1.2.0: unify hex parsing (one parser) + robust encrypted-unit sampling

- One workspace hex parser (libfreemkv::hex): the keydb / online / mapfile
  parsers had divergent prefix handling (0x vs 0X vs none) — a key written with
  a prefix one parser didn't expect was silently dropped. All three now call it.
- read_encrypted_units probes several points spread across each extent instead
  of only the midpoint-forward window, so a late-starting / sparse encrypted
  body still yields samples (empty samples make decrypt_with skip wrong-key
  validation). A read error at one probe no longer abandons the extent.

src/disc/mapfile.rs

@@ -651,35 +651,9 @@ impl Drop for Mapfile {
 /// caller treats a bad VID comment as simply absent rather than an
 /// error, so a corrupt header never fails a mapfile load.
 fn parse_vid_hex(s: &str) -> Option<[u8; 16]> {
-    let s = s.strip_prefix("0x").unwrap_or(s);
-    // Parse on bytes, not on the &str: slicing a &str by byte index
-    // (`&s[i*2..i*2+2]`) panics when the cut lands inside a multi-byte
-    // UTF-8 char. A hand-edited/corrupt `# freemkv-vid:` comment of
-    // exactly 32 bytes containing a multi-byte char would otherwise
-    // kill the whole load. ASCII hex is one byte per char, so anything
-    // non-ASCII is simply rejected here as malformed.
-    let bytes = s.as_bytes();
-    if bytes.len() != 32 {
-        return None;
-    }
-    let mut out = [0u8; 16];
-    for (i, b) in out.iter_mut().enumerate() {
-        let hi = hex_nibble(bytes[i * 2])?;
-        let lo = hex_nibble(bytes[i * 2 + 1])?;
-        *b = (hi << 4) | lo;
-    }
-    Some(out)
-}
-
-/// Map a single ASCII hex digit byte to its 0-15 value. Returns `None`
-/// for any non-hex byte (including any non-ASCII / multi-byte lead byte).
-fn hex_nibble(c: u8) -> Option<u8> {
-    match c {
-        b'0'..=b'9' => Some(c - b'0'),
-        b'a'..=b'f' => Some(c - b'a' + 10),
-        b'A'..=b'F' => Some(c - b'A' + 10),
-        _ => None,
-    }
+    // The one workspace hex parser (accepts an optional `0x`/`0X` prefix,
+    // byte-based so a multi-byte `# freemkv-vid:` comment rejects, never panics).
+    crate::hex::parse_hex_fixed::<16>(s)
 }
 
 /// Parse a `# freemkv-uk:` value `<cps>:<32hex>` into `(cps_unit, key)`. Returns

