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/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 }