libelectrum2descriptors/
electrum_extended_priv_key.rs

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use crate::{Descriptors, Electrum2DescriptorError, ElectrumExtendedKey};
use bitcoin::base58;
use bitcoin::bip32::{ChainCode, ChildNumber, ExtendedPrivKey, Fingerprint};
use bitcoin::secp256k1;
use bitcoin::Network;
use std::convert::TryInto;
use std::str::FromStr;

pub struct ElectrumExtendedPrivKey {
    xprv: ExtendedPrivKey,
    kind: String,
}

type SentinelMap = Vec<([u8; 4], Network, String)>;
fn initialize_sentinels() -> SentinelMap {
    // electrum testnet
    // https://github.com/spesmilo/electrum/blob/928e43fc530ba5befa062db788e4e04d56324161/electrum/constants.py#L110-L116
    //     XPRV_HEADERS = {
    //         'standard':    0x04358394,  # tprv
    //         'p2wpkh-p2sh': 0x044a4e28,  # uprv
    //         'p2wsh-p2sh':  0x024285b5,  # Uprv
    //         'p2wpkh':      0x045f18bc,  # vprv
    //         'p2wsh':       0x02575048,  # Vprv
    //     }
    // electrum mainnet
    // https://github.com/spesmilo/electrum/blob/928e43fc530ba5befa062db788e4e04d56324161/electrum/constants.py#L74-L80
    //     XPRV_HEADERS = {
    //         'standard':    0x0488ade4,  # xprv
    //         'p2wpkh-p2sh': 0x049d7878,  # yprv
    //         'p2wsh-p2sh':  0x0295b005,  # Yprv
    //         'p2wpkh':      0x04b2430c,  # zprv
    //         'p2wsh':       0x02aa7a99,  # Zprv
    //     }

    vec![
        (
            [0x04u8, 0x35, 0x83, 0x94],
            Network::Testnet,
            "pkh".to_string(),
        ), // tprv
        (
            [0x04u8, 0x4a, 0x4e, 0x28],
            Network::Testnet,
            "sh(wpkh".to_string(),
        ), // uprv
        (
            [0x02u8, 0x42, 0x85, 0xb5],
            Network::Testnet,
            "sh(wsh".to_string(),
        ), // Uprv
        (
            [0x04u8, 0x5f, 0x18, 0xbc],
            Network::Testnet,
            "wpkh".to_string(),
        ), // vprv
        (
            [0x02u8, 0x57, 0x50, 0x48],
            Network::Testnet,
            "wsh".to_string(),
        ), // Vprv
        (
            [0x04u8, 0x88, 0xad, 0xE4],
            Network::Bitcoin,
            "pkh".to_string(),
        ), // xprv
        (
            [0x04u8, 0x9d, 0x78, 0x78],
            Network::Bitcoin,
            "sh(wpkh".to_string(),
        ), // yprv
        (
            [0x02u8, 0x95, 0xb0, 0x05],
            Network::Bitcoin,
            "sh(wsh".to_string(),
        ), // Yprv
        (
            [0x04u8, 0xb2, 0x43, 0x0c],
            Network::Bitcoin,
            "wpkh".to_string(),
        ), // zprv
        (
            [0x02u8, 0xaa, 0x7a, 0x99],
            Network::Bitcoin,
            "wsh".to_string(),
        ), // Zprv
    ]
}

impl FromStr for ElectrumExtendedPrivKey {
    type Err = Electrum2DescriptorError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let data = base58::decode_check(s)?;

        if data.len() != 78 {
            return Err(Electrum2DescriptorError::Base58Error(
                base58::Error::InvalidLength(data.len()),
            ));
        }

        let cn_int = u32::from_be_bytes(data[9..13].try_into().unwrap());
        let child_number: ChildNumber = ChildNumber::from(cn_int);
        let (network, kind) = match_electrum_xprv(&data[0..4])?;
        let key = secp256k1::SecretKey::from_slice(&data[46..78])?;

        let xprv = ExtendedPrivKey {
            network,
            depth: data[4],
            parent_fingerprint: Fingerprint::from(&data[5..9].try_into().unwrap()),
            child_number,
            chain_code: ChainCode::from(&data[13..45].try_into().unwrap()),
            private_key: key,
        };
        Ok(ElectrumExtendedPrivKey { xprv, kind })
    }
}

impl ElectrumExtendedKey for ElectrumExtendedPrivKey {
    /// Returns the kind
    fn kind(&self) -> &str {
        &self.kind
    }

    /// Returns the xprv as String
    fn xkey_str(&self) -> String {
        self.xprv.to_string()
    }

    /// Returns internal and external descriptor
    fn to_descriptors(&self) -> Descriptors {
        let xprv = self.xprv.to_string();
        let closing_parenthesis = if self.kind.contains('(') { ")" } else { "" };
        let [external, change] =
            [0, 1].map(|i| format!("{}({}/{}/*){}", self.kind, xprv, i, closing_parenthesis));
        Descriptors { external, change }
    }
}

impl ElectrumExtendedPrivKey {
    /// Constructs a new instance
    pub fn new(xprv: ExtendedPrivKey, kind: String) -> Self {
        ElectrumExtendedPrivKey { xprv, kind }
    }

