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//! The `mulX_POLYVAL()` function.

use crate::Block;

/// The `mulX_POLYVAL()` function as defined in [RFC 8452 Appendix A][1].
///
/// Performs a doubling (a.k.a. "multiply by x") over GF(2^128).
/// This is useful for implementing GHASH in terms of POLYVAL.
///
/// [1]: https://tools.ietf.org/html/rfc8452#appendix-A
#[cfg_attr(docsrs, doc(cfg(feature = "mulx")))]
pub fn mulx(block: &Block) -> Block {
    let mut v = u128::from_le_bytes((*block).into());
    let v_hi = v >> 127;

    v <<= 1;
    v ^= v_hi ^ (v_hi << 127) ^ (v_hi << 126) ^ (v_hi << 121);
    v.to_le_bytes().into()
}

#[cfg(test)]
mod tests {
    use super::{mulx, Block};
    use hex_literal::hex;

    /// Test vector given in RFC 8452 Appendix A.
    ///
    /// NOTE: the vector in the RFC actually contains a typo which has been
    /// reported (and accepted) as RFC errata, so we use the vector from the
    /// errata instead:
    ///
    /// <https://www.rfc-editor.org/errata_search.php?rfc=8452>
    #[test]
    fn rfc8452_vector() {
        let input = Block::from(hex!("9c98c04df9387ded828175a92ba652d8"));
        let expected_output = Block::from(hex!("3931819bf271fada0503eb52574ca572"));
        let actual_output = mulx(&input);
        assert_eq!(expected_output, actual_output);
    }

    /// Test against the `MULX_TEST_VECTORS` given below, which cover the full
    /// size of a POLYVAL field element.
    #[test]
    fn mulx_vectors() {
        // One
        let mut r = Block::from(hex!("01000000000000000000000000000000"));

        for vector in MULX_TEST_VECTORS {
            r = mulx(&r);
            assert_eq!(&r, Block::from_slice(vector));
        }
    }

