DLLs are not encrypted, so there's no "standard". You can use what you wish. Just keep in mind that when you want to execute them, you'll obviously have to decrypt them. Windows will not decrypt DLLs for you, and you cannot run encrypted DLLs for obvious reasons.
If you're searching for a "security" feature, then this is not one. First, you'll have to include key and algorithm for decryption (from which the encryption algorithm can be also deduced, btw) on on your application, and also anyone can retrieve the unencrypted DLL from RAM.