expression: move checksum validation into expression parsing

Right now our character validation logic is spread all over the place.
We have `expression::check_valid_chars` and `checksum::verify_checksum`
and also `checksum::Engine::input` which all do the same checks,
sharing the public array `expression::VALID_CHARS` which is arguably an
implementation detail and shouldn't be exposed.

In fact, whenever we parse any Miniscript object (a script, a
descriptor, a policy of either type), we are first validating the
checksum and then parsing into a tree, and our tree-parsing re-does
character validation that was already done as part of the checksum
check.

This commit moves the checksum error logic into expression, where it
belongs, and folds the checksum error variant into the expression
parsing error, where it belongs (removing it from the mega-enum at the
root).

However it leaves the Taproot parsing alone, which is still a bit of a
mess and will be addressed in a later PR.

Benchmarks show that this significantly slows down expression parsing
(as expected, since it is now doing checksumming) but significantly
speeds up descriptor parsing (which is no longer doing multiple checks
for character validity).

Author: apoelstra

src/descriptor/bare.rs

@@ -12,7 +12,6 @@ use core::fmt;
 use bitcoin::script::{self, PushBytes};
 use bitcoin::{Address, Network, ScriptBuf, Weight};
 
-use super::checksum::verify_checksum;
 use crate::descriptor::{write_descriptor, DefiniteDescriptorKey};
 use crate::expression::{self, FromTree};
 use crate::miniscript::context::{ScriptContext, ScriptContextError};
@@ -186,8 +185,7 @@ impl<Pk: FromStrKey> FromTree for Bare<Pk> {
 impl<Pk: FromStrKey> core::str::FromStr for Bare<Pk> {
     type Err = Error;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let desc_str = verify_checksum(s)?;
-        let top = expression::Tree::from_str(desc_str)?;
+        let top = expression::Tree::from_str(s)?;
         Self::from_tree(&top)
     }
 }
@@ -387,8 +385,7 @@ impl<Pk: FromStrKey> FromTree for Pkh<Pk> {
 impl<Pk: FromStrKey> core::str::FromStr for Pkh<Pk> {
     type Err = Error;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let desc_str = verify_checksum(s)?;
-        let top = expression::Tree::from_str(desc_str)?;
+        let top = expression::Tree::from_str(s)?;
         Self::from_tree(&top)
     }
 }

src/descriptor/checksum.rs

@@ -14,7 +14,6 @@ use core::iter::FromIterator;
 use bech32::primitives::checksum::PackedFe32;
 use bech32::{Checksum, Fe32};
 
-pub use crate::expression::VALID_CHARS;
 use crate::prelude::*;
 
 const CHECKSUM_LENGTH: usize = 8;

src/descriptor/mod.rs

@@ -21,7 +21,6 @@ use bitcoin::{
 };
 use sync::Arc;
 
-use self::checksum::verify_checksum;
 use crate::miniscript::decode::Terminal;
 use crate::miniscript::{satisfy, Legacy, Miniscript, Segwitv0};
 use crate::plan::{AssetProvider, Plan};
@@ -988,8 +987,7 @@ impl<Pk: FromStrKey> FromStr for Descriptor<Pk> {
         let desc = if s.starts_with("tr(") {
             Ok(Descriptor::Tr(Tr::from_str(s)?))
         } else {
-            let desc_str = verify_checksum(s)?;
-            let top = expression::Tree::from_str(desc_str)?;
+            let top = expression::Tree::from_str(s)?;
             expression::FromTree::from_tree(&top)
         }?;
 
@@ -1840,7 +1838,7 @@ mod tests {
             ($secp: ident,$($desc: expr),*) => {
                 $(
                     match Descriptor::parse_descriptor($secp, $desc) {
-                        Err(Error::Checksum(_)) => {},
+                        Err(Error::ParseTree(crate::ParseTreeError::Checksum(_))) => {},
                         Err(e) => panic!("Expected bad checksum for {}, got '{}'", $desc, e),
                         _ => panic!("Invalid checksum treated as valid: {}", $desc),
                     };

src/descriptor/segwitv0.rs

@@ -10,7 +10,6 @@ use core::fmt;
 
 use bitcoin::{Address, Network, ScriptBuf, Weight};
 
