diff --git a/crates/kms/src/backends/contract_tests.rs b/crates/kms/src/backends/contract_tests.rs
index 7e118121e..ed1342f1f 100644
--- a/crates/kms/src/backends/contract_tests.rs
+++ b/crates/kms/src/backends/contract_tests.rs
@@ -183,6 +183,7 @@ async fn assert_state_machine_contract(backend: &dyn KmsBackend, key_id: &str) {
         .await
         .expect("decrypt with a disabled key must keep working");
     assert_eq!(decrypted.plaintext, data_key.plaintext_key, "decrypt must recover the original data key");
+    assert_eq!(decrypted.key_id, key_id, "decrypt must report the master key that opened the envelope");
     // ...disable stays idempotent, cancel has nothing to cancel, and enable recovers.
     backend.disable_key(key_id).await.expect("disable must be idempotent");
     expect_invalid_key_state(backend.cancel_key_deletion(cancel_request(key_id)).await, "not pending deletion");
diff --git a/crates/kms/src/backends/local.rs b/crates/kms/src/backends/local.rs
index 239966a34..836c6d979 100644
--- a/crates/kms/src/backends/local.rs
+++ b/crates/kms/src/backends/local.rs
@@ -1529,14 +1529,7 @@ impl LocalKmsClient {
         ensure_key_status_permits(&request.master_key_id, &key_info.status, StateGatedOperation::GenerateDataKey)?;
 
         // Generate random data key material
-        let key_length = match request.key_spec.as_str() {
-            "AES_256" => 32,
-            "AES_128" => 16,
-            _ => return Err(KmsError::unsupported_algorithm(&request.key_spec)),
-        };
-
-        let mut plaintext_key = vec![0u8; key_length];
-        rand::rng().fill(&mut plaintext_key[..]);
+        let plaintext_key = generate_key_material(&request.key_spec)?;
 
         // Encrypt the data key with the master key
         let (encrypted_key, nonce) = self.encrypt_with_master_key(&request.master_key_id, &plaintext_key).await?;
@@ -1596,11 +1589,19 @@ impl LocalKmsClient {
         })
     }
 
-    pub(crate) async fn decrypt(&self, request: &DecryptRequest, _context: Option<&OperationContext>) -> Result<Vec<u8>> {
+    /// Open a data-key envelope, returning the plaintext and the master key
+    /// that wrapped it.
+    pub(crate) async fn decrypt(
+        &self,
+        request: &DecryptRequest,
+        _context: Option<&OperationContext>,
+    ) -> Result<(Vec<u8>, String)> {
         debug!("Decrypting data");
 
-        // Parse the data key envelope from ciphertext
-        let envelope: DataKeyEnvelope = serde_json::from_slice(&request.ciphertext)?;
+        // Parse the data key envelope from ciphertext. Mapped to the same
+        // error class the other backends report for unparseable ciphertext.
+        let envelope: DataKeyEnvelope = serde_json::from_slice(&request.ciphertext)
+            .map_err(|error| KmsError::cryptographic_error("parse", format!("Failed to parse data key envelope: {error}")))?;
 
         // NOTE: this comparison is an authorization check, not a cryptographic
         // binding. `DekCrypto` seals only the plaintext, so `encryption_context`
@@ -1634,7 +1635,7 @@ impl LocalKmsClient {
             .await?;
 
         debug!("Local KMS data decrypted");
-        Ok(plaintext)
+        Ok((plaintext, envelope.master_key_id))
     }
 
     /// Test-only lifecycle driver: the product path goes through [`KmsBackend`].
@@ -1994,16 +1995,11 @@ impl KmsBackend for LocalKmsBackend {
     }
 
     async fn decrypt(&self, request: DecryptRequest) -> Result<DecryptResponse> {
-        let plaintext = self.client.decrypt(&request, None).await?;
-
-        // The envelope that was just opened names the master key that opened it.
-        // Reporting "unknown" left every caller unable to tell which key was
-        // actually used, which is what audit and key-rotation checks read.
-        let envelope: DataKeyEnvelope = serde_json::from_slice(&request.ciphertext)?;
+        let (plaintext, key_id) = self.client.decrypt(&request, None).await?;
 
         Ok(DecryptResponse {
             plaintext,
-            key_id: envelope.master_key_id,
+            key_id,
             encryption_algorithm: Some("AES-256-GCM".to_string()),
         })
     }
@@ -2017,12 +2013,19 @@ impl KmsBackend for LocalKmsBackend {
             grant_tokens: Vec::new(),
         };
 
-        let data_key = self.client.generate_data_key(&generate_request, None).await?;
+        let mut data_key = self.client.generate_data_key(&generate_request, None).await?;
 
