improve multipart decryption (#20324)

This commit simplifies and optimizes the decryption of large (multipart)
objects. This PR does two things:
 
- Re-write the init logic for the decryption reader
- Reduce the number of OEK decryptions

Before, the init logic copied some SSE HTTP request headers to 
parse them later. This is simplified to parsing them right away. This
removes some fields from the decryption reader struct.

Further, the decryption reader decrypted the OEK using the client-provided 
key (SSE-C) or the KMS (SSE-S3 / SSE-KMS) for each part. This is redundant 
since the OEK is the same for all parts. In particular, a KMS call might be a 
network request. Now, the OEK is decrypted once for the entire multipart object.

This should improve latency when reading encrypted multipart objects 
and reduce requests to the KMS.

Signed-off-by: Andreas Auernhammer <github@aead.dev>

cmd/encryption-v1.go

@@ -601,28 +601,44 @@ func DecryptBlocksRequestR(inputReader io.Reader, h http.Header, seqNumber uint3
 	partEncRelOffset := int64(seqNumber) * (SSEDAREPackageBlockSize + SSEDAREPackageMetaSize)
 
 	w := &DecryptBlocksReader{
-		reader:            inputReader,
-		startSeqNum:       seqNumber,
-		partDecRelOffset:  partDecRelOffset,
-		partEncRelOffset:  partEncRelOffset,
-		parts:             oi.Parts,
-		partIndex:         partStart,
-		header:            h,
-		bucket:            bucket,
-		object:            object,
-		customerKeyHeader: h.Get(xhttp.AmzServerSideEncryptionCustomerKey),
-		copySource:        copySource,
-		metadata:          cloneMSS(oi.UserDefined),
+		reader:           inputReader,
+		startSeqNum:      seqNumber,
+		partDecRelOffset: partDecRelOffset,
+		partEncRelOffset: partEncRelOffset,
+		parts:            oi.Parts,
+		partIndex:        partStart,
+	}
+
+	// In case of SSE-C, we have to decrypt the OEK using the client-provided key.
+	// In case of a SSE-C server-side copy, the client might provide two keys,
+	// one for the source and one for the target. This reader is the source.
+	var ssecClientKey []byte
+	if crypto.SSEC.IsEncrypted(oi.UserDefined) {
+		if copySource && crypto.SSECopy.IsRequested(h) {
+			key, err := crypto.SSECopy.ParseHTTP(h)
+			if err != nil {
+				return nil, err
+			}
+			ssecClientKey = key[:]
+		} else {
+			key, err := crypto.SSEC.ParseHTTP(h)
+			if err != nil {
+				return nil, err
+			}
+			ssecClientKey = key[:]
+		}
 	}
 
-	if w.copySource {
-		w.customerKeyHeader = h.Get(xhttp.AmzServerSideEncryptionCopyCustomerKey)
+	// Decrypt the OEK once and reuse it for all subsequent parts.
+	objectEncryptionKey, err := decryptObjectMeta(ssecClientKey, bucket, object, oi.UserDefined)
+	if err != nil {
+		return nil, err
 	}
+	w.objectEncryptionKey = objectEncryptionKey
 
 	if err := w.buildDecrypter(w.parts[w.partIndex].Number); err != nil {
 		return nil, err
 	}
-
 	return w, nil
 }
 
@@ -638,48 +654,17 @@ type DecryptBlocksReader struct {
 	// Current part index
 	partIndex int
 	// Parts information
-	parts          []ObjectPartInfo
-	header         http.Header
-	bucket, object string
-	metadata       map[string]string
+	parts []ObjectPartInfo
 
+	objectEncryptionKey                []byte
 	partDecRelOffset, partEncRelOffset int64
-
-	copySource bool
-	// Customer Key
-	customerKeyHeader string
 }
 
 func (d *DecryptBlocksReader) buildDecrypter(partID int) error {
-	m := cloneMSS(d.metadata)
-	// Initialize the first decrypter; new decrypters will be
-	// initialized in Read() operation as needed.
-	var key []byte
-	var err error
-	if d.copySource {
-		if crypto.SSEC.IsEncrypted(d.metadata) {
-			d.header.Set(xhttp.AmzServerSideEncryptionCopyCustomerKey, d.customerKeyHeader)
-			key, err = ParseSSECopyCustomerRequest(d.header, d.metadata)
-		}
-	} else {
-		if crypto.SSEC.IsEncrypted(d.metadata) {
-			d.header.Set(xhttp.AmzServerSideEncryptionCustomerKey, d.customerKeyHeader)
-			key, err = ParseSSECustomerHeader(d.header)
-		}
-	}
-	if err != nil {
-		return err
-	}
-
-	objectEncryptionKey, err := decryptObjectMeta(key, d.bucket, d.object, m)
-	if err != nil {
-		return err
-	}
-
 	var partIDbin [4]byte
 	binary.LittleEndian.PutUint32(partIDbin[:], uint32(partID)) // marshal part ID
 
-	mac := hmac.New(sha256.New, objectEncryptionKey) // derive part encryption key from part ID and object key
+	mac := hmac.New(sha256.New, d.objectEncryptionKey) // derive part encryption key from part ID and object key
 	mac.Write(partIDbin[:])
 	partEncryptionKey := mac.Sum(nil)