fix(sse): read MinIO single-part encrypted objects

Two independent defects kept them unreadable, both reached only once the fixture harness could load an inlined object at all.

The stream was keyed as if the object were multipart. is_multipart_encrypted_object falls back to an ETag length test when there is a single part, and MinIO stores an *encrypted* ETag for SSE objects — 96 characters for a single-part upload, not the 32 of a plain MD5 — so every such object read as multipart and had a per-part key derived for a stream sealed with the object key itself. MinIO records the answer outright in X-Minio-Internal-Encrypted-Multipart, so that marker is now consulted first, and on a MinIO-written object its absence is treated as just as informative as its presence.

The size was then reported as the ciphertext's. MinIO writes an explicit plaintext size only for multipart uploads, deriving it from the DARE stream otherwise, so both the read plan and ObjectInfo::decrypted_size fell back to the physical size — the reader waited for the encoding overhead as if it were payload, and a client would have seen a Content-Length overstated by exactly that much. dare_v2_decrypted_size inverts the package layout the way MinIO's sio.DecryptedSize does. It lives in rustfs-utils so the read plan and the size accessor cannot drift apart, and it returns None for a length no DARE stream can have rather than assigning a malformed object a plausible one.

The interop suite is 9/9 with the two single-part cases added, and reverting the classification to the ETag heuristic turns both red. rustfs-ecstore's object_api tests and rustfs-utils are unaffected.

Refs rustfs/backlog#1638.

crates/ecstore/src/object_api/readers.rs

@@ -630,7 +630,7 @@ impl ReadPlan {
             let material = resolved;
             #[cfg(feature = "rio-v2")]
             let uses_legacy_encryption = matches!(material.mode, ReadEncryptionMode::Direct { .. });
-            let is_multipart = is_multipart_encrypted_object(&oi.parts, oi.etag.as_deref());
+            let is_multipart = is_multipart_encrypted_object(&oi.parts, oi.etag.as_deref(), &oi.user_defined);
             let recorded_plaintext_size = oi.encryption_original_size()?;
             let plaintext_size = encrypted_plaintext_size(oi, is_multipart, is_compressed, recorded_plaintext_size)?;
             let full_plaintext_size =
@@ -1285,14 +1285,59 @@ fn encrypted_plaintext_size(
             .unwrap_or(oi.size));
     }
 
-    Ok(recorded_plaintext_size.unwrap_or(oi.size))
+    if let Some(recorded) = recorded_plaintext_size {
+        return Ok(recorded);
+    }
+
+    // A MinIO single-part object records no plaintext size: MinIO writes
+    // `X-Minio-Internal-actual-size` only for multipart uploads and otherwise
+    // derives the size from the DARE stream itself. Falling back to `oi.size`
+    // hands back the *physical* size, so the reader waits for the encoding
+    // overhead as if it were payload and the body ends short by exactly that
+    // much (rustfs/backlog#1638).
+    if rustfs_utils::http::has_minio_internal_sse_metadata(&oi.user_defined)
+        && let Some(plaintext) = rustfs_utils::http::dare_v2_decrypted_size(oi.size)
+    {
+        return Ok(plaintext);
+    }
+
+    Ok(oi.size)
 }
 
-fn is_multipart_encrypted_object(parts: &[ObjectPartInfo], etag: Option<&str>) -> bool {
+/// MinIO's explicit multipart marker, and the internal SSE prefix that
+/// identifies an object as MinIO-written in the first place.
+const MINIO_INTERNAL_ENCRYPTED_MULTIPART_KEY: &str = "X-Minio-Internal-Encrypted-Multipart";
+const MINIO_INTERNAL_SSE_PREFIX: &str = "X-Minio-Internal-Server-Side-Encryption-";
+
+/// Whether an encrypted object's stream is keyed per part.
+///
+/// `user_defined` is consulted before the ETag because MinIO records the answer
+/// outright, in `X-Minio-Internal-Encrypted-Multipart`. The ETag heuristic
+/// cannot stand in for it: MinIO stores an *encrypted* ETag for SSE objects —
+/// 96 characters for a single-part upload, not the 32 of a plain MD5 — so the
+/// length test reads such an object as multipart and derives a per-part key for
+/// a stream that was sealed with the object key itself, which then fails
+/// authentication (rustfs/backlog#1638).
+fn is_multipart_encrypted_object(parts: &[ObjectPartInfo], etag: Option<&str>, user_defined: &HashMap<String, String>) -> bool {
     if parts.len() > 1 {
         return true;
     }
 
+    if user_defined
+        .keys()
+        .any(|key| key.eq_ignore_ascii_case(MINIO_INTERNAL_ENCRYPTED_MULTIPART_KEY))
+    {
+        return true;
+    }
+    // On a MinIO-written object the marker's absence is as informative as its
+    // presence, so the ETag is not consulted at all.
+    if user_defined
+        .keys()
+        .any(|key| rustfs_utils::http::starts_with_ignore_ascii_case(key, MINIO_INTERNAL_SSE_PREFIX))
+    {
+        return false;
+    }
+
     etag.map(|etag| etag.trim_matches('"').len() != 32).unwrap_or(false)
 }
 

crates/ecstore/src/object_api/types.rs

@@ -614,7 +614,20 @@ impl ObjectInfo {
     }
 
     pub fn decrypted_size(&self) -> std::io::Result<i64> {
-        Ok(self.encryption_original_size()?.unwrap_or(self.size))
+        if let Some(recorded) = self.encryption_original_size()? {
+            return Ok(recorded);
+        }
+        // A MinIO single-part object records no plaintext size — MinIO writes one
+        // only for multipart uploads — so `self.size` here is the *physical*
+        // size, encoding overhead included. Reporting that as the object's
+        // length overstates it by exactly that overhead, which is what a client
+        // sees as Content-Length (rustfs/backlog#1638).
+        if rustfs_utils::http::has_minio_internal_sse_metadata(&self.user_defined)
+            && let Some(plaintext) = rustfs_utils::http::dare_v2_decrypted_size(self.size)
+        {
+            return Ok(plaintext);
+        }
+        Ok(self.size)
     }
 
     pub fn get_actual_size(&self) -> std::io::Result<i64> {

crates/kms/src/config.rs

@@ -324,11 +324,18 @@ impl Default for LocalConfig {
 /// wraps data encryption keys — there is no HMAC-SHA256 derivation step — and
 /// each wrapped DEK is serialized as a RustFS `DataKeyEnvelope` JSON blob.
 ///
-/// This mirrors the *concept* of MinIO's builtin/static single-key KMS, but is
-/// not wire-compatible with it: MinIO wraps DEKs in a different (`{"aead": ...}`)
-/// blob that this backend neither produces nor accepts, so KMS ciphertext
-/// written by MinIO cannot be opened here. Reading MinIO-written SSE objects is
-/// tracked separately in rustfs/backlog#1638.
+/// This mirrors the *concept* of MinIO's builtin/static single-key KMS but is
+/// not wire-compatible with it. MinIO seals a DEK as `sealed || iv[16] ||
+/// nonce[12]` under a per-ciphertext key derived from the master secret, with
+/// a legacy JSON encoding of the same layout; this backend neither produces
+/// nor accepts either, so pointing it at MinIO's master key does **not** make
+/// MinIO-written objects readable through it.
+///
+/// Reading MinIO-written SSE objects is a property of the object read path, not
+/// of this backend: that path decodes MinIO's format directly, keyed by
+/// `RUSTFS_SSE_S3_MASTER_KEY`. See the migration section of
+/// `docs/operations/kms-backend-security.md` for which object shapes are
+/// covered, and rustfs/backlog#1638 for the remainder.
 #[derive(Clone, Default, Serialize, Deserialize)]
 pub struct StaticConfig {
     /// Key identifier (name) for the single configured key