+        // Fields are taken, not destructured or cloned: `DataKeyInfo` has a
+        // `Drop` impl, and a clone would leave a second un-zeroized plaintext
+        // DEK on the heap.
+        let plaintext_key = data_key
+            .plaintext
+            .take()
+            .ok_or_else(|| KmsError::internal_error("Generated data key is missing plaintext"))?;
         Ok(GenerateDataKeyResponse {
             key_id: request.key_id,
-            plaintext_key: data_key.plaintext.clone().unwrap_or_default(),
-            ciphertext_blob: data_key.ciphertext.clone(),
+            plaintext_key,
+            ciphertext_blob: std::mem::take(&mut data_key.ciphertext),
         })
     }
 
@@ -2423,8 +2426,9 @@ mod tests {
         let decrypt_request =
             DecryptRequest::new(data_key.ciphertext.clone()).with_context("bucket".to_string(), "test-bucket".to_string());
 
-        let decrypted = client.decrypt(&decrypt_request, None).await.expect("Failed to decrypt");
+        let (decrypted, opened_by) = client.decrypt(&decrypt_request, None).await.expect("Failed to decrypt");
         assert_eq!(decrypted, data_key.plaintext.clone().expect("No plaintext"));
+        assert_eq!(opened_by, key_id, "decrypt must report the master key that opened the envelope");
     }
 
     #[tokio::test]
@@ -2455,7 +2459,7 @@ mod tests {
         // Pre-fix, each of those regenerated the master key, so this unwrap fails with an AEAD
         // error. Post-fix, the original material is preserved and the DEK still decrypts.
         let decrypt_request = DecryptRequest::new(ciphertext).with_context("bucket".to_string(), "b".to_string());
-        let decrypted = client
+        let (decrypted, _opened_by) = client
             .decrypt(&decrypt_request, None)
             .await
             .expect("DEK must still decrypt after status transitions");
@@ -2796,7 +2800,7 @@ mod tests {
         assert!(matches!(error, KmsError::InvalidOperation { .. }));
 
         for (index, (ciphertext, plaintext)) in batch.iter().enumerate() {
-            let decrypted = client
+            let (decrypted, _opened_by) = client
                 .decrypt(&DecryptRequest::new(ciphertext.clone()), None)
                 .await
                 .unwrap_or_else(|error| panic!("batch member {index} must decrypt: {error}"));
diff --git a/crates/kms/src/backends/static_kms.rs b/crates/kms/src/backends/static_kms.rs
index 02d29bc36..3c82fb345 100644
--- a/crates/kms/src/backends/static_kms.rs
+++ b/crates/kms/src/backends/static_kms.rs
@@ -26,7 +26,7 @@
 
 use crate::backends::{BackendCapabilities, KmsBackend, empty_key_page, list_keys_page_size};
 use crate::config::{BackendConfig, KmsConfig};
-use crate::encryption::{DataKeyEnvelope, context_aad};
+use crate::encryption::{DataKeyEnvelope, context_aad, generate_key_material};
 use crate::error::{KmsError, Result};
 use crate::types::*;
 use aes_gcm::{
@@ -124,16 +124,7 @@ impl StaticKmsBackend {
         // The requested spec decides the DEK length; a caller that asked for
         // AES_128 and silently got 256 bits would build objects whose recorded
         // spec does not match their key material.
-        // Lengths track `KeySpec::key_size`; the request carries the spec as a
-        // string, so the mapping is repeated here rather than shared.
-        let key_length = match request.key_spec.as_str() {
-            "AES_256" | "ChaCha20" => 32,
-            "AES_128" => 16,
-            _ => return Err(KmsError::unsupported_algorithm(&request.key_spec)),
-        };
-
-        let mut plaintext = vec![0u8; key_length];
-        rand::rng().fill(&mut plaintext[..]);
+        let plaintext = generate_key_material(&request.key_spec)?;
 
         // Encrypt DEK with AES-256-GCM using the static key directly
         let key = Key::<Aes256Gcm>::from(*self.key);
@@ -167,7 +158,7 @@ impl StaticKmsBackend {
         Ok(DataKeyInfo::new(
             self.key_id.clone(),
             0,
-            Some(plaintext.to_vec()),
+            Some(plaintext),
             ciphertext,
             request.key_spec.clone(),
         ))
@@ -360,17 +351,20 @@ impl KmsBackend for StaticKmsBackend {
             encryption_context: request.encryption_context,
             grant_tokens: Vec::new(),
         };
-        let data_key = self.generate_data_key_envelope(&gen_req)?;
+        let mut data_key = self.generate_data_key_envelope(&gen_req)?;
 
