fix: honor SSE-C chunk offsets in decryption for large chunked uploads (#8216)

* fix: honor SSE-C chunk offsets in decryption for large chunked uploads

Fixes issue #8215 where SSE-C decryption for large objects could corrupt
data by ignoring per-chunk PartOffset values.

Changes:
- Add TestSSECLargeObjectChunkReassembly unit test to verify correct
  decryption of 19MB object split into 8MB chunks using PartOffset
- Update decryptSSECChunkView and createMultipartSSECDecryptedReaderDirect
  to extract PartOffset from SSE-C metadata and pass to
  CreateSSECDecryptedReaderWithOffset for offset-aware decryption
- Fix createCTRStreamWithOffset to use calculateIVWithOffset for proper
  block-aligned counter advancement, matching SSE-KMS/S3 behavior
- Update comments to clarify SSE-C IV handling uses per-chunk offsets
  (unlike base IV approach used by KMS/S3)

All tests pass: go test ./weed/s3api ✓

* fix: close chunkReader on error paths in createMultipartSSECDecryptedReader

Address resource leak issue reported in PR #8216: ensure chunkReader is
properly closed before returning on all error paths, including:
- DeserializeSSECMetadata failures
- IV decoding errors
- Invalid PartOffset values
- SSE-C reader creation failures
- Missing per-chunk metadata

This prevents leaking network connections and file handles during
SSE-C multipart decryption error scenarios.

* docs: clarify SSE-C IV handling in decryptSSECChunkView comment

Replace misleading warning 'Do NOT call calculateIVWithOffset' with
accurate explanation that:
- CreateSSECDecryptedReaderWithOffset internally uses calculateIVWithOffset
  to advance the CTR counter to reach PartOffset
- calculateIVWithOffset is applied only to the per-part IV, NOT to derive
  a global base IV for all parts
- This differs fundamentally from SSE-KMS/SSE-S3 which use base IV +
  calculateIVWithOffset(ChunkOffset)

This clarifies the IV advancement mechanism while contrasting it with
the base IV approach used by other encryption schemes.

weed/s3api/s3_sse_c.go

@@ -263,18 +263,13 @@ func CreateSSECDecryptedReaderWithOffset(r io.Reader, customerKey *SSECustomerKe
 
 // createCTRStreamWithOffset creates a CTR stream positioned at a specific counter offset
 func createCTRStreamWithOffset(block cipher.Block, iv []byte, counterOffset uint64) cipher.Stream {
-	// Create a copy of the IV to avoid modifying the original
-	offsetIV := make([]byte, len(iv))
-	copy(offsetIV, iv)
-
-	// Calculate the counter offset in blocks (AES block size is 16 bytes)
-	blockOffset := counterOffset / 16
-
-	// Add the block offset to the counter portion of the IV
-	// In AES-CTR, the last 8 bytes of the IV are typically used as the counter
-	addCounterToIV(offsetIV, blockOffset)
-
-	return cipher.NewCTR(block, offsetIV)
+	adjustedIV, skip := calculateIVWithOffset(iv, int64(counterOffset))
+	stream := cipher.NewCTR(block, adjustedIV)
+	if skip > 0 {
+		dummy := make([]byte, skip)
+		stream.XORKeyStream(dummy, dummy)
+	}
+	return stream
 }
 
 // addCounterToIV adds a counter value to the IV (treating last 8 bytes as big-endian counter)

