kat_tag.as_bytes(), &mut want)?;
                ensure!(
                    ic_core::ct::verify(&want, tag.as_ref()),
                    SelfTestFailed,
                    $id
                );
                Ok(())
            }
        }
    };
}

/// The RFC 4231 test-case-1 key: twenty `0x0b` bytes.
const KAT_KEY: &str = "0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b";

// Known-answer vectors, all over the RFC 4231 test-case-1 input
// (a 20-byte 0x0b key over "Hi There"):
//
// * SHA-256 / SHA-384 / SHA-512 tags are RFC 4231 test case 1.
// * SHA-512/256 and the SHA-3 tags are the corresponding NIST HMAC sample
//   values for the same input.
hmac_alias!(
    HmacSha256,
    ic_hash::Sha256,
    "hmac-sha2-256",
    "HMAC-SHA-256",
    32,
    KAT_KEY,
    b"Hi There",
    "b0344c61d8db38535ca8afceaf0bf12b881dc200c9833da726e9376c2e32cff7"
);
hmac_alias!(
    HmacSha384,
    ic_hash::Sha384,
    "hmac-sha2-384",
    "HMAC-SHA-384",
    48,
    KAT_KEY,
    b"Hi There",
    "afd03944d84895626b0825f4ab46907f15f9dadbe4101ec682aa034c7cebc59cfaea9ea9076ede7f4af152e8b2fa9cb6"
);
hmac_alias!(
    HmacSha512,
    ic_hash::Sha512,
    "hmac-sha2-512",
    "HMAC-SHA-512",
    64,
    KAT_KEY,
    b"Hi There",
    "87aa7cdea5ef619d4ff0b4241a1d6cb02379f4e2ce4ec2787ad0b30545e17cdedaa833b7d6b8a702038b274eaea3f4e4be9d914eeb61f1702e696c203a126854"
);
hmac_alias!(
    HmacSha512_256,
    ic_hash::Sha512_256,
    "hmac-sha2-512-256",
    "HMAC-SHA-512/256",
    32,
    KAT_KEY,
    b"Hi There",
    "9f9126c3d9c3c330d760425ca8a217e31feae31bfe70196ff81642b868402eab"
);
hmac_alias!(
    HmacSha3_256,
    ic_hash::Sha3_256,
    "hmac-sha3-256",
    "HMAC-SHA3-256",
    32,
    KAT_KEY,
    b"Hi There",
    "ba85192310dffa96e2a3a40e69774351140bb7185e1202cdcc917589f95e16bb"
);
hmac_alias!(
    HmacSha3_512,
    ic_hash::Sha3_512,
    "hmac-sha3-512",
    "HMAC-SHA3-512",
    64,
    KAT_KEY,
    b"Hi There",
    "eb3fbd4b2eaab8f5c504bd3a41465aacec15770a7cabac531e482f860b5ec7ba47ccb2c6f2afce8f88d22b6dc61380f23a668fd3888bb80537c0a0b86407689e"
);

#[cfg(test)]
mod tests {
    use super::*;
    use ic_core::codec::hex;

    #[test]
    fn rfc4231_case_1() {
        let key = [0x0bu8; 20];
        assert_eq!(
            hex(HmacSha256::mac(&key, b"Hi There").unwrap().as_ref()),
            "b0344c61d8db38535ca8afceaf0bf12b881dc200c9833da726e9376c2e32cff7"
        );
        assert_eq!(
            hex(HmacSha512::mac(&key, b"Hi There").unwrap().as_ref()),
            "87aa7cdea5ef619d4ff0b4241a1d6cb02379f4e2ce4ec2787ad0b30545e17cdedaa833b7d6b8a702038b274eaea3f4e4be9d914eeb61f1702e696c203a126854"
        );
    }

    #[test]
    fn rfc4231_case_2_short_key() {
        assert_eq!(
            hex(HmacSha256::mac(b"Jefe", b"what do ya want for nothing?")
                .unwrap()
                .as_ref()),
            "5bdcc146bf60754e6a042426089575c75a003f089d2739839dec58b964ec3843"
        );
    }

    /// Case 3 uses a key and message that both exceed one block.
    #[test]
    fn rfc4231_case_3_long_data() {
        let key = [0xaau8; 20];
        let data = [0xddu8; 50];
        assert_eq!(
            hex(HmacSha256::mac(&key, &data).unwrap().as_ref()),
            "773ea91e36800e46854db8ebd09181a72959098b3ef8c122d9635514ced565fe"
        );
    }

    /// Case 6: a 131-byte key, longer than the SHA-256 block, so it is hashed
    /// down first.
    #[test]
    fn rfc4231_case_6_oversized_key() {
        let key = [0xaau8; 131];
        assert_eq!(
            hex(HmacSha256::mac(
                &key,
                b"Test Using Larger Than Block-Size Key - Hash Key First"
            )
            .unwrap()
            .as_ref()),
            "60e431591ee0b67f0d8a26aacbf5b77f8e0bc6213728c5140546040f0ee37f54"
        );
    }

    /// Case 6 again, for SHA-384 and SHA-512: 131 bytes is also longer than
    /// their 128-byte block. These are the digests whose key setup runs out of
    /// line, so this is what checks the long-key path there. RFC 4231 section
    /// 4.7; both tags also checked against Python's `hmac` module.
    #[test]
    fn rfc4231_case_6_oversized_key_sha2_512_family() {
        let key = [0xaau8; 131];
        let msg = b"Test Using Larger Than Block-Size Key - Hash Key First";
        assert_eq!(
            hex(HmacSha384::mac(&key, msg).unwrap().as_ref()),
            "4ece084485813e9088d2c63a041bc5b44f9ef1012a2b588f3cd11f05033ac4c60c2ef6ab4030fe8296248df163f44952"
        );
        assert_eq!(
            hex(HmacSha512::mac(&key, msg).unwrap().as_ref()),
            "80b24263c7c1a3ebb71493c1dd7be8b49b46d1f41b4aeec1121b013783f8f3526b56d037e05f2598bd0fd2215d6a1e5295e64f73f63f0aec8b915a985d786598"
        );
    }

    #[test]
    fn empty_key_and_message() {
        assert_eq!(
            hex(HmacSha256::mac(b"", b"").unwrap().as_ref()),
            "b613679a0814d9ec772f95d778c35fc5ff1697c493715653c6c712144292c5ad"
        );
    }

    #[test]
    fn streaming_matches_one_shot() {
        let data: Vec<u8> = (0..200u8).collect();
        for split in [0usize, 1, 63, 64, 128, 200] {
            let mut m = HmacSha256::new(b"k").unwrap();
            m.update(&data[..split]);
            m.update(&data[split..]);
            assert_eq!(m.finalize(), HmacSha256::mac(b"k", &data).unwrap());
        }
    }

    #[test]
    fn verify_rejects_wrong_tag_and_length() {
        let tag = HmacSha256::mac(b"k", b"m").unwrap();
        HmacSha256::verify(b"k", b"m", tag.as_ref()).unwrap();
        let mut bad = tag;
        bad[0] ^= 1;
        assert!(HmacSha256::verify(b"k", b"m", bad.as_ref()).is_err());
        assert!(HmacSha256::verify(b"k", b"m", &tag.as_ref()[..31]).is_err());
    }

    #[test]
    fn self_tests_pass() {
        HmacSha256::self_test().unwrap();
        HmacSha384::self_test().unwrap();
        HmacSha512::self_test().unwrap();
        HmacSha512_256::self_test().unwrap();
        HmacSha3_256::self_test().unwrap();
        HmacSha3_512::self_test().unwrap();
    }
}