+        // Fields are taken, not destructured or cloned: `DataKeyInfo` has a
+        // `Drop` impl, and a clone would leave a second un-zeroized plaintext
+        // DEK on the heap.
         let plaintext_key = data_key
             .plaintext
-            .clone()
+            .take()
             .ok_or_else(|| KmsError::internal_error("Generated data key is missing plaintext"))?;
 
         Ok(GenerateDataKeyResponse {
-            key_id: data_key.key_id.clone(),
+            key_id: std::mem::take(&mut data_key.key_id),
             plaintext_key,
-            ciphertext_blob: data_key.ciphertext.clone(),
+            ciphertext_blob: std::mem::take(&mut data_key.ciphertext),
         })
     }
 
diff --git a/crates/kms/src/backends/vault.rs b/crates/kms/src/backends/vault.rs
index 2cca931b8..188f2fac8 100644
--- a/crates/kms/src/backends/vault.rs
+++ b/crates/kms/src/backends/vault.rs
@@ -875,7 +875,13 @@ impl VaultKmsClient {
         })
     }
 
-    pub(crate) async fn decrypt(&self, request: &DecryptRequest, _context: Option<&OperationContext>) -> Result<Vec<u8>> {
+    /// Open a data-key envelope, returning the plaintext and the master key
+    /// that wrapped it.
+    pub(crate) async fn decrypt(
+        &self,
+        request: &DecryptRequest,
+        _context: Option<&OperationContext>,
+    ) -> Result<(Vec<u8>, String)> {
         debug!("Decrypting data");
 
         // Parse the data key envelope from ciphertext
@@ -929,7 +935,7 @@ impl VaultKmsClient {
         };
 
         debug!("Vault KMS data decrypted");
-        Ok(plaintext)
+        Ok((plaintext, envelope.master_key_id))
     }
 
     /// Report which master key version wraps an envelope, and whether that is
@@ -1626,16 +1632,11 @@ impl KmsBackend for VaultKmsBackend {
     }
 
     async fn decrypt(&self, request: DecryptRequest) -> Result<DecryptResponse> {
-        let plaintext = self.client.decrypt(&request, None).await?;
-
-        // The envelope that was just opened names the master key that opened it.
-        // Reporting "unknown" left every caller unable to tell which key was
-        // actually used, which is what audit and key-rotation checks read.
-        let envelope: DataKeyEnvelope = serde_json::from_slice(&request.ciphertext)?;
+        let (plaintext, key_id) = self.client.decrypt(&request, None).await?;
 
         Ok(DecryptResponse {
             plaintext,
-            key_id: envelope.master_key_id,
+            key_id,
             encryption_algorithm: Some("AES-256-GCM".to_string()),
         })
     }
@@ -1660,12 +1661,19 @@ impl KmsBackend for VaultKmsBackend {
             grant_tokens: Vec::new(),
         };
 
-        let data_key = self.client.generate_data_key(&generate_request, None).await?;
+        let mut data_key = self.client.generate_data_key(&generate_request, None).await?;
 
+        // Fields are taken, not destructured or cloned: `DataKeyInfo` has a
+        // `Drop` impl, and a clone would leave a second un-zeroized plaintext
+        // DEK on the heap.
+        let plaintext_key = data_key
+            .plaintext
+            .take()
+            .ok_or_else(|| KmsError::internal_error("Generated data key is missing plaintext"))?;
         Ok(GenerateDataKeyResponse {
             key_id: request.key_id,
-            plaintext_key: data_key.plaintext.clone().unwrap_or_default(),
-            ciphertext_blob: data_key.ciphertext.clone(),
+            plaintext_key,
+            ciphertext_blob: std::mem::take(&mut data_key.ciphertext),
         })
     }
 
@@ -2525,7 +2533,7 @@ mod tests {
 
         // A mixed batch of envelopes from every historical version must decrypt.
         for (data_key, label) in [(&dk_v1, "v1"), (&dk_v3, "v3"), (&dk_v2, "v2"), (&dk_v1, "v1 again")] {
-            let plaintext = client
+            let (plaintext, _opened_by) = client
                 .decrypt(&integration_decrypt_request(data_key.ciphertext.clone()), None)
                 .await
                 .unwrap_or_else(|error| panic!("envelope wrapped under {label} must stay decryptable: {error}"));
@@ -2561,7 +2569,7 @@ mod tests {
 
         // The baseline rule must route the legacy envelope to the frozen version 1
         // material even though the current version has moved on.
-        let plaintext = client
+        let (plaintext, _opened_by) = client
             .decrypt(&integration_decrypt_request(legacy_ciphertext), None)
             .await
             .expect("legacy envelope must stay decryptable after rotation");
@@ -2605,7 +2613,7 @@ mod tests {
         );
 
