crypto: update SSE-S3 and SSE-C key derivation (#6152)

This commit updates the key derivation to reflect the
latest change of crypto/doc.go. This includes handling
the insecure legacy KDF.

Since #6064 is fixed, the 3. test case for object key
generation is enabled again.

cmd/crypto/error.go

@@ -16,6 +16,13 @@ package crypto
 
 import "errors"
 
+// Error is the generic type for any error happening during decrypting
+// an object. It indicates that the object itself or its metadata was
+// modified accidentally or maliciously.
+type Error struct{ msg string }
+
+func (e Error) Error() string { return e.msg }
+
 var (
 	// ErrInvalidEncryptionMethod indicates that the specified SSE encryption method
 	// is not supported.