crates/kms/src/lib.rs

@@ -255,6 +255,10 @@ pub use cache::KmsCacheStats;
 pub use config::*;
 pub use deletion_worker::DeletionReferenceChecker;
 pub use encryption::is_data_key_envelope;
+// Re-exported so the object layer binds encryption context exactly the way the
+// KMS backends do. A second canonicalization is how the object layer once
+// serialized a HashMap directly while the Static backend already sorted keys.
+pub use encryption::context_aad;
 pub use error::{KmsError, KmsUnavailableError, Result};
 pub use key_impact::{KeyImpactReport, KeyReference, KeyReferenceKind, ReferenceCompleteness, ReferenceCoverage, ReferenceScope};
 pub use manager::KmsManager;

crates/utils/src/http/object_encryption_keys.rs

@@ -53,6 +53,50 @@ pub const MINIO_INTERNAL_ENCRYPTION_KMS_KEY_ID_HEADER: &str = "X-Minio-Internal-
 pub const MINIO_INTERNAL_ENCRYPTION_KMS_DATA_KEY_HEADER: &str = "X-Minio-Internal-Server-Side-Encryption-S3-Kms-Sealed-Key";
 pub const MINIO_INTERNAL_ENCRYPTION_KMS_CONTEXT_HEADER: &str = "X-Minio-Internal-Server-Side-Encryption-Context";
 
+/// Plaintext length of a DARE v2 stream of `ciphertext_size` bytes.
+///
+/// The stream is a sequence of packages, each a 16-byte header, up to 64 KiB of
+/// payload, and a 16-byte tag; only the last may be short. This is the same
+/// arithmetic as MinIO's `sio.DecryptedSize`, and it is the only way to size a
+/// MinIO single-part object: MinIO writes an explicit plaintext size only for
+/// multipart uploads and otherwise derives it from the stream.
+///
+/// Returns `None` for a size no DARE stream can have — a final package carrying
+/// overhead but no payload — so a malformed object is not silently assigned a
+/// plausible length.
+pub fn dare_v2_decrypted_size(ciphertext_size: i64) -> Option<i64> {
+    const HEADER_LEN: i64 = 16;
+    const TAG_LEN: i64 = 16;
+    const MAX_PAYLOAD: i64 = 64 * 1024;
+    const PACKAGE_LEN: i64 = HEADER_LEN + MAX_PAYLOAD + TAG_LEN;
+
+    if ciphertext_size < 0 {
+        return None;
+    }
+    if ciphertext_size == 0 {
+        return Some(0);
+    }
+
+    let full_packages = ciphertext_size / PACKAGE_LEN;
+    let remainder = ciphertext_size % PACKAGE_LEN;
+    if remainder == 0 {
+        return Some(full_packages * MAX_PAYLOAD);
+    }
+    if remainder <= HEADER_LEN + TAG_LEN {
+        return None;
+    }
+    Some(full_packages * MAX_PAYLOAD + remainder - HEADER_LEN - TAG_LEN)
+}
+
+/// True when the metadata was written by MinIO's SSE path.
+pub fn has_minio_internal_sse_metadata<S: std::hash::BuildHasher>(
+    metadata: &std::collections::HashMap<String, String, S>,
+) -> bool {
+    metadata
+        .keys()
+        .any(|key| super::starts_with_ignore_ascii_case(key, "x-minio-internal-server-side-encryption-"))
+}
+
 /// Reserved RustFS-branded twin of the MinIO-internal SSE key family.
 ///
 /// No RustFS writer emits these keys today — the SSE writer persists the
@@ -323,6 +367,31 @@ mod tests {
         assert!(!format!("{projected:?}").contains("secret-key"));
     }
 
+    #[test]
+    fn dare_v2_size_inverts_the_package_layout() {
+        const PACKAGE: i64 = 16 + 64 * 1024 + 16;
+
+        // Exactly one full package, and exactly two.
+        assert_eq!(dare_v2_decrypted_size(PACKAGE), Some(64 * 1024));
+        assert_eq!(dare_v2_decrypted_size(2 * PACKAGE), Some(128 * 1024));
+
+        // The shape that motivated this: a 64 KiB object stored as 65568 bytes.
+        assert_eq!(dare_v2_decrypted_size(65568), Some(65536));
+
+        // A short trailing package carries its own header and tag.
+        assert_eq!(dare_v2_decrypted_size(PACKAGE + 16 + 1 + 16), Some(64 * 1024 + 1));
+        assert_eq!(dare_v2_decrypted_size(16 + 1 + 16), Some(1));
+
+        assert_eq!(dare_v2_decrypted_size(0), Some(0));
+
+        // Sizes no DARE stream can have: overhead with no payload behind it.
+        // Refused rather than rounded into a plausible length.
+        assert_eq!(dare_v2_decrypted_size(1), None);
+        assert_eq!(dare_v2_decrypted_size(32), None);
+        assert_eq!(dare_v2_decrypted_size(PACKAGE + 32), None);
+        assert_eq!(dare_v2_decrypted_size(-1), None);
+    }
+
     #[test]
     fn transport_metadata_roundtrip_restores_stored_keys() {
         let mut headers = http::HeaderMap::new();

docs/operations/kms-backend-security.md

@@ -14,24 +14,43 @@ For how the Vault backends authenticate (static token, AppRole, Kubernetes, Vaul
 | Vault Transit | `VaultTransit` | Key-encryption keys never leave Vault; only Transit ciphertext is visible outside | Vault Transit engine (cryptographic isolation) | Delegated to Vault storage | Via Vault Transit key versioning | Deployments that need key material to be unreadable through storage APIs |
 | AWS KMS | `AWS` (alias `AwsKms`) | Key material never leaves AWS KMS; RustFS mirrors no key state | AWS KMS (cryptographic isolation) + IAM | Delegated to AWS | On-demand `RotateKeyOnDemand`; prior backing keys stay usable for decryption | Deployments already rooted in AWS IAM that want AWS as the cryptographic root — read [AWS KMS: deviations from the shared backend contract](#aws-kms-deviations-from-the-shared-backend-contract) first |
 
-## Migrating from MinIO: encrypted objects do not carry over
+## Migrating from MinIO: what carries over, and what does not
 
-> **Warning: RustFS does not currently support reading objects that MinIO encrypted.**
-> This applies to SSE-S3, SSE-KMS, and SSE-C, in every released binary and container image, and it holds regardless of which KMS backend you configure. Configuring the `Static` backend with the same key material MinIO used does **not** make those objects readable — MinIO wraps data keys in a different envelope format that no RustFS backend produces or accepts (`crates/kms/src/config.rs:304-308`). Plan for this **before** moving data. Tracked in rustfs/backlog#1638.
+> **Read this before moving data.** Some MinIO-encrypted objects are readable by RustFS and some are not, and the boundary is not where you would guess. Verify against a sample of your own objects rather than assuming either answer. Tracked in rustfs/backlog#1638.
 