         // The untampered envelope still decrypts through its recorded version.
-        let plaintext = client
+        let (plaintext, _opened_by) = client
             .decrypt(&integration_decrypt_request(data_key.ciphertext.clone()), None)
             .await
             .expect("untampered envelope must still decrypt");
@@ -2870,7 +2878,7 @@ mod tests {
         assert_eq!(envelope.master_key_id, "wired-key");
         assert_eq!(envelope.master_key_version, Some(1));
 
-        let decrypted = client
+        let (decrypted, opened_by) = client
             .decrypt(
                 &DecryptRequest {
                     ciphertext: encrypted.ciphertext.clone(),
@@ -2882,6 +2890,7 @@ mod tests {
             .await
             .expect("decrypt must round-trip the envelope");
         assert_eq!(decrypted, b"kv2-direct-encrypt".to_vec());
+        assert_eq!(opened_by, "wired-key", "decrypt must report the master key that opened the envelope");
 
         // A different object context must not decrypt (checked before any
         // Vault read, so no scripted response is consumed).
@@ -4181,7 +4190,7 @@ mod tests {
 
         let (vault, client) = scripted_client(vec![ScriptedResponse::ok(kv2_read_data(&key_data))]).await;
 
-        let plaintext = client
+        let (plaintext, _opened_by) = client
             .decrypt(
                 &DecryptRequest {
                     ciphertext,
@@ -4315,7 +4324,7 @@ mod tests {
         }
         let (vault, client) = scripted_client(responses).await;
 
-        let plaintext = client
+        let (plaintext, _opened_by) = client
             .decrypt(
                 &DecryptRequest {
                     ciphertext: ciphertext.to_vec(),
diff --git a/crates/kms/src/backends/vault_transit.rs b/crates/kms/src/backends/vault_transit.rs
index 5894bd561..3e29541a1 100644
--- a/crates/kms/src/backends/vault_transit.rs
+++ b/crates/kms/src/backends/vault_transit.rs
@@ -817,7 +817,13 @@ impl VaultTransitKmsClient {
         })
     }
 
-    pub(crate) async fn decrypt(&self, request: &DecryptRequest, _context: Option<&OperationContext>) -> Result<Vec<u8>> {
+    /// Open a data-key envelope, returning the plaintext and the master key
+    /// that wrapped it.
+    pub(crate) async fn decrypt(
+        &self,
+        request: &DecryptRequest,
+        _context: Option<&OperationContext>,
+    ) -> Result<(Vec<u8>, String)> {
         let envelope: DataKeyEnvelope = serde_json::from_slice(&request.ciphertext)
             .map_err(|e| KmsError::cryptographic_error("parse", format!("Failed to parse data key envelope: {e}")))?;
 
@@ -839,7 +845,7 @@ impl VaultTransitKmsClient {
             .transit_decrypt(&envelope.master_key_id, encrypted_key, &envelope.encryption_context)
             .await
         {
-            Ok(plaintext) => Ok(plaintext),
+            Ok(plaintext) => Ok((plaintext, envelope.master_key_id)),
             Err(error) => {
                 self.invalidate_metadata_on_state_error(&envelope.master_key_id, &error).await;
                 Err(error)
@@ -1384,11 +1390,10 @@ impl KmsBackend for VaultTransitKmsBackend {
     }
 
     async fn decrypt(&self, request: DecryptRequest) -> Result<DecryptResponse> {
-        let envelope: DataKeyEnvelope = serde_json::from_slice(&request.ciphertext)?;
-        let plaintext = self.client.decrypt(&request, None).await?;
+        let (plaintext, key_id) = self.client.decrypt(&request, None).await?;
         Ok(DecryptResponse {
             plaintext,
-            key_id: envelope.master_key_id,
+            key_id,
             encryption_algorithm: Some("vault-transit".to_string()),
         })
     }
@@ -1413,13 +1418,19 @@ impl KmsBackend for VaultTransitKmsBackend {
             grant_tokens: Vec::new(),
         };
 
-        let data_key = self.client.generate_data_key(&generate_request, None).await?;
-        let plaintext_key = data_key.plaintext.clone().unwrap_or_default();
-        let ciphertext_blob = data_key.ciphertext.clone();
+        let mut data_key = self.client.generate_data_key(&generate_request, None).await?;
+
+        // Fields are taken, not destructured or cloned: `DataKeyInfo` has a
+        // `Drop` impl, and a clone would leave a second un-zeroized plaintext
+        // DEK on the heap.
+        let plaintext_key = data_key
+            .plaintext
+            .take()
+            .ok_or_else(|| KmsError::internal_error("Generated data key is missing plaintext"))?;
         Ok(GenerateDataKeyResponse {
             key_id: request.key_id,
             plaintext_key,
-            ciphertext_blob,
+            ciphertext_blob: std::mem::take(&mut data_key.ciphertext),
         })
     }
 
