crypto: add functions for sealing/unsealing the etag for SSE (#6618)

This commit adds two functions for sealing/unsealing the
etag (a.k.a. content MD5) in case of SSE single-part upload.

Sealing the ETag is neccessary in case of SSE-S3 to preserve
the security guarantees. In case of SSE-S3 AWS returns the
content-MD5 of the plaintext object as ETag. However, we
must not store the MD5 of the plaintext for encrypted objects.
Otherwise it becomes possible for an attacker to detect
equal/non-equal encrypted objects. Therefore we encrypt
the ETag before storing on the backend. But we only need
to encrypt the ETag (content-MD5) if the client send it -
otherwise the client cannot verify it anyway.

cmd/crypto/key.go

@@ -140,3 +140,37 @@ func (key ObjectKey) DerivePartKey(id uint32) (partKey [32]byte) {
 	mac.Sum(partKey[:0])
 	return partKey
 }
+
+// SealETag seals the etag using the object key.
+// It does not encrypt empty ETags because such ETags indicate
+// that the S3 client hasn't sent an ETag = MD5(object) and
+// the backend can pick an ETag value.
+func (key ObjectKey) SealETag(etag []byte) []byte {
+	if len(etag) == 0 { // don't encrypt empty ETag - only if client sent ETag = MD5(object)
+		return etag
+	}
+	var buffer bytes.Buffer
+	mac := hmac.New(sha256.New, key[:])
+	mac.Write([]byte("SSE-etag"))
+	if _, err := sio.Encrypt(&buffer, bytes.NewReader(etag), sio.Config{Key: mac.Sum(nil)}); err != nil {
+		logger.CriticalIf(context.Background(), errors.New("Unable to encrypt ETag using object key"))
+	}
+	return buffer.Bytes()
+}
+
+// UnsealETag unseals the etag using the provided object key.
+// It does not try to decrypt the ETag if len(etag) == 16
+// because such ETags indicate that the S3 client hasn't sent
+// an ETag = MD5(object) and the backend has picked an ETag value.
+func (key ObjectKey) UnsealETag(etag []byte) ([]byte, error) {
+	if !IsETagSealed(etag) {
+		return etag, nil
+	}
+	var buffer bytes.Buffer
+	mac := hmac.New(sha256.New, key[:])
+	mac.Write([]byte("SSE-etag"))
+	if _, err := sio.Decrypt(&buffer, bytes.NewReader(etag), sio.Config{Key: mac.Sum(nil)}); err != nil {
+		return nil, err
+	}
+	return buffer.Bytes(), nil
+}

cmd/crypto/key_test.go

@@ -166,3 +166,31 @@ func TestDerivePartKey(t *testing.T) {
 		}
 	}
 }
+
+var sealUnsealETagTests = []string{
+	"",
+	"90682b8e8cc7609c",
+	"90682b8e8cc7609c4671e1d64c73fc30",
+	"90682b8e8cc7609c4671e1d64c73fc307fb3104f",
+}
+
+func TestSealETag(t *testing.T) {
+	var key ObjectKey
+	for i := range key {
+		key[i] = byte(i)
+	}
+	for i, etag := range sealUnsealETagTests {
+		tag, err := hex.DecodeString(etag)
+		if err != nil {
+			t.Errorf("Test %d: failed to decode etag: %s", i, err)
+		}
+		sealedETag := key.SealETag(tag)
+		unsealedETag, err := key.UnsealETag(sealedETag)
+		if err != nil {
+			t.Errorf("Test %d: failed to decrypt etag: %s", i, err)
+		}
+		if !bytes.Equal(unsealedETag, tag) {
+			t.Errorf("Test %d: unsealed etag does not match: got %s - want %s", i, hex.EncodeToString(unsealedETag), etag)
+		}
+	}
+}

cmd/crypto/metadata.go

@@ -219,3 +219,6 @@ func (ssec) ParseMetadata(metadata map[string]string) (sealedKey SealedKey, err
 	copy(sealedKey.Key[:], encryptedKey)
 	return sealedKey, nil
 }
+
+// IsETagSealed returns true if the etag seems to be encrypted.
+func IsETagSealed(etag []byte) bool { return len(etag) > 16 }

cmd/crypto/metadata_test.go

@@ -17,6 +17,7 @@ package crypto
 import (
 	"bytes"
 	"encoding/base64"
+	"encoding/hex"
 	"testing"
 
 	"github.com/minio/minio/cmd/logger"
@@ -364,3 +365,25 @@ func TestSSECCreateMetadata(t *testing.T) {
 	}()
 	_ = SSEC.CreateMetadata(nil, SealedKey{Algorithm: InsecureSealAlgorithm})
 }
+
+var isETagSealedTests = []struct {
+	ETag     string
+	IsSealed bool
+}{
+	{ETag: "", IsSealed: false},                                                                                                // 0
+	{ETag: "90682b8e8cc7609c4671e1d64c73fc30", IsSealed: false},                                                                // 1
+	{ETag: "f201040c9dc593e39ea004dc1323699bcd", IsSealed: true},                                                               // 2 not valid ciphertext but looks like sealed ETag
+	{ETag: "20000f00fba2ee2ae4845f725964eeb9e092edfabc7ab9f9239e8344341f769a51ce99b4801b0699b92b16a72fa94972", IsSealed: true}, // 3
+}
+
+func TestIsETagSealed(t *testing.T) {
+	for i, test := range isETagSealedTests {
+		etag, err := hex.DecodeString(test.ETag)
+		if err != nil {
+			t.Errorf("Test %d: failed to decode etag: %s", i, err)
+		}
+		if sealed := IsETagSealed(etag); sealed != test.IsSealed {
+			t.Errorf("Test %d: got %v - want %v", i, sealed, test.IsSealed)
+		}
+	}
+}