-The read does fail closed — ciphertext is never served as plaintext. MinIO's internal encryption headers mark the object as encrypted (`crates/utils/src/http/header_compat.rs:50-67`), so the read path demands encryption material and refuses when none resolves (`crates/ecstore/src/object_api/readers.rs:559-568`). Two properties still make the problem easy to discover late:
+Support is stated per shape below because that is how far it has been *measured* — against fixtures captured from a real MinIO server (`minio/minio:RELEASE.2025-09-07T16-13-09Z`), not inferred from the code:
+
+| MinIO object | RustFS read | Evidence |
+| --- | --- | --- |
+| SSE-S3, multipart | **Yes** | `reads_minio_generated_sse_s3_multipart_fixture` |
+| SSE-KMS, multipart | **Yes** | `reads_minio_generated_sse_kms_multipart_fixture` |
+| SSE-C, multipart | **Yes** | `reads_minio_generated_sse_c_multipart_fixture` |
+| SSE-S3, single-part | **Yes** | `reads_minio_generated_sse_s3_singlepart_fixture` |
+| SSE-KMS, single-part | **Yes** | `reads_minio_generated_sse_kms_singlepart_fixture` |
+| SSE-C, single-part | **Unverified** | No fixture coverage |
+| Sealed by KES, a KMS plugin, or MinKMS | **No**, and not planned | Re-encrypt at the source before migrating |
+
+SSE-C needs no KMS at all: the customer supplies the key on each request, exactly as against MinIO. Note that a MinIO SSE-C object stores no customer-key MD5, so the usual early "these parameters do not match" rejection cannot fire for it — a wrong key is refused by the decryption itself instead, which is a different error but the same outcome.
+
+Reading a supported *managed* object (SSE-S3, SSE-KMS) requires RustFS to hold the same master key MinIO used, supplied through `RUSTFS_SSE_S3_MASTER_KEY` (the production entry point, exercised by `reads_minio_generated_sse_s3_fixture_through_production_master_key_env`). MinIO's builtin KMS derives a per-ciphertext sealing key from that master secret, so the *same* secret is required — not merely an equivalently configured backend.
+
+**"Unverified" means unknown, not broken.** The remaining row has no fixture coverage, so it has never been read in a test either way. Do not read the table's "Yes" rows as covering it.
+
+Whatever the table says, verify before you commit: **read a sample of encrypted objects, not just their listings.** A read that is not supported fails closed — ciphertext is never served as plaintext — but two properties still make it easy to discover late:
 
 - **The error does not say what happened.** It surfaces as a 500 `InternalError`, which reads as a RustFS fault rather than "another implementation encrypted this object".
 - **Surrounding metadata migrates fine.** The object's `xl.meta` parses, so encrypted objects list and HEAD normally and report plausible sizes. The failure appears only when something reads the payload.
 
-Read a sample of encrypted objects, not just their listings, before decommissioning the MinIO deployment.
-
-Current options for a migration whose source contains encrypted objects:
+For any shape that does not read, the options are unchanged:
 
 - Decrypt on the MinIO side first, migrate plaintext, then let RustFS re-encrypt with its own KMS.
 - Copy through the S3 API rather than moving drives — MinIO decrypts on read, and RustFS encrypts on write. This re-encrypts rather than preserving ciphertext and costs a full data transfer.
 - Leave encrypted objects on MinIO and migrate only unencrypted data.
 
+### The reverse direction does not work
+
+MinIO cannot read objects RustFS encrypted, and that is a deliberate, documented position rather than a gap awaiting a fix. RustFS fills MinIO's metadata slots — the sealed-key and IV headers are MinIO-shaped — but the data key in `X-Minio-Internal-Server-Side-Encryption-S3-Kms-Sealed-Key` is a RustFS envelope, which MinIO's KMS cannot open. **Treat the MinIO-branded headers on a RustFS-written object as RustFS-internal.** Their presence is not a statement that MinIO can read the object, and no coexistence plan should assume two-way reads.
+
 Inventory the source before choosing: bucket default-encryption settings mean objects can be encrypted without any client having sent SSE headers.
 
 The same limitation applies in reverse — objects RustFS encrypts are not readable by MinIO. For the code-level breakdown of which seams block each SSE mode, see [MinIO file-format interoperability, Part C](../architecture/minio-file-format-compat.md#part-c--server-side-encryption-sse).

rustfs/src/storage/minio_generated_read_test.rs

@@ -4,7 +4,7 @@ use std::fs;
 use std::io::Cursor;
 use std::path::{Path, PathBuf};
 
-use super::sse::SseObjectEncryptionResolver;
+use super::sse::{SseObjectEncryptionResolver, reset_sse_dek_provider};
 use super::storage_api::ecstore_test_support::{
     DiskAPI as _, DiskOption, Endpoint, Erasure, GetObjectReader, ObjectInfo, ObjectOptions, create_bitrot_reader, new_disk,
 };
@@ -92,6 +92,37 @@ fn object_xl_meta_path(case_dir: &Path, manifest: &ManifestRecord) -> PathBuf {
     case_dir.join("backend").join(relative)
 }
 
+/// Metadata as each disk stored it, indexed by disk position.
+///
+/// The object's primary `FileInfo` is read from one disk, but inline shard data
+/// and bitrot checksums are per disk, so both have to come from the disk they
+/// belong to.
+fn load_per_disk_file_info(case_dir: &Path, manifest: &ManifestRecord, disk_count: usize) -> Vec<Option<FileInfo>> {
+    (0..disk_count)
+        .map(|idx| {
+            let path = case_dir
+                .join("backend")
+                .join(format!("disk{}", idx + 1))
+                .join(&manifest.bucket)
+                .join(&manifest.object)
+                .join("xl.meta");
+            let bytes = fs::read(&path).ok()?;
+            get_file_info(
+                &bytes,
+                &manifest.bucket,
+                &manifest.object,
+                "",
+                FileInfoOpts {
+                    data: true,
+                    include_free_versions: true,
+                    include_part_checksums: false,
+                },
+            )
+            .ok()
+        })
+        .collect()
+}
+
 fn load_file_info(case_dir: &Path, manifest: &ManifestRecord) -> FileInfo {
     let xl_meta_path = object_xl_meta_path(case_dir, manifest);
     let xl_meta = fs::read(&xl_meta_path).unwrap_or_else(|err| panic!("read {}: {err}", xl_meta_path.display()));
@@ -131,6 +162,13 @@ async fn load_fixture_reader_input(case_id: &str) -> (ObjectInfo, Vec<u8>, Strin
 async fn read_fixture_plaintext(encrypted: Vec<u8>, object_info: ObjectInfo, kms_key_b64: String) -> Result<Vec<u8>, String> {
     let object_size = object_info.size;
 
