Add Codex32 checksum algo back in

src/lib.rs

@@ -18,6 +18,8 @@
 //! - Non-segwit stuff and you do *not* have an allocator, use the
 //!   [`primitives::decode::CheckedHrpstring`] type for decoding. For encoding we provide various
 //!   top level functions of the form `_to_fmt`.
+//! - To define your own checksum algorithm implement [`bech32::primitives::Checksum`].
+//!   See the implementation on [`primitives::checksum::Codex32`] for an example.
 //!
 //! The original description in [BIP-0173](https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki)
 //! has more details. See also [BIP-0350](https://github.com/bitcoin/bips/blob/master/bip-0350.mediawiki).

src/primitives/mod.rs

@@ -26,6 +26,11 @@ pub enum Bech32 {}
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub enum Bech32m {}
 
+/// The codex32 checksum algorithm, defined in [BIP-93].
+/// [BIP-350]: <https://github.com/bitcoin/bips/blob/master/bip-0093.mediawiki>
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+enum Codex32 {}
+
 impl Checksum for NoChecksum {
     type MidstateRepr = PackedNull;
     const CHECKSUM_LENGTH: usize = 0;
@@ -50,6 +55,20 @@ impl Checksum for Bech32m {
     const TARGET_RESIDUE: u32 = 0x2bc830a3;
 }
 
+impl Checksum for Codex32 {
+    type MidstateRepr = u128;
+    const CHECKSUM_LENGTH: usize = 13;
+    // Copied from BIP-93
+    const GENERATOR_SH: [u128; 5] = [
+        0x19dc500ce73fde210,
+        0x1bfae00def77fe529,
+        0x1fbd920fffe7bee52,
+        0x1739640bdeee3fdad,
+        0x07729a039cfc75f5a,
+    ];
+    const TARGET_RESIDUE: u128 = 0x10ce0795c2fd1e62a;
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;