weed/s3api/s3_sse_multipart_test.go

@@ -422,6 +422,58 @@ func TestMultipartSSEMixedScenarios(t *testing.T) {
 	})
 }
 
+func TestSSECLargeObjectChunkReassembly(t *testing.T) {
+	keyPair := GenerateTestSSECKey(1)
+	customerKey := &SSECustomerKey{
+		Algorithm: "AES256",
+		Key:       keyPair.Key,
+		KeyMD5:    keyPair.KeyMD5,
+	}
+
+	const chunkSize = 8 * 1024 * 1024 // matches putToFiler chunk size
+	totalSize := chunkSize*2 + 3*1024*1024
+	plaintext := make([]byte, totalSize)
+	for i := range plaintext {
+		plaintext[i] = byte(i % 251)
+	}
+
+	encryptedReader, iv, err := CreateSSECEncryptedReader(bytes.NewReader(plaintext), customerKey)
+	if err != nil {
+		t.Fatalf("Failed to create encrypted reader: %v", err)
+	}
+	encryptedData, err := io.ReadAll(encryptedReader)
+	if err != nil {
+		t.Fatalf("Failed to read encrypted data: %v", err)
+	}
+
+	var reconstructed bytes.Buffer
+	offset := int64(0)
+	for offset < int64(len(encryptedData)) {
+		end := offset + chunkSize
+		if end > int64(len(encryptedData)) {
+			end = int64(len(encryptedData))
+		}
+
+		chunkIV := make([]byte, len(iv))
+		copy(chunkIV, iv)
+		chunkReader := bytes.NewReader(encryptedData[offset:end])
+		decryptedReader, decErr := CreateSSECDecryptedReaderWithOffset(chunkReader, customerKey, chunkIV, uint64(offset))
+		if decErr != nil {
+			t.Fatalf("Failed to create decrypted reader for offset %d: %v", offset, decErr)
+		}
+		decryptedChunk, decErr := io.ReadAll(decryptedReader)
+		if decErr != nil {
+			t.Fatalf("Failed to read decrypted chunk at offset %d: %v", offset, decErr)
+		}
+		reconstructed.Write(decryptedChunk)
+		offset = end
+	}
+
+	if !bytes.Equal(reconstructed.Bytes(), plaintext) {
+		t.Fatalf("Reconstructed data mismatch: expected %d bytes, got %d", len(plaintext), reconstructed.Len())
+	}
+}
+
 // TestMultipartSSEPerformance tests performance characteristics of SSE with multipart
 func TestMultipartSSEPerformance(t *testing.T) {
 	if testing.Short() {

weed/s3api/s3api_object_handlers.go

@@ -1512,17 +1512,18 @@ func writeZeroBytes(w io.Writer, n int64) error {
 //
 // IV Handling for SSE-C:
 // ----------------------
-// SSE-C multipart encryption (see lines 2772-2781) differs fundamentally from SSE-KMS/SSE-S3:
+// SSE-C multipart encryption differs from SSE-KMS/SSE-S3:
 //
-// 1. Encryption: CreateSSECEncryptedReader generates a RANDOM IV per part/chunk
-//   - Each part starts with a fresh random IV
+// 1. Encryption: CreateSSECEncryptedReader generates a RANDOM IV per part
+//   - Each part starts with a fresh random IV (NOT derived from a base IV)
 //   - CTR counter starts from 0 for each part: counter₀, counter₁, counter₂, ...
-//   - PartOffset is stored in metadata but NOT applied during encryption
+//   - PartOffset is stored in metadata to describe where this chunk sits in that encrypted stream
 //
-// 2. Decryption: Use the stored IV directly WITHOUT offset adjustment
-//   - The stored IV already represents the start of this part's encryption
-//   - Applying calculateIVWithOffset would shift to counterₙ, misaligning the keystream
-//   - Result: XOR with wrong keystream = corrupted plaintext
+// 2. Decryption: Use the stored per-part IV and advance the CTR by PartOffset
+//   - CreateSSECDecryptedReaderWithOffset internally uses calculateIVWithOffset to advance
+//     the CTR counter to reach PartOffset within the per-part encrypted stream
+//   - calculateIVWithOffset is applied to the per-part IV, NOT to derive a global base IV
+//   - Do NOT compute a single base IV for all parts (unlike SSE-KMS/SSE-S3)
 //
 // This contrasts with SSE-KMS/SSE-S3 which use: base IV + calculateIVWithOffset(ChunkOffset)
 func (s3a *S3ApiServer) decryptSSECChunkView(ctx context.Context, fileChunk *filer_pb.FileChunk, chunkView *filer.ChunkView, customerKey *SSECustomerKey) (io.Reader, error) {
@@ -1544,11 +1545,14 @@ func (s3a *S3ApiServer) decryptSSECChunkView(ctx context.Context, fileChunk *fil
 			return nil, fmt.Errorf("failed to fetch full chunk: %w", err)
 		}
 
