dlc 0.8.0

Creation, signing and verification of Discreet Log Contracts (DLC) transactions.
Documentation 
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//! Crypto utilities providing necessary DLC specific functions not available in
//! rust-secp256k1 or rust-secp256k1-zkp.

use crate::Error;
use bitcoin::hashes::{sha256t_hash_newtype, Hash};
use core::ptr;
use secp256k1_sys::{
    types::{c_int, c_uchar, c_void, size_t},
    CPtr, SchnorrSigExtraParams,
};
use secp256k1_zkp::{
    schnorr::Signature as SchnorrSignature, Keypair, Message, PublicKey, Scalar, Secp256k1,
    Signing, Verification, XOnlyPublicKey,
};

const BIP340_MIDSTATE: [u8; 32] = [
    0x9c, 0xec, 0xba, 0x11, 0x23, 0x92, 0x53, 0x81, 0x11, 0x67, 0x91, 0x12, 0xd1, 0x62, 0x7e, 0x0f,
    0x97, 0xc8, 0x75, 0x50, 0x00, 0x3c, 0xc7, 0x65, 0x90, 0xf6, 0x11, 0x64, 0x33, 0xe9, 0xb6, 0x6a,
];

sha256t_hash_newtype! {
    /// BIP340 Hash Tag
    pub struct BIP340HashTag = raw(BIP340_MIDSTATE, 64);

    /// BIP340 Hash
    #[hash_newtype(backward)]
    pub struct BIP340Hash(_);
}

/// Create a Schnorr signature using the provided nonce instead of generating one.
pub fn schnorrsig_sign_with_nonce<S: Signing>(
    secp: &Secp256k1<S>,
    msg: &Message,
    keypair: &Keypair,
    nonce: &[u8; 32],
) -> SchnorrSignature {
    unsafe {
        let mut sig = [0u8; secp256k1_zkp::constants::SCHNORR_SIGNATURE_SIZE];
        let extra_params =
            SchnorrSigExtraParams::new(Some(constant_nonce_fn), nonce.as_c_ptr() as *const c_void);
        assert_eq!(
            1,
            secp256k1_sys::secp256k1_schnorrsig_sign_custom(
                secp.ctx().as_ref(),
                sig.as_mut_c_ptr(),
                msg.as_c_ptr(),
                32_usize,
                keypair.as_c_ptr(),
                &extra_params,
            )
        );

        SchnorrSignature::from_slice(&sig).unwrap()
    }
}

/// Compute a signature point for the given public key, nonce and message.
pub fn schnorrsig_compute_sig_point<C: Verification>(
    secp: &Secp256k1<C>,
    pubkey: &XOnlyPublicKey,
    nonce: &XOnlyPublicKey,
    message: &Message,
) -> Result<PublicKey, Error> {
    let hash = create_schnorr_hash(message, nonce, pubkey);
    let pk = schnorr_pubkey_to_pubkey(pubkey)?;
    let scalar = Scalar::from_be_bytes(hash).unwrap();
    let tweaked = pk.mul_tweak(secp, &scalar)?;
    let npk = schnorr_pubkey_to_pubkey(nonce)?;
    Ok(npk.combine(&tweaked)?)
}

/// Decompose a bip340 signature into a nonce and a secret key (as byte array)
pub fn schnorrsig_decompose(
    signature: &SchnorrSignature,
) -> Result<(XOnlyPublicKey, &[u8]), Error> {
    let bytes = signature.as_ref();
    Ok((XOnlyPublicKey::from_slice(&bytes[0..32])?, &bytes[32..64]))
}

extern "C" fn constant_nonce_fn(
    nonce32: *mut c_uchar,
    _msg32: *const c_uchar,
    _msg_len: size_t,
    _key32: *const c_uchar,
    _xonly_pk32: *const c_uchar,
    _algo16: *const c_uchar,
    _algo_len: size_t,
    data: *mut c_void,
) -> c_int {
    unsafe {
        ptr::copy_nonoverlapping(data as *const c_uchar, nonce32, 32);
    }
    1
}

fn create_schnorr_hash(msg: &Message, nonce: &XOnlyPublicKey, pubkey: &XOnlyPublicKey) -> [u8; 32] {
    let mut buf = Vec::<u8>::new();
    buf.extend(nonce.serialize());
    buf.extend(pubkey.serialize());
    buf.extend(msg.as_ref().to_vec());
    BIP340Hash::hash(&buf).to_byte_array()
}

fn schnorr_pubkey_to_pubkey(schnorr_pubkey: &XOnlyPublicKey) -> Result<PublicKey, Error> {
    let mut buf = Vec::<u8>::with_capacity(33);
    buf.push(0x02);
    buf.extend(schnorr_pubkey.serialize());
    Ok(PublicKey::from_slice(&buf)?)
}