-use super::checksum::verify_checksum;
 use super::SortedMultiVec;
 use crate::descriptor::{write_descriptor, DefiniteDescriptorKey};
 use crate::expression::{self, FromTree};
@@ -288,8 +287,7 @@ impl<Pk: MiniscriptKey> fmt::Display for Wsh<Pk> {
 impl<Pk: FromStrKey> core::str::FromStr for Wsh<Pk> {
     type Err = Error;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let desc_str = verify_checksum(s)?;
-        let top = expression::Tree::from_str(desc_str)?;
+        let top = expression::Tree::from_str(s)?;
         Wsh::<Pk>::from_tree(&top)
     }
 }
@@ -505,8 +503,7 @@ impl<Pk: FromStrKey> crate::expression::FromTree for Wpkh<Pk> {
 impl<Pk: FromStrKey> core::str::FromStr for Wpkh<Pk> {
     type Err = Error;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let desc_str = verify_checksum(s)?;
-        let top = expression::Tree::from_str(desc_str)?;
+        let top = expression::Tree::from_str(s)?;
         Self::from_tree(&top)
     }
 }

src/descriptor/sh.rs

@@ -13,7 +13,6 @@ use core::fmt;
 use bitcoin::script::PushBytes;
 use bitcoin::{script, Address, Network, ScriptBuf, Weight};
 
-use super::checksum::verify_checksum;
 use super::{SortedMultiVec, Wpkh, Wsh};
 use crate::descriptor::{write_descriptor, DefiniteDescriptorKey};
 use crate::expression::{self, FromTree};
@@ -109,8 +108,7 @@ impl<Pk: FromStrKey> crate::expression::FromTree for Sh<Pk> {
 impl<Pk: FromStrKey> core::str::FromStr for Sh<Pk> {
     type Err = Error;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let desc_str = verify_checksum(s)?;
-        let top = expression::Tree::from_str(desc_str)?;
+        let top = expression::Tree::from_str(s)?;
         Self::from_tree(&top)
     }
 }

src/descriptor/tr.rs

@@ -557,7 +557,9 @@ impl<Pk: FromStrKey> crate::expression::FromTree for Tr<Pk> {
 impl<Pk: FromStrKey> core::str::FromStr for Tr<Pk> {
     type Err = Error;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
-        let desc_str = verify_checksum(s)?;
+        let desc_str = verify_checksum(s)
+            .map_err(From::from)
+            .map_err(Error::ParseTree)?;
         let top = parse_tr_tree(desc_str)?;
         Self::from_tree(&top)
     }
@@ -587,8 +589,6 @@ impl<Pk: MiniscriptKey> fmt::Display for Tr<Pk> {
 
 // Helper function to parse string into miniscript tree form
 fn parse_tr_tree(s: &str) -> Result<expression::Tree, Error> {
-    expression::check_valid_chars(s)?;
-
     if s.len() > 3 && &s[..3] == "tr(" && s.as_bytes()[s.len() - 1] == b')' {
         let rest = &s[3..s.len() - 1];
         if !rest.contains(',') {

src/expression/error.rs

@@ -4,26 +4,22 @@
 
 use core::fmt;
 
+use crate::descriptor::checksum;
 use crate::prelude::*;
 use crate::ThresholdError;
 
 /// An error parsing an expression tree.
 #[derive(Debug, PartialEq, Eq)]
 pub enum ParseTreeError {
+    /// Error validating the checksum or character set.
+    Checksum(checksum::Error),
     /// Expression tree had depth exceeding our hard cap.
     MaxRecursionDepthExceeded {
         /// The depth of the tree that was attempted to be parsed.
         actual: usize,
         /// The maximum depth.
         maximum: u32,
     },
-    /// Character occurred which was not part of the valid descriptor character set.
-    InvalidCharacter {
-        /// The character in question.
-        ch: char,
-        /// Its byte-index into the string.
-        pos: usize,
-    },
     /// After a close-paren, the only valid next characters are close-parens and commas. Got
     /// something else.
     ExpectedParenOrComma {
@@ -66,15 +62,17 @@ pub enum ParseTreeError {
     },
 }
 
+impl From<checksum::Error> for ParseTreeError {
+    fn from(e: checksum::Error) -> Self { Self::Checksum(e) }
+}
+
 impl fmt::Display for ParseTreeError {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self {
+            ParseTreeError::Checksum(ref e) => e.fmt(f),
             ParseTreeError::MaxRecursionDepthExceeded { actual, maximum } => {
                 write!(f, "maximum recursion depth exceeded (max {}, got {})", maximum, actual)
             }
-            ParseTreeError::InvalidCharacter { ch, pos } => {
-                write!(f, "character `{}` (position {}) not allowed in descriptor", ch, pos)
-            }
             ParseTreeError::ExpectedParenOrComma { ch, pos } => {
                 write!(
                     f,
@@ -103,7 +101,17 @@ impl fmt::Display for ParseTreeError {
 }
 #[cfg(feature = "std")]
 impl std::error::Error for ParseTreeError {
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        match self {
+            ParseTreeError::Checksum(ref e) => Some(e),
+            ParseTreeError::MaxRecursionDepthExceeded { .. }
+            | ParseTreeError::ExpectedParenOrComma { .. }
+            | ParseTreeError::UnmatchedOpenParen { .. }
+            | ParseTreeError::UnmatchedCloseParen { .. }
+            | ParseTreeError::MismatchedParens { .. }
+            | ParseTreeError::TrailingCharacter { .. } => None,
+        }
+    }
 }
 