src/hex.rs

@@ -0,0 +1,106 @@
+//! The single hex → bytes parser for the whole workspace.
+//!
+//! Key material arrives as hex from three third-party sources — the keydb, an
+//! online key service, and the mapfile's `# freemkv-vid:` comment — and each
+//! used to parse it slightly differently (one stripped `0x`/`0X`, one stripped
+//! nothing, one stripped `0x` only). A key written with a prefix one parser
+//! didn't expect was silently dropped → "can't decrypt" with no error. This is
+//! the one parser they all call, so the prefix/case/validation rules live in
+//! exactly one place.
+//!
+//! Operates on BYTES, not `&str` char indices: the inputs are untrusted, so a
+//! multi-byte UTF-8 scalar must reject as malformed, never panic on a
+//! mid-codepoint slice.
+
+/// Parse a hex string into bytes. Accepts an optional `0x`/`0X` prefix
+/// (case-insensitive), then requires an even run of ASCII hex digits. Any
+/// non-hex byte, or an odd length, yields `None`.
+pub fn parse_hex_bytes(s: &str) -> Option<Vec<u8>> {
+    let body = strip_prefix(s.trim());
+    let bytes = body.as_bytes();
+    // Empty → empty Vec (a legitimately-empty variable-length field); odd length
+    // is malformed. (`parse_hex_fixed` enforces a concrete length separately.)
+    if bytes.len() % 2 != 0 {
+        return None;
+    }
+    let mut out = Vec::with_capacity(bytes.len() / 2);
+    for pair in bytes.chunks_exact(2) {
+        out.push(byte(pair[0], pair[1])?);
+    }
+    Some(out)
+}
+
+/// Parse a hex string into a fixed `[u8; N]`. Accepts an optional `0x`/`0X`
+/// prefix; requires EXACTLY `2*N` ASCII hex digits after it. `None` on any
+/// non-hex byte or a length mismatch.
+pub fn parse_hex_fixed<const N: usize>(s: &str) -> Option<[u8; N]> {
+    let body = strip_prefix(s.trim());
+    let bytes = body.as_bytes();
+    if bytes.len() != 2 * N {
+        return None;
+    }
+    let mut out = [0u8; N];
+    for (i, slot) in out.iter_mut().enumerate() {
+        *slot = byte(bytes[2 * i], bytes[2 * i + 1])?;
+    }
+    Some(out)
+}
+
+/// Strip a single leading `0x` / `0X` if present (case-insensitive).
+fn strip_prefix(s: &str) -> &str {
+    s.strip_prefix("0x")
+        .or_else(|| s.strip_prefix("0X"))
+        .unwrap_or(s)
+}
+
+/// Combine two ASCII hex-digit bytes into one byte. `as char` is intentional:
+/// for a non-ASCII byte it produces a Latin-1 scalar that `to_digit(16)` then
+/// rejects — so non-hex (incl. `+`/`-` sign chars) and multi-byte input fail
+/// cleanly rather than slipping through `from_str_radix`'s sign handling.
+fn byte(hi: u8, lo: u8) -> Option<u8> {
+    let hi = (hi as char).to_digit(16)?;
+    let lo = (lo as char).to_digit(16)?;
+    Some((hi * 16 + lo) as u8)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fixed_accepts_0x_0x_and_bare_same_result() {
+        let want = [0x00, 0x11, 0xab, 0xCD, 0xef, 0x42, 0x99, 0x00];
+        let bare = "0011abcdef429900";
+        assert_eq!(parse_hex_fixed::<8>(bare), Some(want));
+        assert_eq!(parse_hex_fixed::<8>(&format!("0x{bare}")), Some(want));
+        // The case that used to be dropped by one parser but not another.
+        assert_eq!(parse_hex_fixed::<8>(&format!("0X{bare}")), Some(want));
+        assert_eq!(parse_hex_fixed::<8>(&format!("  0X{bare}  ")), Some(want));
+    }
+
+    #[test]
+    fn fixed_rejects_wrong_length_and_non_hex_and_signs() {
+        assert_eq!(parse_hex_fixed::<16>("00"), None); // too short
+        assert_eq!(parse_hex_fixed::<2>("00112233"), None); // too long
+        assert_eq!(parse_hex_fixed::<2>("zz11"), None); // non-hex
+        assert_eq!(parse_hex_fixed::<2>("+5-A"), None); // sign chars
+    }
+
+    #[test]
+    fn does_not_panic_on_multibyte_of_exact_byte_length() {
+        // "中" is 3 bytes; + 29 'a' = 32 bytes → would mis-slice a &str-indexed
+        // parser. Must reject, not panic.
+        let s = "中".to_string() + &"a".repeat(29);
+        assert_eq!(s.len(), 32);
+        assert_eq!(parse_hex_fixed::<16>(&s), None);
+    }
+
+    #[test]
+    fn bytes_variable_length_and_odd_rejected() {
+        assert_eq!(parse_hex_bytes("0xAABBCC"), Some(vec![0xAA, 0xBB, 0xCC]));
+        assert_eq!(parse_hex_bytes("AABBC"), None); // odd
+        // Empty (or prefix-only) → empty Vec: a legitimately-empty field.
+        assert_eq!(parse_hex_bytes(""), Some(vec![]));
+        assert_eq!(parse_hex_bytes("0x"), Some(vec![]));
+    }
+}