    /// `mulX_POLYVAL()` test vectors.
    ///
    /// These were generated by this crate when in a known-correct state,
    /// verified by a GHASH implementation based on a POLYVAL core successfully
    /// passing the NIST test vectors.
    const MULX_TEST_VECTORS: &[[u8; 16]] = &[
        hex!("02000000000000000000000000000000"),
        hex!("04000000000000000000000000000000"),
        hex!("08000000000000000000000000000000"),
        hex!("10000000000000000000000000000000"),
        hex!("20000000000000000000000000000000"),
        hex!("40000000000000000000000000000000"),
        hex!("80000000000000000000000000000000"),
        hex!("00010000000000000000000000000000"),
        hex!("00020000000000000000000000000000"),
        hex!("00040000000000000000000000000000"),
        hex!("00080000000000000000000000000000"),
        hex!("00100000000000000000000000000000"),
        hex!("00200000000000000000000000000000"),
        hex!("00400000000000000000000000000000"),
        hex!("00800000000000000000000000000000"),
        hex!("00000100000000000000000000000000"),
        hex!("00000200000000000000000000000000"),
        hex!("00000400000000000000000000000000"),
        hex!("00000800000000000000000000000000"),
        hex!("00001000000000000000000000000000"),
        hex!("00002000000000000000000000000000"),
        hex!("00004000000000000000000000000000"),
        hex!("00008000000000000000000000000000"),
        hex!("00000001000000000000000000000000"),
        hex!("00000002000000000000000000000000"),
        hex!("00000004000000000000000000000000"),
        hex!("00000008000000000000000000000000"),
        hex!("00000010000000000000000000000000"),
        hex!("00000020000000000000000000000000"),
        hex!("00000040000000000000000000000000"),
        hex!("00000080000000000000000000000000"),
        hex!("00000000010000000000000000000000"),
        hex!("00000000020000000000000000000000"),
        hex!("00000000040000000000000000000000"),
        hex!("00000000080000000000000000000000"),
        hex!("00000000100000000000000000000000"),
        hex!("00000000200000000000000000000000"),
        hex!("00000000400000000000000000000000"),
        hex!("00000000800000000000000000000000"),
        hex!("00000000000100000000000000000000"),
        hex!("00000000000200000000000000000000"),
        hex!("00000000000400000000000000000000"),
        hex!("00000000000800000000000000000000"),
        hex!("00000000001000000000000000000000"),
        hex!("00000000002000000000000000000000"),
        hex!("00000000004000000000000000000000"),
        hex!("00000000008000000000000000000000"),
        hex!("00000000000001000000000000000000"),
        hex!("00000000000002000000000000000000"),
        hex!("00000000000004000000000000000000"),
        hex!("00000000000008000000000000000000"),
        hex!("00000000000010000000000000000000"),
        hex!("00000000000020000000000000000000"),
        hex!("00000000000040000000000000000000"),
        hex!("00000000000080000000000000000000"),
        hex!("00000000000000010000000000000000"),
        hex!("00000000000000020000000000000000"),
        hex!("00000000000000040000000000000000"),
        hex!("00000000000000080000000000000000"),
        hex!("00000000000000100000000000000000"),
        hex!("00000000000000200000000000000000"),
        hex!("00000000000000400000000000000000"),
        hex!("00000000000000800000000000000000"),
        hex!("00000000000000000100000000000000"),
        hex!("00000000000000000200000000000000"),
        hex!("00000000000000000400000000000000"),
        hex!("00000000000000000800000000000000"),
        hex!("00000000000000001000000000000000"),
        hex!("00000000000000002000000000000000"),
        hex!("00000000000000004000000000000000"),
        hex!("00000000000000008000000000000000"),
        hex!("00000000000000000001000000000000"),
        hex!("00000000000000000002000000000000"),
        hex!("00000000000000000004000000000000"),
        hex!("00000000000000000008000000000000"),
        hex!("00000000000000000010000000000000"),
        hex!("00000000000000000020000000000000"),
        hex!("00000000000000000040000000000000"),
        hex!("00000000000000000080000000000000"),
        hex!("00000000000000000000010000000000"),
        hex!("00000000000000000000020000000000"),
        hex!("00000000000000000000040000000000"),
        hex!("00000000000000000000080000000000"),
        hex!("00000000000000000000100000000000"),
        hex!("00000000000000000000200000000000"),
        hex!("00000000000000000000400000000000"),
        hex!("00000000000000000000800000000000"),
        hex!("00000000000000000000000100000000"),
        hex!("00000000000000000000000200000000"),
        hex!("00000000000000000000000400000000"),
        hex!("00000000000000000000000800000000"),
        hex!("00000000000000000000001000000000"),
        hex!("00000000000000000000002000000000"),
        hex!("00000000000000000000004000000000"),
        hex!("00000000000000000000008000000000"),
        hex!("00000000000000000000000001000000"),
        hex!("00000000000000000000000002000000"),
        hex!("00000000000000000000000004000000"),
        hex!("00000000000000000000000008000000"),
        hex!("00000000000000000000000010000000"),
        hex!("00000000000000000000000020000000"),
        hex!("00000000000000000000000040000000"),
        hex!("00000000000000000000000080000000"),
        hex!("00000000000000000000000000010000"),
        hex!("00000000000000000000000000020000"),
        hex!("00000000000000000000000000040000"),
        hex!("00000000000000000000000000080000"),
        hex!("00000000000000000000000000100000"),
        hex!("00000000000000000000000000200000"),
        hex!("00000000000000000000000000400000"),
        hex!("00000000000000000000000000800000"),
        hex!("00000000000000000000000000000100"),
        hex!("00000000000000000000000000000200"),
        hex!("00000000000000000000000000000400"),
        hex!("00000000000000000000000000000800"),
        hex!("00000000000000000000000000001000"),
        hex!("00000000000000000000000000002000"),
        hex!("00000000000000000000000000004000"),
        hex!("00000000000000000000000000008000"),
        hex!("00000000000000000000000000000001"),
        hex!("00000000000000000000000000000002"),
        hex!("00000000000000000000000000000004"),
        hex!("00000000000000000000000000000008"),
        hex!("00000000000000000000000000000010"),
        hex!("00000000000000000000000000000020"),
        hex!("00000000000000000000000000000040"),
        hex!("00000000000000000000000000000080"),
        hex!("010000000000000000000000000000c2"),
    ];
}