+    // The DEK provider is cached process-wide once built, so without this reset
+    // a case that ran earlier in the same binary keeps serving its master key to
+    // every later case — which silently turned the wrong-key negative below into
+    // a test that could not fail. Reset before each read so the provider is
+    // built from the key this case actually configured.
+    reset_sse_dek_provider();
+
     async_with_vars(
         [
             ("__RUSTFS_SSE_SIMPLE_CMK", Some(kms_key_b64)),
@@ -187,10 +225,20 @@ async fn encrypted_fixture_bytes(case_dir: &Path, manifest: &ManifestRecord, fil
         .unwrap_or_else(|err| panic!("open fixture disk {disk_number}: {err}"));
         disks.push(disk);
     }
+    // Pair each disk with the metadata that disk stored, then place both at the
+    // erasure block slot that metadata claims — the shuffle the read path does
+    // before it reads anything (`shuffle_disks_and_parts_metadata_by_index`).
+    // The harness needs the per-disk metadata, not just the per-disk disk: below
+    // the small-file threshold an object has no part file and each disk carries
+    // its shard inline in its own xl.meta, and the bitrot checksums are per disk
+    // too.
+    let per_disk_meta = load_per_disk_file_info(case_dir, manifest, disks.len());
     let mut disk_order = vec![None; disks.len()];
+    let mut meta_order: Vec<Option<FileInfo>> = vec![None; disks.len()];
     for (idx, disk) in disks.iter().enumerate() {
         let block_index = file_info.erasure.distribution[idx];
         disk_order[block_index - 1] = Some(disk);
+        meta_order[block_index - 1] = per_disk_meta.get(idx).cloned().flatten();
     }
     let data_dir = file_info
         .data_dir
@@ -199,13 +247,21 @@ async fn encrypted_fixture_bytes(case_dir: &Path, manifest: &ManifestRecord, fil
 
     let mut encrypted = Vec::new();
     for part in &file_info.parts {
-        let checksum_info = file_info.erasure.get_checksum_info(part.number);
+        let primary_checksum_info = file_info.erasure.get_checksum_info(part.number);
         let path = format!("{}/{}/part.{}", manifest.object, data_dir, part.number);
         let shard_read_len = file_info.erasure.shard_file_size(part.size as i64);
         let mut readers = Vec::with_capacity(disks.len());
         for (idx, disk) in disk_order.iter().enumerate() {
+            // Below MinIO's small-file threshold there is no part file at all:
+            // each disk keeps its erasure shard inline in its own xl.meta, with
+            // the same bitrot framing the reader expects from a file.
+            let disk_meta = meta_order[idx].as_ref();
+            let inline_shard = disk_meta.and_then(|meta| meta.data.as_deref());
+            let checksum_info = disk_meta
+                .map(|meta| meta.erasure.get_checksum_info(part.number))
+                .unwrap_or_else(|| primary_checksum_info.clone());
             let reader = create_bitrot_reader(
-                None,
+                inline_shard,
                 *disk,
                 &manifest.bucket,
                 &path,
@@ -253,6 +309,133 @@ async fn reads_minio_generated_sse_kms_multipart_fixture() {
     assert_fixture_round_trip("sse-kms-multipart-8m", 8 * 1024 * 1024).await;
 }
 
+/// Read an SSE-C fixture, supplying the customer key the way a client does.
+///
+/// SSE-C needs no KMS at all — the key arrives on the request — so this path
+/// shares nothing with the managed-SSE reads above beyond the fixture loader.
+async fn read_ssec_fixture_plaintext(
+    encrypted: Vec<u8>,
+    object_info: ObjectInfo,
+    customer_key_b64: &str,
+    customer_key_md5_b64: &str,
+) -> Result<Vec<u8>, String> {
+    let object_size = object_info.size;
+    reset_sse_dek_provider();
+
+    let mut headers = http::HeaderMap::new();
+    headers.insert(
+        http::HeaderName::from_static("x-amz-server-side-encryption-customer-algorithm"),
+        http::HeaderValue::from_static("AES256"),
+    );
+    headers.insert(
+        http::HeaderName::from_static("x-amz-server-side-encryption-customer-key"),
+        http::HeaderValue::from_str(customer_key_b64).expect("fixture customer key is a header value"),
+    );
+    headers.insert(
+        http::HeaderName::from_static("x-amz-server-side-encryption-customer-key-md5"),
+        http::HeaderValue::from_str(customer_key_md5_b64).expect("fixture customer key md5 is a header value"),
+    );
+
+    let resolver = SseObjectEncryptionResolver;
+    let (mut reader, offset, length) = GetObjectReader::new_with_resolver(
+        Box::new(Cursor::new(encrypted)),
+        None,
+        &object_info,
+        &ObjectOptions::default(),
+        &headers,
+        Some(&resolver),
+    )
+    .await
+    .map_err(|err| format!("construct GetObjectReader from MinIO SSE-C fixture: {err:?}"))?;
+
+    if offset != 0 || length != object_size {
+        return Err(format!("unexpected fixture range offset={offset} length={length} size={object_size}"));
+    }
+
+    let mut plaintext = Vec::new();
+    reader
+        .read_to_end(&mut plaintext)
+        .await
+        .map_err(|err| format!("read plaintext from MinIO SSE-C fixture: {err}"))?;
+    Ok(plaintext)
+}
+
+/// The interop claim must hold on the production key entry point, not only on
+/// the test-only injection channel every other case here uses.
+#[tokio::test]
+#[ignore = "requires generated MinIO fixture data and a local static KMS key"]
+async fn reads_minio_generated_sse_s3_fixture_through_production_master_key_env() {
+    let (object_info, encrypted, expected_sha256) = load_fixture_reader_input("sse-s3-multipart-8m").await;
+
+    let plaintext = read_fixture_plaintext_via_production_env(encrypted, object_info, minio_static_kms_key_b64())
+        .await
+        .expect("fixture must restore through RUSTFS_SSE_S3_MASTER_KEY");
+
+    assert_eq!(sha256_hex(&plaintext), expected_sha256);
+}
+
+/// Objects small enough that MinIO inlined them into xl.meta instead of writing
+/// a part file — the ordinary shape for everyday small objects, and the one
+/// whose encrypted ETag misleads the multipart heuristic.
+#[tokio::test]
+#[ignore = "requires generated MinIO fixture data and a local static KMS key"]
+async fn reads_minio_generated_sse_s3_singlepart_fixture() {
+    assert_fixture_round_trip("sse-s3-singlepart-64k", 64 * 1024).await;
+}
+
+#[tokio::test]
+#[ignore = "requires generated MinIO fixture data and a local static KMS key"]
+async fn reads_minio_generated_sse_kms_singlepart_fixture() {
+    assert_fixture_round_trip("sse-kms-singlepart-64k", 64 * 1024).await;
+}
+
+/// SSE-C is the one managed shape needing no KMS: the customer supplies the key
+/// on every request, so this measures the read path alone.
+#[tokio::test]
+#[ignore = "requires generated MinIO fixture data"]
+async fn reads_minio_generated_sse_c_multipart_fixture() {
+    // The fixture lab's fixed SSE-C key; recorded in the case's request.json.
+    const SSEC_KEY_B64: &str = "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8=";
+    const SSEC_KEY_MD5_B64: &str = "tP/LI3N87DFaSk0aoqYgzg==";
+
+    let (object_info, encrypted, expected_sha256) = load_fixture_reader_input("sse-c-multipart-8m").await;
+
+    let plaintext = read_ssec_fixture_plaintext(encrypted, object_info, SSEC_KEY_B64, SSEC_KEY_MD5_B64)
+        .await
+        .expect("MinIO SSE-C fixture must restore with the customer key");
+
+    assert_eq!(sha256_hex(&plaintext), expected_sha256);
+}
+
+/// The read path skips the stored-MD5 comparison for MinIO SSE-C objects,
+/// which store no MD5. This holds the line that made that safe: the customer
+/// key is still proven by the object-key unseal, so a wrong key must fail even
+/// with nothing to compare it against.
+#[tokio::test]
+#[ignore = "requires generated MinIO fixture data"]
+async fn sse_c_wrong_customer_key_still_fails_without_a_stored_md5() {
+    // A well-formed 32-byte key that is not the one the fixture was sealed
+    // with, sent with its own correct MD5 so the request itself is valid.
+    const WRONG_KEY_B64: &str = "AQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQEBAQE=";
+    const WRONG_KEY_MD5_B64: &str = "0YB4bMPPCf9SNlqiKmM0uQ==";
+
+    let (object_info, encrypted, expected_sha256) = load_fixture_reader_input("sse-c-multipart-8m").await;
+
+    let result = read_ssec_fixture_plaintext(encrypted, object_info, WRONG_KEY_B64, WRONG_KEY_MD5_B64).await;
+
+    match result {
+        Err(_) => {}
+        // Never reached today, and asserted rather than assumed: if a future
+        // change let a wrong key through, returning the real plaintext would be
+        // the worst possible outcome.
+        Ok(plaintext) => assert_ne!(
+            sha256_hex(&plaintext),
+            expected_sha256,
+            "a wrong SSE-C customer key must never restore the original plaintext"
+        ),
+    }
+}
+
 #[tokio::test]
 #[ignore = "requires generated MinIO fixture data and a local static KMS key"]
 async fn rejects_minio_generated_sse_s3_fixture_with_wrong_kms_key() {
@@ -281,6 +464,55 @@ async fn rejects_minio_generated_sse_s3_fixture_with_truncated_ciphertext() {
     }
 }
 