-		// CRITICAL: Use stored IV directly WITHOUT offset adjustment
-		// The stored IV is the random IV used at encryption time for this specific part
-		// SSE-C does NOT apply calculateIVWithOffset during encryption, so we must not apply it during decryption
-		// (See documentation above and at lines 2772-2781 for detailed explanation)
-		decryptedReader, decryptErr := CreateSSECDecryptedReader(fullChunkReader, customerKey, chunkIV)
+		partOffset := ssecMetadata.PartOffset
+		if partOffset < 0 {
+			fullChunkReader.Close()
+			return nil, fmt.Errorf("invalid SSE-C part offset %d for chunk %s", partOffset, chunkView.FileId)
+		}
+
+		// Use stored IV and advance CTR stream by PartOffset within the encrypted stream
+		decryptedReader, decryptErr := CreateSSECDecryptedReaderWithOffset(fullChunkReader, customerKey, chunkIV, uint64(partOffset))
 		if decryptErr != nil {
 			fullChunkReader.Close()
 			return nil, fmt.Errorf("failed to create decrypted reader: %w", decryptErr)
@@ -2844,15 +2848,20 @@ func (s3a *S3ApiServer) createMultipartSSECDecryptedReaderDirect(ctx context.Con
 
 			// Note: SSE-C multipart behavior (differs from SSE-KMS/SSE-S3):
 			// - Upload: CreateSSECEncryptedReader generates RANDOM IV per part (no base IV + offset)
-			// - Metadata: PartOffset is stored but not used during encryption
-			// - Decryption: Use stored random IV directly (no offset adjustment needed)
+			// - Metadata: PartOffset tracks position within the encrypted stream
+			// - Decryption: Use stored IV and advance CTR stream by PartOffset
 			//
 			// This differs from:
 			// - SSE-KMS/SSE-S3: Use base IV + calculateIVWithOffset(partOffset) during encryption
 			// - CopyObject: Applies calculateIVWithOffset to SSE-C (which may be incorrect)
 			//
 			// TODO: Investigate CopyObject SSE-C PartOffset handling for consistency
-			decryptedChunkReader, decErr := CreateSSECDecryptedReader(chunkReader, customerKey, chunkIV)
+			partOffset := ssecMetadata.PartOffset
+			if partOffset < 0 {
+				chunkReader.Close()
+				return nil, fmt.Errorf("invalid SSE-C part offset %d for chunk %s", partOffset, chunk.GetFileIdString())
+			}
+			decryptedChunkReader, decErr := CreateSSECDecryptedReaderWithOffset(chunkReader, customerKey, chunkIV, uint64(partOffset))
 			if decErr != nil {
 				chunkReader.Close()
 				return nil, fmt.Errorf("failed to decrypt chunk: %v", decErr)
@@ -3235,26 +3244,32 @@ func (s3a *S3ApiServer) createMultipartSSECDecryptedReader(r *http.Request, prox
 				// Deserialize the SSE-C metadata stored in the unified metadata field
 				ssecMetadata, decErr := DeserializeSSECMetadata(chunk.GetSseMetadata())
 				if decErr != nil {
+					chunkReader.Close()
 					return nil, fmt.Errorf("failed to deserialize SSE-C metadata for chunk %s: %v", chunk.GetFileIdString(), decErr)
 				}
 
 				// Decode the IV from the metadata
 				iv, ivErr := base64.StdEncoding.DecodeString(ssecMetadata.IV)
 				if ivErr != nil {
+					chunkReader.Close()
 					return nil, fmt.Errorf("failed to decode IV for SSE-C chunk %s: %v", chunk.GetFileIdString(), ivErr)
 				}
 
-				// Note: For multipart SSE-C, each part was encrypted with offset=0
-				// So we use the stored IV directly without offset adjustment
-				// PartOffset is stored for informational purposes, but encryption uses offset=0
-				chunkIV := iv
+				partOffset := ssecMetadata.PartOffset
+				if partOffset < 0 {
+					chunkReader.Close()
+					return nil, fmt.Errorf("invalid SSE-C part offset %d for chunk %s", partOffset, chunk.GetFileIdString())
+				}
 
-				decryptedReader, decErr := CreateSSECDecryptedReader(chunkReader, customerKey, chunkIV)
+				// Use stored IV and advance CTR stream by PartOffset within the encrypted stream
+				decryptedReader, decErr := CreateSSECDecryptedReaderWithOffset(chunkReader, customerKey, iv, uint64(partOffset))
 				if decErr != nil {
+					chunkReader.Close()
 					return nil, fmt.Errorf("failed to create SSE-C decrypted reader for chunk %s: %v", chunk.GetFileIdString(), decErr)
 				}
 				readers = append(readers, decryptedReader)
 			} else {
+				chunkReader.Close()
 				return nil, fmt.Errorf("SSE-C chunk %s missing required metadata", chunk.GetFileIdString())
 			}
 		} else {