dkim: tidy up handling of l= body length tag

We should parse at the edge and pass down the parsed number

crates/dkim/src/hash.rs

@@ -99,19 +99,12 @@ impl HashImpl {
 /// https://datatracker.ietf.org/doc/html/rfc6376#section-3.7
 pub(crate) fn compute_body_hash<'a>(
     canonicalization_type: canonicalization::Type,
-    length: Option<&str>,
+    length: Option<usize>,
     hash_algo: HashAlgo,
     email: &'a ParsedEmail<'a>,
 ) -> Result<String, DKIMError> {
     let body = email.get_body_bytes();
-
-    let limit = if let Some(length) = length {
-        length
-            .parse::<usize>()
-            .map_err(|err| DKIMError::SignatureSyntaxError(format!("invalid length: {}", err)))?
-    } else {
-        usize::MAX
-    };
+    let limit = length.unwrap_or(usize::MAX);
 
     let mut hasher = LimitHasher {
         hasher: HashImpl::from_algo(hash_algo),
@@ -260,7 +253,7 @@ Hello Alice
         let email = ParsedEmail::parse_bytes(email.as_bytes()).unwrap();
 
         let canonicalization_type = canonicalization::Type::Relaxed;
-        let length = Some("3");
+        let length = Some(3);
         let hash_algo = HashAlgo::RsaSha1;
         assert_eq!(
             compute_body_hash(canonicalization_type, length, hash_algo, &email).unwrap(),

crates/dkim/src/header.rs

@@ -1,6 +1,7 @@
 use crate::{parser, DKIMError};
 use indexmap::map::IndexMap;
 use std::io::Write;
+use std::str::FromStr;
 
 pub(crate) const HEADER: &str = "DKIM-Signature";
 const REQUIRED_TAGS: &[&str] = &["v", "a", "b", "bh", "d", "h", "s"];
@@ -84,6 +85,26 @@ impl DKIMHeader {
         self.tags.get(name).map(|v| v.value.as_str())
     }
 
+    /// Get the named tag.
+    /// Attempt to parse it into an `R`
+    pub fn parse_tag<R>(&self, name: &str) -> Result<Option<R>, DKIMError>
+    where
+        R: FromStr,
+        <R as FromStr>::Err: std::fmt::Display,
+    {
+        match self.get_tag(name) {
+            None => Ok(None),
+            Some(value) => {
+                let value: R = value.parse().map_err(|err| {
+                    DKIMError::SignatureSyntaxError(format!(
+                        "invalid \"{name}\" tag value: {err:#}"
+                    ))
+                })?;
+                Ok(Some(value))
+            }
+        }
+    }
+
     pub fn get_raw_tag(&self, name: &str) -> Option<&str> {
         self.tags.get(name).map(|v| v.raw_value.as_str())
     }

crates/dkim/src/lib.rs

@@ -93,7 +93,7 @@ async fn verify_email_header<'a>(
     let hash_algo = parser::parse_hash_algo(&dkim_header.get_required_tag("a"))?;
     let computed_body_hash = hash::compute_body_hash(
         body_canonicalization_type,
-        dkim_header.get_tag("l"),
+        dkim_header.parse_tag("l")?,
         hash_algo,
         email,
     )?;