+/// Read a fixture through the **production** provider selection.
+///
+/// [`read_fixture_plaintext`] injects the master key through
+/// `__RUSTFS_SSE_SIMPLE_CMK`, which is `#[cfg(test)]`-only, so on its own it
+/// proves nothing about a deployment: it never reaches
+/// `LocalSseDekProvider::new_from_env`. This variant sets only
+/// `RUSTFS_SSE_S3_MASTER_KEY` — the sole production entry point — so the
+/// interop claim rests on the path operators actually run (backlog#1638).
+async fn read_fixture_plaintext_via_production_env(
+    encrypted: Vec<u8>,
+    object_info: ObjectInfo,
+    master_key_b64: String,
+) -> Result<Vec<u8>, String> {
+    let object_size = object_info.size;
+    reset_sse_dek_provider();
+
+    async_with_vars(
+        [
+            ("__RUSTFS_SSE_SIMPLE_CMK", None::<String>),
+            ("RUSTFS_SSE_S3_MASTER_KEY", Some(master_key_b64)),
+        ],
+        async move {
+            let resolver = SseObjectEncryptionResolver;
+            let (mut reader, offset, length) = GetObjectReader::new_with_resolver(
+                Box::new(Cursor::new(encrypted)),
+                None,
+                &object_info,
+                &ObjectOptions::default(),
+                &http::HeaderMap::new(),
+                Some(&resolver),
+            )
+            .await
+            .map_err(|err| format!("construct GetObjectReader from MinIO raw fixture: {err:?}"))?;
+
+            if offset != 0 || length != object_size {
+                return Err(format!("unexpected fixture range offset={offset} length={length} size={object_size}"));
+            }
+
+            let mut plaintext = Vec::new();
+            reader
+                .read_to_end(&mut plaintext)
+                .await
+                .map_err(|err| format!("read plaintext from MinIO raw fixture: {err}"))?;
+            Ok(plaintext)
+        },
+    )
+    .await
+}
+
 async fn assert_fixture_round_trip(case_id: &str, expected_size: i64) {
     let (object_info, encrypted, expected_sha256) = load_fixture_reader_input(case_id).await;
     // `ObjectInfo.size` is the on-disk size. For SSE objects that is the

rustfs/src/storage/sse.rs

@@ -1460,6 +1460,16 @@ fn managed_sse_domain(sse_type: SSEType) -> &'static str {
     }
 }
 
+/// The public `x-amz-server-side-encryption` value a managed scheme reports.
+fn managed_sse_public_header(sse_type: SSEType) -> &'static str {
+    match sse_type {
+        SSEType::SseKms => ServerSideEncryption::AWS_KMS,
+        // SSE-C never reaches the managed path; reporting AES256 keeps this
+        // total without inventing a third public value.
+        SSEType::SseS3 | SSEType::SseC => ServerSideEncryption::AES256,
+    }
+}
+
 fn canonical_kms_bucket_path(bucket: &str, key: &str) -> String {
     path_join_buf(&[bucket, key])
 }
@@ -2051,10 +2061,20 @@ pub async fn sse_prepare_encryption(request: PrepareEncryptionRequest<'_>) -> Re
 /// }
 /// ```
 pub async fn sse_decryption(request: DecryptionRequest<'_>) -> Result<Option<DecryptionMaterial>, ApiError> {
-    // Check for SSE-C encryption
+    // Check for SSE-C encryption.
+    //
+    // The stored customer-algorithm marker is what RustFS writes, but a
+    // MinIO-written object has only the internal sealed-key slot: MinIO keeps
+    // the customer algorithm on the request and synthesizes it back onto the
+    // response, never persisting it. Recognizing that slot as well is what lets
+    // a migrated SSE-C object be read at all; the customer key still has to be
+    // supplied, and is still checked against the stored MD5 below.
     if request
         .metadata
         .contains_key("x-amz-server-side-encryption-customer-algorithm")
+        || request
+            .metadata
+            .contains_key(MINIO_INTERNAL_ENCRYPTION_SSEC_SEALED_KEY_HEADER)
     {
         let (key, key_md5) = match (request.sse_customer_key, request.sse_customer_key_md5) {
             (Some(k), Some(md5)) => (k, md5),
@@ -2068,7 +2088,21 @@ pub async fn sse_decryption(request: DecryptionRequest<'_>) -> Result<Option<Dec
 
