descriptor: add Alekos' checksum implementation

This is a translation of bitcoind's checksum function for descriptors.

This file is entirely taken from Magical Bitcoin / Bitcoindevkit, and
was written by Alekos Filini.
The MIT license was stripped with Alekos' agreement, see
#195 (comment),
in order to make it compatible with rust-miniscript's license.

Co-Authored-by: Alekos Filini <[email protected]>
Signed-off-by: Antoine Poinsot <[email protected]>

Author: darosior

src/descriptor/checksum.rs

@@ -0,0 +1,102 @@
+//! 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::descriptor::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
+pub fn get_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::InvalidDescriptorCharacter(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))
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use crate::descriptor::get_checksum;
+
+    // test get_checksum() function; it should return the same value as Bitcoin Core
+    #[test]
+    fn test_get_checksum() {
+        let desc = "wpkh(tprv8ZgxMBicQKsPdpkqS7Eair4YxjcuuvDPNYmKX3sCniCf16tHEVrjjiSXEkFRnUH77yXc6ZcwHHcLNfjdi5qUvw3VDfgYiH5mNsj5izuiu2N/1/2/*)";
+        assert_eq!(get_checksum(desc).unwrap(), "tqz0nc62");
+
+        let desc = "pkh(tpubD6NzVbkrYhZ4XHndKkuB8FifXm8r5FQHwrN6oZuWCz13qb93rtgKvD4PQsqC4HP4yhV3tA2fqr2RbY5mNXfM7RxXUoeABoDtsFUq2zJq6YK/44'/1'/0'/0/*)";
+        assert_eq!(get_checksum(desc).unwrap(), "lasegmfs");
+    }
+
+    #[test]
+    fn test_get_checksum_invalid_character() {
+        let sparkle_heart = vec![240, 159, 146, 150];
+        let sparkle_heart = std::str::from_utf8(&sparkle_heart)
+            .unwrap()
+            .chars()
+            .next()
+            .unwrap();
+        let invalid_desc = format!("wpkh(tprv8ZgxMBicQKsPdpkqS7Eair4YxjcuuvDPNYmKX3sCniCf16tHEVrjjiSXEkFRnUH77yXc6ZcwHHcL{}fjdi5qUvw3VDfgYiH5mNsj5izuiu2N/1/2/*)", sparkle_heart);
+
+        assert!(matches!(
+            get_checksum(&invalid_desc).err(),
+            Some(Error::InvalidDescriptorCharacter(invalid_char)) if invalid_char == sparkle_heart
+        ));
+    }
+}