checksum.rs

//! Descriptor checksum
//!
//! This module contains a re-implementation of the function used by Bitcoin Core to calculate the
//! checksum of a descriptor

use std::iter::FromIterator;

use crate::Error;

const INPUT_CHARSET: &str =  "0123456789()[],'/*abcdefgh@:$%{}IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~ijklmnopqrstuvwxyzABCDEFGH`#\"\\ ";
const CHECKSUM_CHARSET: &str = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";

fn poly_mod(mut c: u64, val: u64) -> u64 {
    let c0 = c >> 35;

    c = ((c & 0x7ffffffff) << 5) ^ val;
    if c0 & 1 > 0 {
        c ^= 0xf5dee51989
    };
    if c0 & 2 > 0 {
        c ^= 0xa9fdca3312
    };
    if c0 & 4 > 0 {
        c ^= 0x1bab10e32d
    };
    if c0 & 8 > 0 {
        c ^= 0x3706b1677a
    };
    if c0 & 16 > 0 {
        c ^= 0x644d626ffd
    };

    c
}

/// Compute the checksum of a descriptor
/// Note that this function does not check if the
/// descriptor string is syntactically correct or not.
/// This only computes the checksum
pub fn desc_checksum(desc: &str) -> Result<String, Error> {
    let mut c = 1;
    let mut cls = 0;
    let mut clscount = 0;

    for ch in desc.chars() {
        let pos = INPUT_CHARSET.find(ch).ok_or(Error::BadDescriptor(format!(
            "Invalid character in checksum: '{}'",
            ch
        )))? as u64;
        c = poly_mod(c, pos & 31);
        cls = cls * 3 + (pos >> 5);
        clscount += 1;
        if clscount == 3 {
            c = poly_mod(c, cls);
            cls = 0;
            clscount = 0;
        }
    }
    if clscount > 0 {
        c = poly_mod(c, cls);
    }
    (0..8).for_each(|_| c = poly_mod(c, 0));
    c ^= 1;

    let mut chars = Vec::with_capacity(8);
    for j in 0..8 {
        chars.push(
            CHECKSUM_CHARSET
                .chars()
                .nth(((c >> (5 * (7 - j))) & 31) as usize)
                .unwrap(),
        );
    }

    Ok(String::from_iter(chars))
}

/// Helper function for FromStr for various
/// descriptor types. Checks and verifies the checksum
/// if it is present and returns the descriptor string
/// without the checksum
pub(super) fn verify_checksum(s: &str) -> Result<&str, Error> {
    for ch in s.as_bytes() {
        if *ch < 20 || *ch > 127 {
            return Err(Error::Unprintable(*ch));
        }
    }

    let mut parts = s.splitn(2, '#');
    let desc_str = parts.next().unwrap();
    if let Some(checksum_str) = parts.next() {
        let expected_sum = desc_checksum(desc_str)?;
        if checksum_str != expected_sum {
            return Err(Error::BadDescriptor(format!(
                "Invalid checksum '{}', expected '{}'",
                checksum_str, expected_sum
            )));
        }
    }
    Ok(desc_str)
}
#[cfg(test)]
mod test {
    use super::*;
    use std::str;

    macro_rules! check_expected {
        ($desc: expr, $checksum: expr) => {
            assert_eq!(desc_checksum($desc).unwrap(), $checksum);
        };
    }

    #[test]
    fn test_valid_descriptor_checksum() {
        check_expected!(
            "wpkh(tprv8ZgxMBicQKsPdpkqS7Eair4YxjcuuvDPNYmKX3sCniCf16tHEVrjjiSXEkFRnUH77yXc6ZcwHHcLNfjdi5qUvw3VDfgYiH5mNsj5izuiu2N/1/2/*)",
            "tqz0nc62"
        );
        check_expected!(
            "pkh(tpubD6NzVbkrYhZ4XHndKkuB8FifXm8r5FQHwrN6oZuWCz13qb93rtgKvD4PQsqC4HP4yhV3tA2fqr2RbY5mNXfM7RxXUoeABoDtsFUq2zJq6YK/44'/1'/0'/0/*)",
            "lasegmfs"
        );

        // https://github.com/bitcoin/bitcoin/blob/7ae86b3c6845873ca96650fc69beb4ae5285c801/src/test/descriptor_tests.cpp#L352-L354
        check_expected!(
            "sh(multi(2,[00000000/111'/222]xprvA1RpRA33e1JQ7ifknakTFpgNXPmW2YvmhqLQYMmrj4xJXXWYpDPS3xz7iAxn8L39njGVyuoseXzU6rcxFLJ8HFsTjSyQbLYnMpCqE2VbFWc,xprv9uPDJpEQgRQfDcW7BkF7eTya6RPxXeJCqCJGHuCJ4GiRVLzkTXBAJMu2qaMWPrS7AANYqdq6vcBcBUdJCVVFceUvJFjaPdGZ2y9WACViL4L/0))",
            "ggrsrxfy"
        );
        check_expected!(
            "sh(multi(2,[00000000/111'/222]xpub6ERApfZwUNrhLCkDtcHTcxd75RbzS1ed54G1LkBUHQVHQKqhMkhgbmJbZRkrgZw4koxb5JaHWkY4ALHY2grBGRjaDMzQLcgJvLJuZZvRcEL,xpub68NZiKmJWnxxS6aaHmn81bvJeTESw724CRDs6HbuccFQN9Ku14VQrADWgqbhhTHBaohPX4CjNLf9fq9MYo6oDaPPLPxSb7gwQN3ih19Zm4Y/0))",
            "tjg09x5t"
        );
    }

    #[test]
    fn test_desc_checksum_invalid_character() {
        let sparkle_heart = vec![240, 159, 146, 150];
        let sparkle_heart = str::from_utf8(&sparkle_heart)
            .unwrap()
            .chars()
            .next()
            .unwrap();
        let invalid_desc = format!("wpkh(tprv8ZgxMBicQKsPdpkqS7Eair4YxjcuuvDPNYmKX3sCniCf16tHEVrjjiSXEkFRnUH77yXc6ZcwHHcL{}fjdi5qUvw3VDfgYiH5mNsj5izuiu2N/1/2/*)", sparkle_heart);

        assert_eq!(
            desc_checksum(&invalid_desc).err().unwrap().to_string(),
            format!(
                "Invalid descriptor: Invalid character in checksum: '{}'",
                sparkle_heart
            )
        );
    }
}