@@ -2089,7 +2100,7 @@ mod tests {
         // Historical ciphertext keeps decrypting per Vault's version semantics,
         // interleaved with post-rotation ciphertext.
         for (data_key, label) in [(&dk_v1, "v1"), (&dk_v2, "v2"), (&dk_v1, "v1 again")] {
-            let plaintext = client
+            let (plaintext, _opened_by) = client
                 .decrypt(
                     &DecryptRequest {
                         ciphertext: data_key.ciphertext.clone(),
@@ -2659,7 +2670,7 @@ mod tests {
         let rotated = client.rotate_key("wired-key", None).await.expect("rotation must commit");
         assert_eq!(rotated.version, 2, "the rotation must record the version bump");
 
-        let plaintext = client
+        let (plaintext, opened_by) = client
             .decrypt(
                 &DecryptRequest {
                     ciphertext: data_key.ciphertext.clone(),
@@ -2675,6 +2686,7 @@ mod tests {
             RECOVERED_DEK.to_vec(),
             "the decrypt must hand back the recovered material, not merely avoid an error"
         );
+        assert_eq!(opened_by, "wired-key", "decrypt must report the master key that opened the envelope");
 
         let requests = vault.requests();
         assert_eq!(requests.len(), 7, "{requests:?}");
diff --git a/crates/kms/src/encryption/dek.rs b/crates/kms/src/encryption/dek.rs
index ca3f2658d..f8f2ae275 100644
--- a/crates/kms/src/encryption/dek.rs
+++ b/crates/kms/src/encryption/dek.rs
@@ -389,16 +389,18 @@ impl Default for AesDekCrypto {
     }
 }
 
-/// Generate random key material for the given algorithm
+/// Generate random key material for the given algorithm.
+///
+/// The lengths must track [`crate::types::KeySpec::key_size`].
 ///
 /// # Arguments
-/// * `algorithm` - The key algorithm (e.g., "AES_256", "AES_128")
+/// * `algorithm` - The key algorithm (e.g., "AES_256", "AES_128", "ChaCha20")
 ///
 /// # Returns
 /// A vector containing the generated key material
 pub fn generate_key_material(algorithm: &str) -> Result<Vec<u8>> {
     let key_size = match algorithm {
-        "AES_256" => 32,
+        "AES_256" | "ChaCha20" => 32,
         "AES_128" => 16,
         _ => return Err(KmsError::unsupported_algorithm(algorithm)),
     };
diff --git a/crates/kms/tests/behavior_crypto.rs b/crates/kms/tests/behavior_crypto.rs
index 30dc6670c..a2606522d 100644
--- a/crates/kms/tests/behavior_crypto.rs
+++ b/crates/kms/tests/behavior_crypto.rs
@@ -33,7 +33,7 @@
 
 mod common;
 