    /// Returns the xprv
    pub fn xprv(&self) -> &ExtendedPrivKey {
        &self.xprv
    }

    /// converts to electrum format
    pub fn electrum_xprv(&self) -> Result<String, Electrum2DescriptorError> {
        let sentinels = initialize_sentinels();
        let sentinel = sentinels
            .iter()
            .find(|sent| sent.1 == self.xprv.network && sent.2 == self.kind)
            .ok_or_else(|| Electrum2DescriptorError::UnknownType)?;
        let mut data = Vec::from(&sentinel.0[..]);
        data.push(self.xprv.depth);
        data.extend(self.xprv.parent_fingerprint.as_bytes());
        let child_number: u32 = self.xprv.child_number.into();
        data.extend(child_number.to_be_bytes());
        data.extend(self.xprv.chain_code.as_bytes());
        data.push(0u8);
        data.extend(self.xprv.private_key.as_ref());

        if data.len() != 78 {
            return Err(Electrum2DescriptorError::Base58Error(
                base58::Error::InvalidLength(data.len()),
            ));
        }

        Ok(base58::encode_check(&data))
    }
}

fn match_electrum_xprv(version: &[u8]) -> Result<(Network, String), base58::Error> {
    let sentinels = initialize_sentinels();
    let sentinel = sentinels
        .iter()
        .find(|sent| sent.0 == version)
        .ok_or_else(|| {
            base58::Error::InvalidExtendedKeyVersion(version[0..4].try_into().unwrap())
        })?;
    Ok((sentinel.1, sentinel.2.clone()))
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::str::FromStr;

    #[test]
    fn test_vprv_from_electrum() {
        let electrum_xprv = ElectrumExtendedPrivKey::from_str("yprvAHwhK6RbpuS3dgCYHM5jc2ZvEKd7Bi61u9FVhYMpgMSuZS613T1xxQeKTffhrHY79hZ5PsskBjcc6C2V7DrnsMsNaGDaWev3GLRQRgV7hxF").unwrap();
        assert_eq!(electrum_xprv.xprv.to_string(),"xprv9y7S1RkggDtZnP1RSzJ7PwUR4MUfF66Wz2jGv9TwJM52WLGmnnrQLLzBSTi7rNtBk4SGeQHBj5G4CuQvPXSn58BmhvX9vk6YzcMm37VuNYD");
        assert_eq!(electrum_xprv.kind, "sh(wpkh");
        let descriptors = electrum_xprv.to_descriptors();
        assert_eq!(descriptors.external, "sh(wpkh(xprv9y7S1RkggDtZnP1RSzJ7PwUR4MUfF66Wz2jGv9TwJM52WLGmnnrQLLzBSTi7rNtBk4SGeQHBj5G4CuQvPXSn58BmhvX9vk6YzcMm37VuNYD/0/*))");
        assert_eq!(descriptors.change, "sh(wpkh(xprv9y7S1RkggDtZnP1RSzJ7PwUR4MUfF66Wz2jGv9TwJM52WLGmnnrQLLzBSTi7rNtBk4SGeQHBj5G4CuQvPXSn58BmhvX9vk6YzcMm37VuNYD/1/*))");
        let xprv = electrum_xprv.xprv();
        assert_eq!(xprv.to_string(), "xprv9y7S1RkggDtZnP1RSzJ7PwUR4MUfF66Wz2jGv9TwJM52WLGmnnrQLLzBSTi7rNtBk4SGeQHBj5G4CuQvPXSn58BmhvX9vk6YzcMm37VuNYD");
    }

    #[test]
    fn test_vprv_to_electrum() {
        let electrum_xprv = ElectrumExtendedPrivKey::new(
            ExtendedPrivKey::from_str("xprv9y7S1RkggDtZnP1RSzJ7PwUR4MUfF66Wz2jGv9TwJM52WLGmnnrQLLzBSTi7rNtBk4SGeQHBj5G4CuQvPXSn58BmhvX9vk6YzcMm37VuNYD").unwrap(),
            "sh(wpkh".to_string(),
        );
        assert_eq!(electrum_xprv.electrum_xprv().unwrap(), "yprvAHwhK6RbpuS3dgCYHM5jc2ZvEKd7Bi61u9FVhYMpgMSuZS613T1xxQeKTffhrHY79hZ5PsskBjcc6C2V7DrnsMsNaGDaWev3GLRQRgV7hxF");
    }

    #[test]
    fn test_vprv_roundtrip() {
        let elxprv = "yprvAHwhK6RbpuS3dgCYHM5jc2ZvEKd7Bi61u9FVhYMpgMSuZS613T1xxQeKTffhrHY79hZ5PsskBjcc6C2V7DrnsMsNaGDaWev3GLRQRgV7hxF";
        let electrum_xprv = ElectrumExtendedPrivKey::from_str(elxprv).unwrap();
        assert_eq!(electrum_xprv.electrum_xprv().unwrap(), elxprv);
        assert_ne!(electrum_xprv.xprv.to_string(), elxprv);
    }
}