         // Verify that the provided key MD5 matches the stored MD5 for security
         let stored_md5 = request.metadata.get("x-amz-server-side-encryption-customer-key-md5");
-        verify_ssec_key_match(key_md5, stored_md5)?;
+        // MinIO stores no customer-key MD5 — it keeps that header on the request
+        // and returns it on the response — so requiring one would make every
+        // migrated SSE-C object unreadable. Skipping the comparison when there is
+        // nothing to compare against does not weaken the check it performs: the
+        // stored MD5 is an early, friendlier rejection, while the key itself is
+        // proven by the object-key unseal below, whose AEAD fails on a wrong key.
+        // `sse_c_wrong_customer_key_still_fails_without_a_stored_md5` holds that
+        // line. Objects that *do* carry a stored MD5 are unaffected.
+        let minio_ssec_without_stored_md5 = stored_md5.is_none()
+            && request
+                .metadata
+                .contains_key(MINIO_INTERNAL_ENCRYPTION_SSEC_SEALED_KEY_HEADER);
+        if !minio_ssec_without_stored_md5 {
+            verify_ssec_key_match(key_md5, stored_md5)?;
+        }
 
         let mut material = apply_ssec_decryption_material(request.bucket, request.key, request.metadata, key, key_md5).await?;
         material.customer_key_md5 = Some(key_md5.clone());
@@ -2445,20 +2479,42 @@ async fn apply_managed_decryption_material_inner(
 ) -> Result<Option<DecryptionMaterial>, ApiError> {
     #[cfg(not(feature = "rio-v2"))]
     let _ = (bucket, key);
-    if !contains_managed_encryption_metadata(metadata) || !metadata.contains_key("x-amz-server-side-encryption") {
+    if !contains_managed_encryption_metadata(metadata) {
         return Ok(None);
     }
 
-    // Safe: presence is guaranteed by the contains_key check above.
-    let server_side_encryption = metadata.get("x-amz-server-side-encryption").cloned().unwrap_or_default();
-    let normalized_metadata = normalize_managed_metadata(metadata, Some(recode_minio_kms_context));
-
-    let encryption_type = match server_side_encryption.as_str() {
-        ServerSideEncryption::AES256 => SSEType::SseS3,
-        ServerSideEncryption::AWS_KMS => SSEType::SseKms,
-        _ => SSEType::SseS3,
+    let encryption_type = match metadata.get("x-amz-server-side-encryption").map(String::as_str) {
+        Some(ServerSideEncryption::AWS_KMS) => SSEType::SseKms,
+        Some(_) => SSEType::SseS3,
+        // MinIO never persists the public scheme header: `crypto.S3.CreateMetadata`
+        // writes only the `X-Minio-Internal-*` family and the public header is
+        // synthesized onto the response by `DecryptObjectInfo`. Requiring it here
+        // is what made every MinIO-encrypted object unreadable (backlog#1638).
+        //
+        // Inferring from the sealed-key slot is self-consistent by construction:
+        // the slot decides which header the unseal reads AND which domain string
+        // the sealing key is derived under, so a scheme that disagrees with the
+        // slot cannot silently derive a wrong key — it finds no key at all.
+        // Inferring from the KMS key id would NOT be safe: MinIO writes
+        // `-S3-Kms-Key-Id` on SSE-S3 objects too.
+        #[cfg(feature = "rio-v2")]
+        None => match infer_minio_managed_sse_type(metadata) {
+            Some(sse_type) => sse_type,
+            // Still fail-closed, and deliberately not an error raised here: the
+            // read plan independently classifies the object as encrypted from
+            // its markers and refuses to serve it without material, so an
+            // object whose scheme cannot be established never degrades into a
+            // plaintext read.
+            None => return Ok(None),
+        },
+        // Without the rio-v2 reader there is no MinIO-format read path to serve
+        // such an object with, so it stays on the fail-closed branch.
+        #[cfg(not(feature = "rio-v2"))]
+        None => return Ok(None),
     };
 
+    let normalized_metadata = normalize_managed_metadata(metadata, Some(recode_minio_kms_context));
+
     // Extract KMS key ID from metadata (optional, used for provider context)
     let kms_key_id = normalized_metadata
         .get(INTERNAL_ENCRYPTION_KEY_ID_HEADER)
@@ -2556,8 +2612,19 @@ async fn apply_managed_decryption_material_inner(
     } else {
         get_local_sse_dek_provider().await?
     };
+    // A MinIO sealed key alone does not mean MinIO wrote the object: RustFS's own
+    // writer fills MinIO's metadata slots too, while still storing a RustFS
+    // envelope in them, so neither the slot nor the header name distinguishes the
+    // two. The data key's own shape does. RustFS envelopes are strictly-parsed
+    // JSON; MinIO's builtin-KMS ciphertext is opaque bytes that match neither, so
+    // recognizing RustFS positively — and treating only the remainder as MinIO —
+    // keeps a RustFS envelope from ever reaching MinIO's decoder.
     #[cfg(feature = "rio-v2")]
-    let decrypted_data_key = if is_legacy_rustfs_managed_metadata(&normalized_metadata) {
+    let decrypted_data_key = if minio_sealed_key.is_some() && !is_rustfs_managed_data_key(&encrypted_data_key) {
+        provider
+            .decrypt_minio_sse_dek(&encrypted_data_key, &kms_key_id, &object_context)
+            .await
+    } else if is_legacy_rustfs_managed_metadata(&normalized_metadata) {
         provider
             .decrypt_legacy_sse_dek(&encrypted_data_key, &kms_key_id, &object_context)
             .await
@@ -2592,7 +2659,11 @@ async fn apply_managed_decryption_material_inner(
 
     Ok(Some(DecryptionMaterial {
         sse_type: encryption_type,
-        server_side_encryption: ServerSideEncryption::from(server_side_encryption),
+        // Synthesized from the resolved scheme rather than read back from
+        // metadata: a MinIO-written object has no stored scheme header, which is
+        // exactly why the gate above had to infer it. MinIO synthesizes the same
+        // header onto its own responses.
+        server_side_encryption: ServerSideEncryption::from(managed_sse_public_header(encryption_type).to_string()),
         kms_key_id: Some(SSEKMSKeyId::from(kms_key_id)),
         algorithm,
         customer_key_md5: None,
@@ -2659,6 +2730,30 @@ pub trait SseDekProvider: Send + Sync {
     ) -> Result<[u8; 32], ApiError> {
         self.decrypt_sse_dek(encrypted_dek, kms_key_id, context).await
     }
+
+    /// Unwrap a data key that MinIO's builtin KMS sealed.
+    ///
+    /// A separate entry point rather than a shape sniff inside
+    /// [`Self::decrypt_sse_dek`]: the caller already knows the object carries a
+    /// MinIO sealed key, and MinIO's raw ciphertext is unstructured bytes that
+    /// no parser can reliably tell apart from anything else. Routing on the
+    /// caller's knowledge keeps a RustFS envelope from ever reaching MinIO's
+    /// decoder, and vice versa.
+    ///
+    /// Defaults to refusing: only a provider holding the MinIO master secret
+    /// can serve these, and a provider that cannot must fail rather than fall
+    /// back to a decoder that would misread the bytes.
+    #[cfg(feature = "rio-v2")]
+    async fn decrypt_minio_sse_dek(
+        &self,
+        _encrypted_dek: &[u8],
+        _kms_key_id: &str,
+        _context: &ObjectEncryptionContext,
+    ) -> Result<[u8; 32], ApiError> {
+        Err(ApiError::from(StorageError::other(
+            "This KMS provider cannot unwrap a data key sealed by MinIO's builtin KMS",
+        )))
+    }
 }
 
