diff --git a/src/aacs/mod.rs b/src/aacs/mod.rs
index 2240534..516c03b 100644
--- a/src/aacs/mod.rs
+++ b/src/aacs/mod.rs
@@ -61,9 +61,25 @@ impl Unlocker for AacsUnlocker {
             return Ok(None);
         }
         crate::fallthrough(
-            handshake::run_cert_handshake(scsi, &self.host_certs).map(|h| Unlocked {
-                vid: Some(h.volume_id),
-                bus_key: h.read_data_key,
+            handshake::run_cert_handshake(scsi, &self.host_certs).map(|h| {
+                // A UHD disc whose bus-key fetch the drive refused (non-transport)
+                // returns Ok with read_data_key: None + read_data_key_err: Some —
+                // otherwise indistinguishable from an AACS-1.0 disc. Surface it.
+                if h.read_data_key.is_none()
+                    && let Some(code) = h.read_data_key_err
+                {
+                    tracing::warn!(
+                        target: "freemkv::disc",
+                        phase = "read_data_key_dropped",
+                        error_code = code,
+                        "AACS auth + VID succeeded but the drive served no read_data_key (bus key); \
+                         unlock reports bus_key: None"
+                    );
+                }
+                Unlocked {
+                    vid: Some(h.volume_id),
+                    bus_key: h.read_data_key,
+                }
             }),
         )
     }
diff --git a/src/ld/platform/mt1959/mod.rs b/src/ld/platform/mt1959/mod.rs
index a867b88..05d5588 100644
--- a/src/ld/platform/mt1959/mod.rs
+++ b/src/ld/platform/mt1959/mod.rs
@@ -586,6 +586,40 @@ mod tests {
         assert_eq!(len, 2208, "MODE SELECT length = firmware.len(), not 2496");
     }
 
+    // Byte-for-byte pin of variant_b's Step-2 metadata-read (READ_BUFFER offset
+    // 0x3000 at cdb[4]=0x30, length 0x10 at cdb[8]) and Step-3 write-extra CDBs.
+    #[test]
+    fn variant_b_meta_read_and_write_extra_cdbs_are_exact_bytes() {
+        let mut profile = fixture_profile([0x11, 0x22, 0x33, 0x44]);
+        profile.firmware = vec![0u8; 2208];
+        let mut mt = Mt1959::new(profile, true);
+        let mut t = RecordingTransport { cdbs: Vec::new() };
+        let _ = variant_b::load_firmware(&mut mt, &mut t);
+
+        // Step 2: READ_BUFFER mode 6, offset 0x3000, length 0x10.
+        let meta = t
+            .cdbs
+            .iter()
+            .find(|c| c.first() == Some(&SCSI_READ_BUFFER) && c.get(4) == Some(&0x30))
+            .expect("Step-2 metadata READ_BUFFER issued");
+        assert_eq!(
+            meta.as_slice(),
+            &[0x3C, 0x06, 0x00, 0x00, 0x30, 0x00, 0x00, 0x00, 0x10, 0x00]
+        );
+
+        // Step 3: WRITE_BUFFER of the 0x10-byte extra block. Distinguish from
+        // the Step-1 firmware upload (WRITE_BUFFER too) by its length byte.
+        let extra = t
+            .cdbs
+            .iter()
+            .find(|c| c.first() == Some(&SCSI_WRITE_BUFFER) && c.get(8) == Some(&0x10))
+            .expect("Step-3 write-extra WRITE_BUFFER issued");
+        assert_eq!(
+            extra.as_slice(),
+            &[0x3B, 0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x10, 0x00]
+        );
+    }
+
     // Pins variant_a's upload sequence: WRITE_BUFFER at the real per-drive
     // length in the CDB's 24-bit length field, then the 0x45 verify
     // READ_BUFFER whose result used to be discarded outright.
@@ -614,6 +648,40 @@ mod tests {
         );
     }
 
