Add fuzz target for ShortId parsing

fuzz/Cargo.toml

@@ -34,3 +34,9 @@ name = "url_decode_url"
 path = "fuzz_targets/url/decode_url.rs"
 doc = false
 bench = false
+
+[[bin]]
+name = "directory_short_id"
+path = "fuzz_targets/directory/short_id.rs"
+doc = false
+bench = false

fuzz/fuzz_targets/directory/short_id.rs

@@ -0,0 +1,52 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use payjoin::directory::ShortId;
+
+fn do_test(data: &[u8]) {
+    // Path 1: raw 8-byte identifier, as stored internally and compared
+    // against database keys. Any length other than 8 must be rejected.
+    match ShortId::try_from(data) {
+        Ok(id) => {
+            assert_eq!(id.as_bytes(), data);
+            assert_eq!(id.as_slice(), data);
+
+            let encoded = id.to_string();
+            let reparsed: ShortId =
+                encoded.parse().expect("Display output of a valid ShortId must re-parse");
+            assert_eq!(reparsed, id, "round-trip mismatch via Display/FromStr: {encoded}");
+        }
+        Err(_) => assert_ne!(data.len(), 8, "an 8-byte slice must always convert"),
+    }
+
+    // Path 2: attacker-controlled URL path segment, decoded as bech32
+    // without a checksum. This is the actual parsing path exercised by
+    // payjoin-mailroom when routing /{id} requests.
+    if let Ok(s) = std::str::from_utf8(data) {
+        if let Ok(id) = s.parse::<ShortId>() {
+            let reencoded = id.to_string();
+            let reparsed: ShortId =
+                reencoded.parse().expect("re-encoding a parsed ShortId must re-parse");
+            assert_eq!(reparsed, id, "round-trip mismatch via FromStr/Display: {s}");
+        }
+    }
+}
+
+fuzz_target!(|data| {
+    do_test(data);
+});
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn empty_input_does_not_crash() { super::do_test(&[]); }
+
+    #[test]
+    fn eight_zero_bytes_round_trip() { super::do_test(&[0u8; 8]); }
+
+    #[test]
+    fn every_length_boundary() {
+        super::do_test(&[0u8; 7]);
+        super::do_test(&[0u8; 9]);
+    }
+}