 // ============================================================================
@@ -2797,6 +2892,163 @@ pub(crate) struct LocalSseDekProvider {
 
 const LOCAL_SSE_DEK_FORMAT_VERSION: u8 = 1;
 
+#[cfg(feature = "rio-v2")]
+/// Returns true when a managed-SSE data key is one RustFS itself wrote.
+///
+/// Both RustFS envelope shapes are strict JSON — the KMS envelope
+/// ([`rustfs_kms::is_data_key_envelope`]) and the local provider's
+/// [`LocalSseDekEnvelope`], whose `deny_unknown_fields` keeps it from accepting
+/// anything else. Recognition is deliberately positive: an unrecognized payload
+/// is left to MinIO's decoder rather than guessed at, and neither decoder is
+/// ever handed the other's format.
+fn is_rustfs_managed_data_key(encrypted_dek: &[u8]) -> bool {
+    if rustfs_kms::is_data_key_envelope(encrypted_dek) {
+        return true;
+    }
+    std::str::from_utf8(encrypted_dek)
+        .ok()
+        .is_some_and(|text| serde_json::from_str::<LocalSseDekEnvelope<'_>>(text).is_ok())
+}
+
+#[cfg(feature = "rio-v2")]
+/// Associated data MinIO binds when sealing a data key.
+///
+/// MinIO passes the object's encryption context as the AEAD's associated data,
+/// serialized as canonical JSON with sorted keys — the same canonicalization
+/// [`rustfs_kms::context_aad`] performs, which is why the context RustFS
+/// already rebuilds for the read can be reused verbatim. For SSE-S3 that
+/// context is `{bucket: "bucket/object"}`; for SSE-KMS it is whatever the
+/// request supplied, recovered from the stored MinIO context header.
+fn minio_kms_associated_data(context: &ObjectEncryptionContext) -> Result<Vec<u8>, ApiError> {
+    let mut ctx = context.encryption_context.clone();
+    ctx.entry(context.bucket.clone())
+        .or_insert_with(|| canonical_kms_bucket_path(&context.bucket, &context.object_key));
+    rustfs_kms::context_aad(&ctx)
+        .map_err(|e| ApiError::from(StorageError::other(format!("Failed to canonicalize MinIO KMS context: {e}"))))
+}
+
+#[cfg(feature = "rio-v2")]
+/// MinIO's builtin-KMS ciphertext in its JSON encoding.
+///
+/// Deliberately its own type rather than a relaxation of
+/// [`LocalSseDekEnvelope`]: widening that envelope's `deny_unknown_fields`
+/// to admit this shape would also admit malformed RustFS envelopes, which
+/// backlog#1567 requires to keep failing closed.
+#[derive(Debug, Deserialize)]
+#[serde(deny_unknown_fields)]
+struct MinioKmsCiphertextJson {
+    aead: String,
+    #[allow(
+        dead_code,
+        reason = "present in MinIO's encoding; the key is identified by metadata instead"
+    )]
+    #[serde(default)]
+    id: String,
+    iv: String,
+    nonce: String,
+    bytes: String,
+}
+
+/// Bytes of trailing randomness every MinIO builtin-KMS ciphertext carries:
+/// a 16-byte IV followed by a 12-byte nonce, *after* the sealed bytes.
+#[cfg(feature = "rio-v2")]
+const MINIO_KMS_RANDOM_LEN: usize = 28;
+#[cfg(feature = "rio-v2")]
+const MINIO_KMS_IV_LEN: usize = 16;
+
+#[cfg(feature = "rio-v2")]
+const MINIO_KMS_AEAD_AES_GCM: &str = "AES-256-GCM-HMAC-SHA-256";
+#[cfg(feature = "rio-v2")]
+const MINIO_KMS_AEAD_CHACHA20: &str = "ChaCha20Poly1305";
+
+#[cfg(feature = "rio-v2")]
+/// Unwrap a data key sealed by MinIO's builtin (static-secret) KMS.
+///
+/// The wire format is `sealed_bytes || iv[16] || nonce[12]` — the randomness
+/// trails the ciphertext rather than leading it, and MinIO's own decoder
+/// normalizes its legacy JSON encoding into exactly that byte order before
+/// opening it (`internal/kms/secret-key.go`, `parseCiphertext`). A raw
+/// (non-JSON) ciphertext is AES-256-GCM by definition there; the JSON form
+/// names its algorithm.
+///
+/// The sealing key is derived per ciphertext rather than being the master key:
+/// `HMAC-SHA256(master, iv)` for AES-256-GCM, `HChaCha20(master, iv)` for
+/// ChaCha20-Poly1305. The encryption context is bound as associated data.
+fn decrypt_minio_kms_data_key(encrypted_dek: &[u8], master_key: &[u8; 32], aad: &[u8]) -> Result<[u8; 32], ApiError> {
+    let (body, algorithm) = match std::str::from_utf8(encrypted_dek) {
+        // MinIO only treats a payload as JSON when it both starts and ends like
+        // an object, and falls back to the raw layout when it does not parse —
+        // mirrored here so a ciphertext that merely looks like JSON is not
+        // rejected outright.
+        Ok(text)
+            if text.starts_with('{')
+                && text.ends_with('}')
+                && let Ok(json) = serde_json::from_str::<MinioKmsCiphertextJson>(text) =>
+        {
+            let decode = |what: &str, value: &str| -> Result<Vec<u8>, ApiError> {
+                BASE64_STANDARD
+                    .decode(value)
+                    .map_err(|e| ApiError::from(StorageError::other(format!("Invalid MinIO KMS {what}: {e}"))))
+            };
+            let mut body = decode("ciphertext", &json.bytes)?;
+            body.extend_from_slice(&decode("iv", &json.iv)?);
+            body.extend_from_slice(&decode("nonce", &json.nonce)?);
+            (body, json.aead)
+        }
+        _ => (encrypted_dek.to_vec(), MINIO_KMS_AEAD_AES_GCM.to_string()),
+    };
+
+    if body.len() <= MINIO_KMS_RANDOM_LEN {
+        return Err(ApiError::from(StorageError::other(
+            "MinIO KMS ciphertext is too short to carry its IV and nonce",
+        )));
+    }
+    let (sealed, random) = body.split_at(body.len() - MINIO_KMS_RANDOM_LEN);
+    let (iv, nonce) = random.split_at(MINIO_KMS_IV_LEN);
+
+    let plaintext = match algorithm.as_str() {
+        MINIO_KMS_AEAD_AES_GCM => {
+            use aes_gcm::{Aes256Gcm, KeyInit, aead::Aead};
+            let mut mac = HmacSha256::new_from_slice(master_key)
+                .map_err(|_| ApiError::from(StorageError::other("MinIO KMS sealing key derivation failed")))?;
+            mac.update(iv);
+            let sealing_key: [u8; 32] = mac.finalize().into_bytes().into();
+            let cipher = Aes256Gcm::new_from_slice(&sealing_key)
+                .map_err(|_| ApiError::from(StorageError::other("MinIO KMS sealing key is not a valid AES-256 key")))?;
+            let nonce = aes_gcm::Nonce::try_from(nonce)
+                .map_err(|_| ApiError::from(StorageError::other("MinIO KMS nonce is not 12 bytes")))?;
+            cipher.decrypt(&nonce, aes_gcm::aead::Payload { msg: sealed, aad })
+        }
+        MINIO_KMS_AEAD_CHACHA20 => {
+            use chacha20poly1305::{KeyInit, XChaCha20Poly1305, aead::Aead};
+            // MinIO derives this branch's key with HChaCha20 over the 16-byte
+            // IV, which is exactly XChaCha20-Poly1305's own construction, so the
+            // extended-nonce cipher does the derivation rather than hand-rolling it.
+            let mut extended = Vec::with_capacity(MINIO_KMS_IV_LEN + nonce.len());
+            extended.extend_from_slice(iv);
+            extended.extend_from_slice(nonce);
+            let cipher = XChaCha20Poly1305::new_from_slice(master_key)
+                .map_err(|_| ApiError::from(StorageError::other("MinIO KMS master key is not a valid ChaCha20 key")))?;
+            let nonce = chacha20poly1305::XNonce::try_from(extended.as_slice())
+                .map_err(|_| ApiError::from(StorageError::other("MinIO KMS extended nonce is not 24 bytes")))?;
+            cipher.decrypt(&nonce, chacha20poly1305::aead::Payload { msg: sealed, aad })
+        }
+        other => {
+            return Err(ApiError::from(StorageError::other(format!(
+                "Unsupported MinIO KMS AEAD algorithm: {other}"
+            ))));
+        }
+    }
+    // An AEAD failure here is authentication, not a decode slip: a wrong master
+    // key, a tampered ciphertext, and an encryption context that does not match
+    // what sealed it all land here and must all fail closed.
+    .map_err(|_| ApiError::from(StorageError::other("MinIO KMS data key failed authentication")))?;
+
+    plaintext.try_into().map_err(|value: Vec<u8>| {
+        ApiError::from(StorageError::other(format!("MinIO KMS data key must be 32 bytes, got {}", value.len())))
+    })
+}
+
 #[derive(Debug, Deserialize, Serialize)]
 #[serde(deny_unknown_fields)]
 struct LocalSseDekEnvelope<'a> {
@@ -3013,6 +3265,17 @@ impl SseDekProvider for LocalSseDekProvider {
         let dek = Self::decrypt_dek(encrypted_dek_str, self.master_key)?;
         Ok(dek)
     }
+
+    #[cfg(feature = "rio-v2")]
+    async fn decrypt_minio_sse_dek(
+        &self,
+        encrypted_dek: &[u8],
+        _kms_key_id: &str,
+        context: &ObjectEncryptionContext,
+    ) -> Result<[u8; 32], ApiError> {
+        let aad = minio_kms_associated_data(context)?;
+        decrypt_minio_kms_data_key(encrypted_dek, &self.master_key, &aad)
+    }
 }
 