+    // Byte-for-byte pin of variant_a's two firmware-upload CDBs (not just the
+    // length field): the WRITE_BUFFER (mode cdb[1]=0x06, reserved, 24-bit len,
+    // control byte 9) and the whole 0x45 verify READ_BUFFER CDB.
+    #[test]
+    fn variant_a_write_buffer_and_verify_cdbs_are_exact_bytes() {
+        let mut profile = fixture_profile([0x11, 0x22, 0x33, 0x44]);
+        profile.firmware = vec![0u8; 2208]; // 0x0008A0
+        let mut mt = Mt1959::new(profile, false);
+        let mut t = RecordingTransport { cdbs: Vec::new() };
+        let _ = variant_a::load_firmware(&mut mt, &mut t);
+
+        let wb = t
+            .cdbs
+            .iter()
+            .find(|c| c.first() == Some(&SCSI_WRITE_BUFFER))
+            .expect("WRITE_BUFFER issued");
+        // 3B [mode 06] [rsvd 00 00 00 00] [len 24-bit BE = 0008A0] [control 00]
+        assert_eq!(
+            wb.as_slice(),
+            &[0x3B, 0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0x08, 0xA0, 0x00]
+        );
+
+        let verify = t
+            .cdbs
+            .iter()
+            .find(|c| c.first() == Some(&SCSI_READ_BUFFER) && c.get(2) == Some(&0x45))
+            .expect("0x45 verify READ_BUFFER issued");
+        // 3C [MODE_A 01] [buffer_id 45] [rsvd ×5] [alloc len 04] [control 00]
+        assert_eq!(
+            verify.as_slice(),
+            &[0x3C, 0x01, 0x45, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x00]
+        );
+    }
+
     /// An EMPTY firmware blob cannot be uploaded — catches a profile whose
     /// firmware failed to decode being pushed at the drive as a zero-length
     /// WRITE_BUFFER.
diff --git a/src/lib.rs b/src/lib.rs
index dbfc31e..16b84c1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -61,7 +61,7 @@ pub enum DiscKind {
 
 /// A host certificate for the AACS cert handshake (raw; the consumer collects
 /// these from its key sources and passes them in).
-#[derive(Debug, Clone)]
+#[derive(Clone)]
 pub struct HostCert {
     /// AACS 1.0 host private key (20 bytes).
     pub private_key: [u8; 20],
@@ -73,6 +73,20 @@ pub struct HostCert {
     pub certificate_v2: Option<Vec<u8>>,
 }
 
+// Manual Debug: `private_key`/`private_key_v2` are the raw host private keys —
+// they must never reach a log in plaintext; presence (Some/None) of the v2 key
+// stays observable. Mirrors the redacting Debug on AacsAuth/Unlocked.
+impl std::fmt::Debug for HostCert {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("HostCert")
+            .field("private_key", &"[redacted]")
+            .field("certificate", &self.certificate)
+            .field("private_key_v2", &self.private_key_v2.map(|_| "[redacted]"))
+            .field("certificate_v2", &self.certificate_v2)
+            .finish()
+    }
+}
+
 /// Per-attempt context the consumer hands to EVERY unlocker, uniformly: the
 /// drive identity and the mounted disc's kind. These are the shared facts the
 /// dispatch loop knows; anything an individual unlocker needs beyond them (the
@@ -250,4 +264,29 @@ mod tests {
         // Presence (Some/None) stays observable.
         assert!(s.contains("Some"), "must still show a key WAS present: {s}");
     }
+
+    // `private_key`/`private_key_v2` are the raw host private keys; `Debug` must
+    // NOT print those bytes. Restoring `#[derive(Debug)]` renders the byte
+    // arrays (e.g. 0xAB → 171 / 0xCD → 205), turning this test red.
+    #[test]
+    fn host_cert_debug_redacts_private_keys() {
+        let c = HostCert {
+            private_key: [0xAB; 20],
+            certificate: vec![0x01, 0x02],
+            private_key_v2: Some([0xCD; 32]),
+            certificate_v2: None,
+        };
+        let s = format!("{c:?}");
+        assert!(!s.contains("171"), "private_key bytes must not appear: {s}");
+        assert!(
+            !s.contains("205"),
+            "private_key_v2 bytes must not appear: {s}"
+        );
+        assert!(s.contains("[redacted]"), "must mark redaction: {s}");
+        // Presence of the v2 key stays observable.
+        assert!(
+            s.contains("Some"),
+            "must still show the v2 key WAS present: {s}"
+        );
+    }
 }
diff --git a/src/renesas/mod.rs b/src/renesas/mod.rs
index fa0378c..463e8f7 100644
--- a/src/renesas/mod.rs
+++ b/src/renesas/mod.rs
@@ -306,6 +306,94 @@ mod tests {
         );
     }
 