-use common::{BackendCase, BackendKind, TestKms, assert_context_mismatch, ctx, flip_middle_bit, for_each_backend, payload};
+use common::{BackendCase, TestKms, assert_context_mismatch, ctx, flip_middle_bit, for_each_backend, payload};
 use rustfs_kms::{
     DecryptRequest, EncryptRequest, GenerateDataKeyRequest, KeySpec, KmsError, ObjectEncryptionContext, is_data_key_envelope,
 };
@@ -259,55 +259,51 @@ async fn data_key_spec_controls_the_length_of_the_generated_key() {
         // A backend that accepts a `key_spec` must honour it. Silently
         // returning a different size means the caller builds a cipher from
         // material it did not ask for, and the envelope records a spec its
-        // payload does not match.
-        // ChaCha20 material is 32 random bytes, exactly like AES_256, so a
-        // backend that mints DEKs itself has no technical reason to refuse it.
-        // Static accepts it; Local and both Vault backends route through
-        // `generate_key_material`, which only knows the two AES specs. That
-        // split is pinned per backend rather than tolerated on both sides: a
-        // blanket "honoured or refused" contract would accept a backend
-        // regressing from working into refusing, which is exactly how a
-        // silently dropped spec would ship.
+        // payload does not match. Every backend in this matrix mints DEKs via
+        // the shared `generate_key_material`, so all three specs must be
+        // honoured; tolerating a refusal would accept a backend regressing
+        // out of that shared mapping.
         for spec in [KeySpec::Aes256, KeySpec::Aes128, KeySpec::ChaCha20] {
-            let must_be_honoured = spec != KeySpec::ChaCha20 || case.kind() == BackendKind::Static;
-            match manager
+            let generated = manager
                 .generate_data_key(GenerateDataKeyRequest {
                     key_id: case.key_id.clone(),
                     key_spec: spec.clone(),
                     encryption_context: context(),
                 })
                 .await
-            {
-                Ok(generated) => assert_eq!(
-                    generated.plaintext_key.len(),
-                    spec.key_size(),
-                    "[{label}] {spec:?} must yield a {}-byte data key",
-                    spec.key_size()
-                ),
-                Err(KmsError::UnsupportedAlgorithm { .. }) if !must_be_honoured => {}
-                Err(error) => panic!("[{label}] {spec:?} must yield a {}-byte data key: {error:?}", spec.key_size()),
-            }
+                .unwrap_or_else(|error| panic!("[{label}] {spec:?} must yield a data key: {error:?}"));
+            assert_eq!(
+                generated.plaintext_key.len(),
+                spec.key_size(),
+                "[{label}] {spec:?} must yield a {}-byte data key",
+                spec.key_size()
+            );
+            assert_eq!(
+                generated.key_id, case.key_id,
+                "[{label}] {spec:?} must name the master key that wrapped the DEK"
+            );
+
+            // The envelope must record the spec it was minted under, or a
+            // reader can no longer tell what the wrapped material is.
+            let envelope: serde_json::Value =
+                serde_json::from_slice(&generated.ciphertext_blob).expect("ciphertext must be a KMS envelope");
+            assert_eq!(
+                envelope.get("key_spec").and_then(|value| value.as_str()),
+                Some(spec.as_str()),
+                "[{label}] the envelope must record the requested spec"
+            );
+
+            // Whatever the length, the blob still round-trips.
+            let decrypted = manager
+                .decrypt(DecryptRequest {
+                    ciphertext: generated.ciphertext_blob,
+                    encryption_context: context(),
+                    grant_tokens: Vec::new(),
+                })
+                .await
+                .unwrap_or_else(|error| panic!("[{label}] {spec:?} blob should decrypt: {error:?}"));
+            assert_eq!(decrypted.plaintext, generated.plaintext_key);
         }
-
-        let aes128 = manager
-            .generate_data_key(GenerateDataKeyRequest {
-                key_id: case.key_id.clone(),
-                key_spec: KeySpec::Aes128,
-                encryption_context: context(),
-            })
-            .await
-            .unwrap_or_else(|error| panic!("[{label}] AES-128 generate should succeed: {error:?}"));
-
-        // Whatever the length, the blob still round-trips.
-        let decrypted = manager
-            .decrypt(DecryptRequest {
-                ciphertext: aes128.ciphertext_blob,
-                encryption_context: context(),
-                grant_tokens: Vec::new(),
-            })
-            .await
-            .unwrap_or_else(|error| panic!("[{label}] AES-128 blob should decrypt: {error:?}"));
-        assert_eq!(decrypted.plaintext, aes128.plaintext_key);
     })
     .await;
 }
@@ -344,7 +340,8 @@ async fn corrupt_ciphertext_fails_cleanly() {
         let tampered = flip_middle_bit(&dek.ciphertext_blob);
         assert!(decrypt(tampered).await.is_err(), "[{label}] a bit-flipped envelope must not decrypt");
 
-        // Truncation, emptiness, and non-envelope bytes are all typed errors.
+        // Truncation, emptiness, and non-envelope bytes are unparseable
+        // ciphertext, and every backend reports that as the same error class.
         for (name, input) in [
             ("truncated", dek.ciphertext_blob[..dek.ciphertext_blob.len() / 2].to_vec()),
             ("empty", Vec::new()),
@@ -355,8 +352,8 @@ async fn corrupt_ciphertext_fails_cleanly() {
                 .await
                 .expect_err(&format!("[{label}] {name} input must be rejected"));
             assert!(
-                !matches!(error, KmsError::InternalError { .. }),
-                "[{label}] {name} input must map to a specific error, not InternalError: {error:?}"
+                matches!(error, KmsError::CryptographicError { .. }),
+                "[{label}] {name} input must be rejected as unparseable ciphertext, got: {error:?}"
             );
         }
 