 // ============================================================================
@@ -3201,6 +3464,23 @@ fn is_legacy_rustfs_managed_metadata(metadata: &HashMap<String, String>) -> bool
         && !metadata.contains_key(MINIO_INTERNAL_ENCRYPTION_KMS_SEALED_KEY_HEADER)
 }
 
+#[cfg(feature = "rio-v2")]
+#[cfg(feature = "rio-v2")]
+/// Infer the managed SSE scheme from the MinIO sealed-key slot that is present.
+///
+/// Returns `None` when no managed MinIO slot is present, which keeps callers on
+/// their fail-closed path. SSE-C is not a managed scheme and is handled by the
+/// SSE-C read path, so its slot is not considered here.
+fn infer_minio_managed_sse_type(metadata: &HashMap<String, String>) -> Option<SSEType> {
+    if metadata.contains_key(MINIO_INTERNAL_ENCRYPTION_S3_SEALED_KEY_HEADER) {
+        Some(SSEType::SseS3)
+    } else if metadata.contains_key(MINIO_INTERNAL_ENCRYPTION_KMS_SEALED_KEY_HEADER) {
+        Some(SSEType::SseKms)
+    } else {
+        None
+    }
+}
+
 #[cfg(feature = "rio-v2")]
 fn parse_minio_managed_sealed_key(
     metadata: &HashMap<String, String>,
@@ -4483,6 +4763,43 @@ mod tests {
     }
 
     #[cfg(feature = "rio-v2")]
+    /// A stored customer-key MD5 must still be compared against the one the
+    /// request presents.
+    ///
+    /// The read path skips that comparison for MinIO SSE-C objects, which store
+    /// no MD5. Nothing pinned the check for objects that *do* store one —
+    /// disabling it outright left this file's 115 tests green — so a later
+    /// widening of that skip would have gone unnoticed. The mismatch has to be
+    /// refused here, at the request boundary, rather than surfacing later as a
+    /// decryption failure.
+    #[tokio::test]
+    async fn ssec_stored_md5_mismatch_is_refused_when_an_md5_is_stored() {
+        let key = SSECustomerKey::from(BASE64_STANDARD.encode([0x11u8; 32]));
+        let provided_md5 = SSECustomerKeyMD5::from(md5_base64(&[0x11u8; 32]));
+
+        let metadata = HashMap::from([
+            (SSEC_ALGORITHM_HEADER.to_string(), "AES256".to_string()),
+            // A stored MD5 that belongs to a different key.
+            ("x-amz-server-side-encryption-customer-key-md5".to_string(), md5_base64(&[0x22u8; 32])),
+        ]);
+
+        let error = sse_decryption(DecryptionRequest {
+            bucket: "bucket",
+            key: "object",
+            metadata: &metadata,
+            sse_customer_key: Some(&key),
+            sse_customer_key_md5: Some(&provided_md5),
+            principal: None,
+        })
+        .await
+        .expect_err("a stored MD5 that does not match the request must be refused");
+
+        assert!(
+            format!("{error:?}").contains("did not match"),
+            "expected the parameter-mismatch refusal, got {error:?}"
+        );
+    }
+
     #[tokio::test]
     async fn test_sse_kms_roundtrip_persists_and_uses_minio_context() {
         use rustfs_kms::types::{CreateKeyRequest, KeyUsage};