Add segwit API

Add a segwit API with the aim that "typical" modern bitcoin usage is
easy and correct.

src/lib.rs

@@ -43,6 +43,7 @@ pub use crate::primitives::{Bech32, Bech32m};
 
 mod error;
 pub mod primitives;
+pub mod segwit;
 
 #[cfg(feature = "arrayvec")]
 use arrayvec::{ArrayVec, CapacityError};

src/primitives/decode.rs

@@ -78,6 +78,7 @@ use crate::primitives::checksum::{self, Checksum};
 use crate::primitives::gf32::Fe32;
 use crate::primitives::hrp::{self, Hrp};
 use crate::primitives::iter::{Fe32IterExt, FesToBytes};
+use crate::primitives::segwit::{self, WitnessLengthError};
 use crate::{write_err, Bech32, Bech32m};
 
 /// Separator between the hrp and payload (as defined by BIP-173).
@@ -264,7 +265,7 @@ impl<'s> CheckedHrpstring<'s> {
         self.data = &self.data[1..]; // Remove the witness version byte from data.
 
         self.validate_padding()?;
-        self.validate_witness_length(witness_version)?;
+        self.validate_witness_program_length(witness_version)?;
 
         Ok(SegwitHrpstring { hrp: self.hrp(), witness_version, data: self.data })
     }
@@ -309,21 +310,11 @@ impl<'s> CheckedHrpstring<'s> {
     /// Validates the segwit witness length rules.
     ///
     /// Must be called after the witness version byte is removed from the data.
-    #[allow(clippy::manual_range_contains)] // For witness length range check.
-    fn validate_witness_length(&self, witness_version: Fe32) -> Result<(), WitnessLengthError> {
-        use WitnessLengthError::*;
-
-        let witness_len = self.byte_iter().len();
-        if witness_len < 2 {
-            return Err(TooShort);
-        }
-        if witness_len > 40 {
-            return Err(TooLong);
-        }
-        if witness_version == Fe32::Q && witness_len != 20 && witness_len != 32 {
-            return Err(InvalidSegwitV0);
-        }
-        Ok(())
+    fn validate_witness_program_length(
+        &self,
+        witness_version: Fe32,
+    ) -> Result<(), WitnessLengthError> {
+        segwit::validate_witness_program_length(self.byte_iter().len(), witness_version)
     }
 }
 
@@ -746,41 +737,6 @@ impl std::error::Error for ChecksumError {
     }
 }
 
-/// Witness program invalid because of incorrect length.
-#[derive(Debug, Clone, PartialEq, Eq)]
-#[non_exhaustive]
-pub enum WitnessLengthError {
-    /// The witness data is too short.
-    TooShort,
-    /// The witness data is too long.
-    TooLong,
-    /// The segwit v0 witness is not 20 or 32 bytes long.
-    InvalidSegwitV0,
-}
-
-impl fmt::Display for WitnessLengthError {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        use WitnessLengthError::*;
-
-        match *self {
-            TooShort => write!(f, "witness program is less than 2 bytes long"),
-            TooLong => write!(f, "witness program is more than 40 bytes long"),
-            InvalidSegwitV0 => write!(f, "the segwit v0 witness is not 20 or 32 bytes long"),
-        }
-    }
-}
-
-#[cfg(feature = "std")]
-impl std::error::Error for WitnessLengthError {
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        use WitnessLengthError::*;
-
-        match *self {
-            TooShort | TooLong | InvalidSegwitV0 => None,
-        }
-    }
-}
-
 /// Error validating the padding bits on the witness data.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum PaddingError {

src/primitives/mod.rs

@@ -8,6 +8,7 @@ pub mod encode;
 pub mod gf32;
 pub mod hrp;
 pub mod iter;
+pub mod segwit;
 
 use checksum::{Checksum, PackedNull};
 

src/primitives/segwit.rs

@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: MIT
+
+//! Segregated Witness functionality - useful for enforcing parts of [`BIP-173`] and [`BIP-350`].
+//!
+//! [BIP-173]: <https://github.com/bitcoin/bips/blob/master/bip-0173.mediawiki>
+//! [BIP-350]: <https://github.com/bitcoin/bips/blob/master/bip-0350.mediawiki>
+
+use core::fmt;
+
+use crate::primitives::gf32::Fe32;
+
+/// Returns true if given field element represents a valid segwit version.
+pub fn is_valid_witness_version(witness_version: Fe32) -> bool {
+    validate_witness_version(witness_version).is_ok()
+}
+
+/// Returns true if `length` represents a valid witness program length for `witness_version`.
+pub fn is_valid_witness_program_length(length: usize, witness_version: Fe32) -> bool {
+    validate_witness_program_length(length, witness_version).is_ok()
+}
+
+/// Checks that the given field element represents a valid segwit witness version.
+pub fn validate_witness_version(witness_version: Fe32) -> Result<(), InvalidWitnessVersionError> {
+    if witness_version.to_u8() > 16 {
+        Err(InvalidWitnessVersionError(witness_version))
+    } else {
+        Ok(())
+    }
+}
+
+/// Validates the segwit witness program `length` rules for witness `version`.
+pub fn validate_witness_program_length(
+    length: usize,
+    version: Fe32,
+) -> Result<(), WitnessLengthError> {
+    use WitnessLengthError::*;
+
+    if length < 2 {
+        return Err(TooShort);
+    }
+    if length > 40 {
+        return Err(TooLong);
+    }
+    if version == Fe32::Q && length != 20 && length != 32 {
+        return Err(InvalidSegwitV0);
+    }
+    Ok(())
+}
+
+/// Field element does not represent a valid witness version.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct InvalidWitnessVersionError(Fe32);
+
+impl fmt::Display for InvalidWitnessVersionError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "field element does not represent a valid witness version")
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for InvalidWitnessVersionError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+}
+
+/// Witness program invalid because of incorrect length.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum WitnessLengthError {
+    /// The witness data is too short.
+    TooShort,
+    /// The witness data is too long.
+    TooLong,
+    /// The segwit v0 witness is not 20 or 32 bytes long.
+    InvalidSegwitV0,
+}
+
+impl fmt::Display for WitnessLengthError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use WitnessLengthError::*;
+
+        match *self {
+            TooShort => write!(f, "witness program is less than 2 bytes long"),
+            TooLong => write!(f, "witness program is more than 40 bytes long"),
+            InvalidSegwitV0 => write!(f, "the segwit v0 witness is not 20 or 32 bytes long"),
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+impl std::error::Error for WitnessLengthError {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        use WitnessLengthError::*;
+
+        match *self {
+            TooShort | TooLong | InvalidSegwitV0 => None,
+        }
+    }
+}