src/keysource.rs

@@ -344,8 +344,9 @@ pub fn key_fetch(
 /// "Encrypted" is decided by [`crate::aacs::ts_sync_destroyed`] — the SAME
 /// predicate the decrypt gate uses — so all sides agree. A clip opens with clear
 /// navigation units (PAT/PMT, menus); only the feature body is scrambled, and a
-/// clear unit proves nothing, so this collects only scrambled ones, sampling the
-/// largest extent at its midpoint forward.
+/// clear unit proves nothing, so this collects only scrambled ones — probing
+/// several points spread across EACH extent so a title whose encrypted body
+/// starts late (or whose midpoint lands in clear nav) still yields samples.
 pub fn read_encrypted_units(
     reader: &mut dyn crate::sector::SectorSource,
     title: &crate::disc::DiscTitle,
@@ -353,7 +354,12 @@ pub fn read_encrypted_units(
 ) -> Vec<Vec<u8>> {
     use crate::aacs::{ALIGNED_UNIT_LEN, ALIGNED_UNIT_SECTORS, ts_sync_destroyed};
     const CHUNK_UNITS: u32 = 15; // 45 sectors/read — under the drive transfer cap
-    const MAX_CHUNKS_PER_EXTENT: u32 = 4; // ~60 units scanned at each extent's midpoint
+    // Probe several evenly-spaced points across EACH extent rather than only the
+    // midpoint-and-forward: a title whose encrypted feature starts late, or whose
+    // midpoint lands in a clear nav stretch, must STILL yield scrambled samples.
+    // Empty samples make `Disc::decrypt_with` skip wrong-key validation, so a
+    // real encrypted title returning nothing here is a silent wrong-key hazard.
+    const PROBES_PER_EXTENT: u32 = 8;
 