diff --git a/crates/kms/tests/behavior_objects.rs b/crates/kms/tests/behavior_objects.rs
index b154a6e6d..979cf0bb8 100644
--- a/crates/kms/tests/behavior_objects.rs
+++ b/crates/kms/tests/behavior_objects.rs
@@ -925,3 +925,181 @@ async fn a_rewritten_context_header_fails_authentication() {
         "a context the object was not sealed under must not open it"
     );
 }
+
+/// The SSE-C flank of the tamper check above: the customer-key path prefers
+/// the stored AAD bytes through the same branch, so a reverted preference —
+/// re-deriving canonical bytes from the parsed map — would open a tampered
+/// object here too, and only an SSE-C probe would notice.
+#[tokio::test]
+async fn a_rewritten_sse_c_context_header_fails_authentication() {
+    let (_kms, service) = service_with_key("sse-c-tampered-context").await;
+    let object_key = "tampered-sse-c.bin";
+    let customer_key = [0x55u8; 32];
+    let data = payload(256);
+
+    let encrypted = service
+        .encrypt_object_with_customer_key(BUCKET, object_key, data.as_slice(), &customer_key, None)
+        .await
+        .expect("SSE-C encrypt should succeed");
+
+    let mut headers = service.metadata_to_headers(&encrypted.metadata);
+    // Same pairs, different serialization: a pure ordering rewrite, so the
+    // rejection can only come from the AAD bytes and not from a changed map.
+    let rewritten = non_canonical_context_json(&encrypted.metadata.encryption_context);
+    assert_ne!(
+        Some(rewritten.as_str()),
+        headers.get("x-rustfs-encryption-context").map(String::as_str),
+        "the rewrite must actually change the stored bytes, or this proves nothing"
+    );
+    headers.insert("x-rustfs-encryption-context".to_string(), rewritten);
+
+    let tampered = service.headers_to_metadata(&headers).expect("tampered headers still parse");
+    assert!(
+        discard(
+            service
+                .decrypt_object_with_customer_key(BUCKET, object_key, encrypted.ciphertext.clone(), &tampered, &customer_key)
+                .await
+        )
+        .is_err(),
+        "a context the SSE-C object was not sealed under must not open it, even with the right key"
+    );
+}
+
+/// An SSE-KMS object written before the internal `x-rustfs-` headers existed
+/// must still open, and must rebuild into a record that names its real cipher.
+///
+/// Back then `x-amz-server-side-encryption: aws:kms` plus the S3 key-id header
+/// was the whole record, and AES-256-GCM was the only cipher in use — which is
+/// exactly the assumption the `aws:kms` fallback in `headers_to_metadata`
+/// encodes. The fallback is a normalization: `aws:kms` also parses as a cipher
+/// alias for AES-256-GCM, so the object opens either way, but only the
+/// normalized record re-projects the cipher header a modern read expects. The
+/// legacy header shape is reconstructed here by rewriting the SSE mode to
+/// `aws:kms`, adding the S3 key-id header, and dropping both internal headers.
+#[tokio::test]
+async fn a_legacy_aws_kms_object_without_the_cipher_header_still_opens() {
+    let (_kms, service) = service_with_key("sse-legacy-mode").await;
+    let object_key = "legacy-aws-kms.bin";
+    let data = payload(512);
+
+    let encrypted = service
+        .encrypt_object(BUCKET, object_key, data.as_slice(), &EncryptionAlgorithm::Aes256, None, None)
+        .await
+        .expect("encrypt should succeed");
+
+    let mut headers = service.metadata_to_headers(&encrypted.metadata);
+    headers.insert("x-amz-server-side-encryption".to_string(), "aws:kms".to_string());
+    headers.insert(
+        "x-amz-server-side-encryption-aws-kms-key-id".to_string(),
+        encrypted.metadata.key_id.clone(),
+    );
+    for internal in ["x-rustfs-encryption-algorithm", "x-rustfs-encryption-key-id"] {
+        headers
+            .remove(internal)
+            .unwrap_or_else(|| panic!("the modern projection must write the {internal} header this test deletes"));
+    }
+
+    let rebuilt = service
+        .headers_to_metadata(&headers)
+        .expect("a pre-internal-header record must still parse");
+    assert_eq!(rebuilt.key_id, encrypted.metadata.key_id, "the S3 key-id header must resolve the key");
+    assert_eq!(
+        rebuilt.algorithm,
+        EncryptionAlgorithm::Aes256.as_str(),
+        "aws:kms with no cipher header must normalize to the only cipher that era wrote"
+    );
+
+    // The normalization is what a re-projection stores: the upgraded record
+    // writes the modern cipher header instead of perpetuating the gap.
+    let reprojected = service.metadata_to_headers(&rebuilt);
+    assert_eq!(
+        reprojected.get("x-rustfs-encryption-algorithm").map(String::as_str),
+        Some(EncryptionAlgorithm::Aes256.as_str()),