+    // A recognized Renesas drive whose A read (RB 0xB0@0x04) is REFUSED but
+    // whose B read (RB 0xB0@0x50) SUCCEEDS after the knock must unlock — and the
+    // knock CDB has to be issued BETWEEN A and B. See docs/renesas-mod.md.
+    #[test]
+    fn knock_runs_between_a_and_b_and_b_opens() {
+        // The exact knock CDB the code issues (pinned wire format).
+        const KNOCK_A5AAAA_CDB: [u8; 10] =
+            [0x3B, 0x02, 0x41, 0xA5, 0xAA, 0xAA, 0x00, 0x00, 0x00, 0x00];
+
+        struct AFailsBSucceeds {
+            cdbs: Vec<Vec<u8>>,
+        }
+        impl ScsiTransport for AFailsBSucceeds {
+            fn execute(
+                &mut self,
+                cdb: &[u8],
+                _dir: DataDirection,
+                data: &mut [u8],
+                _timeout_ms: u32,
+            ) -> Result<ScsiResult> {
+                self.cdbs.push(cdb.to_vec());
+                // Gate 0xF1 → SAT identity.
+                if cdb.get(2) == Some(&0xF1) {
+                    let p = renesas_payload();
+                    let n = p.len().min(data.len());
+                    data[..n].copy_from_slice(&p[..n]);
+                    return Ok(ScsiResult {
+                        status: 0,
+                        bytes_transferred: n,
+                        sense: [0u8; 32],
+                    });
+                }
+                // A read (RB 0xB0@0x04) → refuse with a drive sense.
+                if cdb.get(2) == Some(&0xB0) && cdb.get(3) == Some(&0x00) {
+                    let mut sense = [0u8; 32];
+                    sense[2] = 0x05; // ILLEGAL REQUEST
+                    sense[12] = 0x20;
+                    return Err(ScsiError {
+                        status: crate::scsi::SCSI_STATUS_CHECK_CONDITION,
+                        sense: Some(sense),
+                    });
+                }
+                // B read (RB 0xB0@0x50), the knock (0x3B/0x41), and the 0xAD VID
+                // read all return GOOD so the open + VID read succeed.
+                let n = data.len().min(36);
+                if !data.is_empty() {
+                    // Non-zero VID so read_aacs_vid yields Some.
+                    for b in data.iter_mut().take(n) {
+                        *b = 0x5A;
+                    }
+                }
+                Ok(ScsiResult {
+                    status: 0,
+                    bytes_transferred: data.len(),
+                    sense: [0u8; 32],
+                })
+            }
+        }
+
+        let mut t = AFailsBSucceeds { cdbs: Vec::new() };
+        let id = crate::DriveId::default();
+        let ctx = UnlockCtx::new(&id, DiscKind::Unknown);
+        let out = Renesas::new()
+            .unlock(&mut t, &ctx)
+            .expect("no fault")
+            .expect("B read opens the drive → unlocked");
+        assert_eq!(out.bus_key, None);
+
+        // The knock must appear, between the A read (0xB0@0x04) and the B read
+        // (0xB0@0x50).
+        let a = t
+            .cdbs
+            .iter()
+            .position(|c| c.get(2) == Some(&0xB0) && c.get(3) == Some(&0x00))
+            .expect("A read issued");
+        let knock = t
+            .cdbs
+            .iter()
+            .position(|c| c.as_slice() == KNOCK_A5AAAA_CDB)
+            .expect("knock CDB issued");
+        let b = t
+            .cdbs
+            .iter()
+            .position(|c| c.get(2) == Some(&0xB0) && c.get(3) == Some(&0x50))
+            .expect("B read issued");
+        assert!(a < knock && knock < b, "knock must run between A and B");
+    }
+
     // A recognized Renesas drive that REFUSES the vendor open read must
     // defer to the next unlocker, not claim the drive. See docs/renesas-mod.md.
     #[test]
diff --git a/src/vid.rs b/src/vid.rs
index 7855781..cceef55 100644
--- a/src/vid.rs
+++ b/src/vid.rs
@@ -172,6 +172,18 @@ mod tests {
         assert_eq!(read_aacs_vid(&mut t).expect("no fault"), None);
     }
 
+    /// A rejection surfaced as `Err` WITH a sense (a non-conforming transport)
+    /// is a drive refusal, not a dead bus → `Ok(None)` VID-miss, never
+    /// `Err(Transport)`.
+    #[test]
+    fn err_with_sense_is_a_vid_miss_not_transport() {
+        let mut t = MockTransport::always(Reply::illegal_request_as_err());
+        assert_eq!(
+            read_aacs_vid(&mut t).expect("a sense is not a dead bus"),
+            None
+        );
+    }
+
     /// Only a dead bus is an error.
     #[test]
     fn transport_fault_propagates() {