 /// Error parsing a threshold expression.

src/expression/mod.rs

@@ -9,32 +9,13 @@ use core::fmt;
 use core::str::FromStr;
 
 pub use self::error::{ParseThresholdError, ParseTreeError};
+use crate::descriptor::checksum::verify_checksum;
 use crate::prelude::*;
 use crate::{errstr, Error, Threshold, MAX_RECURSION_DEPTH};
 
 /// Allowed characters are descriptor strings.
 pub const INPUT_CHARSET: &str = "0123456789()[],'/*abcdefgh@:$%{}IJKLMNOPQRSTUVWXYZ&+-.;<=>?!^_|~ijklmnopqrstuvwxyzABCDEFGH`#\"\\ ";
 
-/// Map of valid characters in descriptor strings.
-#[rustfmt::skip]
-pub const VALID_CHARS: [Option<u8>; 128] = [
-    None, None, None, None, None, None, None, None, None, None, None, None, None,
-    None, None, None, None, None, None, None, None, None, None, None, None, None,
-    None, None, None, None, None, None, Some(94), Some(59), Some(92), Some(91),
-    Some(28), Some(29), Some(50), Some(15), Some(10), Some(11), Some(17), Some(51),
-    Some(14), Some(52), Some(53), Some(16), Some(0), Some(1), Some(2), Some(3),
-    Some(4), Some(5), Some(6), Some(7), Some(8), Some(9), Some(27), Some(54),
-    Some(55), Some(56), Some(57), Some(58), Some(26), Some(82), Some(83),
-    Some(84), Some(85), Some(86), Some(87), Some(88), Some(89), Some(32), Some(33),
-    Some(34), Some(35), Some(36), Some(37), Some(38), Some(39), Some(40), Some(41),
-    Some(42), Some(43), Some(44), Some(45), Some(46), Some(47), Some(48), Some(49),
-    Some(12), Some(93), Some(13), Some(60), Some(61), Some(90), Some(18), Some(19),
-    Some(20), Some(21), Some(22), Some(23), Some(24), Some(25), Some(64), Some(65),
-    Some(66), Some(67), Some(68), Some(69), Some(70), Some(71), Some(72), Some(73),
-    Some(74), Some(75), Some(76), Some(77), Some(78), Some(79), Some(80), Some(81),
-    Some(30), Some(62), Some(31), Some(63), None,
-];
-
 #[derive(Debug)]
 /// A token of the form `x(...)` or `x`
 pub struct Tree<'a> {
@@ -145,19 +126,17 @@ impl<'a> Tree<'a> {
         Self::from_slice_delim(sl, 0u32, '(')
     }
 
-    fn parse_pre_check(s: &str, open: u8, close: u8) -> Result<(), ParseTreeError> {
-        // First, scan through string to make sure it is well-formed.
-        let mut max_depth = 0;
+    /// Check that a string is a well-formed expression string, with optional
+    /// checksum.
+    ///
+    /// Returns the string with the checksum removed.
+    fn parse_pre_check(s: &str, open: u8, close: u8) -> Result<&str, ParseTreeError> {
         // Do ASCII check first; after this we can use .bytes().enumerate() rather
         // than .char_indices(), which is *significantly* faster.
-        for (pos, ch) in s.char_indices() {
-            if !(32..128).contains(&u32::from(ch)) {
-                return Err(ParseTreeError::InvalidCharacter { ch, pos });
-            }
-        }
+        let s = verify_checksum(s)?;
 
+        let mut max_depth = 0;
         let mut open_paren_stack = Vec::with_capacity(128);
-
         for (pos, ch) in s.bytes().enumerate() {
             if ch == open {
                 open_paren_stack.push((ch, pos));
@@ -232,7 +211,7 @@ impl<'a> Tree<'a> {
             });
         }
 