+        "re-projecting the rebuilt record must write the cipher header"
+    );
+
+    let decrypted = read_all(
+        service
+            .decrypt_object(BUCKET, object_key, encrypted.ciphertext.clone(), &rebuilt, None)
+            .await
+            .expect("a legacy aws:kms object must still open"),
+    )
+    .await;
+    assert_eq!(decrypted, data, "the rebuilt record must recover the full plaintext");
+}
+
+/// Metadata persisted before `context_aad` existed deserializes with `None`
+/// there, and decrypt must then re-derive the AAD from the parsed context.
+/// That derived path only opens the object because the seal side canonicalizes
+/// the very same way — this is the independent probe of that pairing, for both
+/// the KMS and the customer-key flavours.
+#[tokio::test]
+async fn metadata_without_stored_context_bytes_still_opens() {
+    let (_kms, service) = service_with_key("sse-derived-aad").await;
+    let data = payload(512);
+    // Several entries, so canonicalization has an ordering to actually decide.
+    let context = ctx(&[("zeta", "26"), ("alpha", "1"), ("mu", "13")]);
+
+    // Byte-equality of the derived and stored AAD is what keeps the `None`
+    // path working, so pin the seal side of that pairing directly: the sealed
+    // record must carry exactly the canonical serialization of its context.
+    let canonical_aad = |context: &HashMap<String, String>| {
+        let canonical: std::collections::BTreeMap<&str, &str> =
+            context.iter().map(|(key, value)| (key.as_str(), value.as_str())).collect();
+        serde_json::to_vec(&canonical).expect("context serializes")
+    };
+
+    let encrypted = service
+        .encrypt_object(
+            BUCKET,
+            "derived-aad.bin",
+            data.as_slice(),
+            &EncryptionAlgorithm::Aes256,
+            None,
+            Some(&context),
+        )
+        .await
+        .expect("encrypt should succeed");
+    assert_eq!(
+        encrypted.metadata.context_aad.as_deref(),
+        Some(canonical_aad(&encrypted.metadata.encryption_context).as_slice()),
+        "the seal must pin the exact canonical AAD bytes it fed the AEAD"
+    );
+    let stripped = EncryptionMetadata {
+        context_aad: None,
+        ..encrypted.metadata.clone()
+    };
+    let decrypted = read_all(
+        service
+            .decrypt_object(BUCKET, "derived-aad.bin", encrypted.ciphertext.clone(), &stripped, None)
+            .await
+            .expect("metadata with no stored AAD bytes must open through the derived path"),
+    )
+    .await;
+    assert_eq!(decrypted, data, "the derived AAD must match the bytes the object was sealed under");
+
+    // The SSE-C record carries the same optional field through the same serde
+    // default, so its derived path needs its own proof.
+    let customer_key = [0x66u8; 32];
+    let sse_c = service
+        .encrypt_object_with_customer_key(BUCKET, "derived-aad-c.bin", data.as_slice(), &customer_key, None)
+        .await
+        .expect("SSE-C encrypt should succeed");
+    assert_eq!(
+        sse_c.metadata.context_aad.as_deref(),
+        Some(canonical_aad(&sse_c.metadata.encryption_context).as_slice()),
+        "the SSE-C seal must pin the exact canonical AAD bytes it fed the AEAD"
+    );
+    let stripped = EncryptionMetadata {
+        context_aad: None,
+        ..sse_c.metadata.clone()
+    };
+    let decrypted = read_all(
+        service
+            .decrypt_object_with_customer_key(BUCKET, "derived-aad-c.bin", sse_c.ciphertext.clone(), &stripped, &customer_key)
+            .await
+            .expect("SSE-C metadata with no stored AAD bytes must open through the derived path"),
+    )
+    .await;
+    assert_eq!(decrypted, data, "the SSE-C derived AAD must match the bytes the object was sealed under");
+}
diff --git a/crates/kms/tests/common/mod.rs b/crates/kms/tests/common/mod.rs
index f8123d803..9d2b8d7ec 100644
--- a/crates/kms/tests/common/mod.rs
+++ b/crates/kms/tests/common/mod.rs
@@ -445,22 +445,6 @@ pub fn assert_configuration_error<T: Debug>(result: Result<T>, message_fragment:
     }
 }
 
-#[track_caller]
-pub fn assert_validation_error<T: Debug>(result: Result<T>) {
-    match result {
-        Err(KmsError::ValidationError { .. }) => {}
-        other => panic!("expected ValidationError, got {other:?}"),
-    }
-}
-
-#[track_caller]
-pub fn assert_cryptographic_error<T: Debug>(result: Result<T>) {
-    match result {
-        Err(KmsError::CryptographicError { .. }) => {}
-        other => panic!("expected CryptographicError, got {other:?}"),
-    }
-}
-
 #[track_caller]
 pub fn assert_invalid_key_size<T: Debug>(result: Result<T>, expected: usize, actual: usize) {
     match result {