The native PKCS11 mechanism which performs padding report error code equivalents to IllegalBlockSizeException when the padding bytes are missing. The internal PKCS5Padding impl used by SunPKCS11 provider should match this behavior so the same exception is thrown regardless whether users are using native PKCS11 mechs which supports padding or the combination of (native PKCS11 mechs which does NOT support padding + PKCS5Padding impl from SunPKCS11 provider).