     let mut out: Vec<Vec<u8>> = Vec::new();
     for ext in &title.extents {
@@ -361,25 +367,30 @@ pub fn read_encrypted_units(
         if total_units == 0 {
             continue;
         }
-        let mut unit = total_units / 2; // midpoint (past the clear nav at the head)
-        for _ in 0..MAX_CHUNKS_PER_EXTENT {
+        for p in 1..=PROBES_PER_EXTENT {
+            // Probe at p/(P+1) of the extent — spreads P points across it while
+            // skipping the clear nav at the very head.
+            let unit = ((total_units as u64 * p as u64) / (PROBES_PER_EXTENT as u64 + 1)) as u32;
             if unit >= total_units {
-                break;
+                continue;
             }
             let units_this = CHUNK_UNITS.min(total_units - unit);
             // Saturate: start_lba comes from attacker-controlled UDF/MPLS
             // extents; a malformed extent near u32::MAX would otherwise panic
             // (debug) or wrap to a wrong LBA (release). An over-capacity LBA then
-            // fails cleanly via the read_sectors().is_err() break below.
+            // fails cleanly via the read_sectors().is_err() skip below.
             let lba = ext
                 .start_lba
                 .saturating_add(unit.saturating_mul(ALIGNED_UNIT_SECTORS));
             let count = (units_this * ALIGNED_UNIT_SECTORS) as u16;
             let mut buf = vec![0u8; count as usize * 2048];
             // `false` = no recovery retries; the reader is the raw drive/file
-            // (no decrypt decorator), so these are the on-disc encrypted bytes.
+            // (no decrypt decorator), so these are the on-disc encrypted bytes. A
+            // read error at one probe skips THAT probe only — it must not abandon
+            // the rest of the extent (the old `break` blinded the sampler on a
+            // single transient miss).
             if reader.read_sectors(lba, count, &mut buf, false).is_err() {
-                break;
+                continue;
             }
             for i in 0..units_this as usize {
                 let o = i * ALIGNED_UNIT_LEN;
@@ -394,7 +405,6 @@ pub fn read_encrypted_units(
                     }
                 }
             }
-            unit += units_this;
         }
     }
     out
@@ -625,4 +635,85 @@ mod tests {
         );
         assert_eq!(*builds.lock().unwrap(), 1, "make_sources invoked per fetch");
     }
+
+    /// #4 regression: encrypted content NOT at the extent midpoint (a late-
+    /// starting feature, or a midpoint landing in clear nav) must still be
+    /// sampled — empty samples make `decrypt_with` skip wrong-key validation.
+    /// The old midpoint-and-forward sampler returned empty; the probe-spread
+    /// finds the early scrambled band.
+    #[test]
+    fn read_encrypted_units_finds_scrambled_content_off_the_midpoint() {
+        use crate::aacs::{ALIGNED_UNIT_LEN, ALIGNED_UNIT_SECTORS, ts_sync_destroyed};
+        use crate::error::Result;
+        use crate::sector::SectorSource;
+
+        // Units in the FIRST SIXTH of the extent are scrambled (0xFF → no TS
+        // sync); everything else (incl. the midpoint) is clear (0x47 syncs).
+        struct BandSource {
+            ext_start: u32,
+            total_units: u32,
+        }
+        impl SectorSource for BandSource {
+            fn capacity_sectors(&self) -> u32 {
+                self.ext_start + self.total_units * ALIGNED_UNIT_SECTORS + 64
+            }
+            fn read_sectors(
+                &mut self,
+                lba: u32,
+                count: u16,
+                buf: &mut [u8],
+                _r: bool,
+            ) -> Result<usize> {
+                let bytes = count as usize * 2048;
+                for (i, chunk) in buf[..bytes].chunks_mut(ALIGNED_UNIT_LEN).enumerate() {
+                    if chunk.len() < ALIGNED_UNIT_LEN {
+                        break;
+                    }
+                    let abs_unit = (lba - self.ext_start) / ALIGNED_UNIT_SECTORS + i as u32;
+                    if abs_unit < self.total_units / 6 {
+                        chunk.fill(0xFF); // scrambled: no TS sync
+                    } else {
+                        chunk.fill(0);
+                        let mut o = 4;
+                        while o < ALIGNED_UNIT_LEN {
+                            chunk[o] = 0x47; // clear TS syncs
+                            o += 192;
+                        }
+                    }
+                }
+                Ok(bytes)
+            }
+        }
+
+        let total_units = 600u32;
+        let ext_start = 1000u32;
+        let mut src = BandSource {
+            ext_start,
+            total_units,
+        };
+        let title = crate::disc::DiscTitle {
+            playlist: String::new(),
+            playlist_id: 0,
+            duration_secs: 0.0,
+            size_bytes: 0,
+            clips: Vec::new(),
+            streams: Vec::new(),
+            chapters: Vec::new(),
+            extents: vec![crate::disc::Extent {
+                start_lba: ext_start,
+                sector_count: total_units * ALIGNED_UNIT_SECTORS,
+            }],
+            content_format: crate::disc::ContentFormat::BdTs,
+            codec_privates: Vec::new(),
+        };
+
+        let samples = read_encrypted_units(&mut src, &title, 4);
+        assert!(
+            !samples.is_empty(),
+            "the probe-spread must sample the early scrambled band the midpoint misses"
+        );
+        for s in &samples {
+            assert!(ts_sync_destroyed(s), "every sample is a scrambled unit");
+        }
+    }
 }

src/lib.rs

@@ -111,6 +111,7 @@ pub mod dvdnav;
 pub mod error;
 pub mod event;
 pub mod halt;
+pub mod hex;
 pub(crate) mod identity;
 pub(crate) mod ifo;
 pub mod io;