-        Ok(())
+        Ok(s)
     }
 
     pub(crate) fn from_slice_delim(
@@ -242,9 +221,9 @@ impl<'a> Tree<'a> {
     ) -> Result<(Tree<'a>, &'a str), Error> {
         if depth == 0 {
             if delim == '{' {
-                Self::parse_pre_check(sl, b'{', b'}').map_err(Error::ParseTree)?;
+                sl = Self::parse_pre_check(sl, b'{', b'}').map_err(Error::ParseTree)?;
             } else {
-                Self::parse_pre_check(sl, b'(', b')').map_err(Error::ParseTree)?;
+                sl = Self::parse_pre_check(sl, b'(', b')').map_err(Error::ParseTree)?;
             }
         }
 
@@ -288,8 +267,6 @@ impl<'a> Tree<'a> {
     /// Parses a tree from a string
     #[allow(clippy::should_implement_trait)] // Cannot use std::str::FromStr because of lifetimes.
     pub fn from_str(s: &'a str) -> Result<Tree<'a>, Error> {
-        check_valid_chars(s)?;
-
         let (top, rem) = Tree::from_slice(s)?;
         if rem.is_empty() {
             Ok(top)
@@ -328,23 +305,6 @@ impl<'a> Tree<'a> {
     }
 }
 
-/// Filter out non-ASCII because we byte-index strings all over the
-/// place and Rust gets very upset when you splinch a string.
-pub fn check_valid_chars(s: &str) -> Result<(), Error> {
-    for ch in s.bytes() {
-        if !ch.is_ascii() {
-            return Err(Error::Unprintable(ch));
-        }
-        // Index bounds: We know that ch is ASCII, so it is <= 127.
-        if VALID_CHARS[ch as usize].is_none() {
-            return Err(Error::Unexpected(
-                "Only characters in INPUT_CHARSET are allowed".to_string(),
-            ));
-        }
-    }
-    Ok(())
-}
-
 /// Parse a string as a u32, for timelocks or thresholds
 pub fn parse_num(s: &str) -> Result<u32, Error> {
     if s.len() > 1 {
@@ -418,15 +378,6 @@ mod tests {
         assert!(parse_num("-6").is_err());
     }
 
-    #[test]
-    fn test_valid_char_map() {
-        let mut valid_chars = [None; 128];
-        for (i, ch) in super::INPUT_CHARSET.chars().enumerate() {
-            valid_chars[ch as usize] = Some(i as u8);
-        }
-        assert_eq!(valid_chars, super::VALID_CHARS);
-    }
-
     #[test]
     fn parse_tree_basic() {
         assert_eq!(Tree::from_str("thresh").unwrap(), leaf("thresh"));

src/lib.rs

@@ -492,8 +492,6 @@ pub enum Error {
     Threshold(ThresholdError),
     /// Invalid threshold.
     ParseThreshold(ParseThresholdError),
-    /// Checksum error parsing a descriptor.
-    Checksum(descriptor::checksum::Error),
     /// Invalid expression tree.
     ParseTree(ParseTreeError),
 }
@@ -557,7 +555,6 @@ impl fmt::Display for Error {
             Error::RelativeLockTime(ref e) => e.fmt(f),
             Error::Threshold(ref e) => e.fmt(f),
             Error::ParseThreshold(ref e) => e.fmt(f),
-            Error::Checksum(ref e) => e.fmt(f),
             Error::ParseTree(ref e) => e.fmt(f),
         }
     }
@@ -609,7 +606,6 @@ impl error::Error for Error {
             RelativeLockTime(e) => Some(e),
             Threshold(e) => Some(e),
             ParseThreshold(e) => Some(e),
-            Checksum(e) => Some(e),
             ParseTree(e) => Some(e),
         }
     }
@@ -650,11 +646,6 @@ impl From<bitcoin::address::P2shError> for Error {
     fn from(e: bitcoin::address::P2shError) -> Error { Error::AddrP2shError(e) }
 }
 
-#[doc(hidden)]
-impl From<descriptor::checksum::Error> for Error {
-    fn from(e: descriptor::checksum::Error) -> Error { Error::Checksum(e) }
-}
-
 #[doc(hidden)]
 #[cfg(feature = "compiler")]
 impl From<crate::policy::compiler::CompilerError> for Error {

src/miniscript/mod.rs

@@ -1315,7 +1315,7 @@ mod tests {
         assert!(Segwitv0Script::from_str_insane("🌏")
             .unwrap_err()
             .to_string()
-            .contains("unprintable character"));
+            .contains("invalid character"));
     }
 
     #[test]