Say there exists some individual who is in danger of prosecution for federal crimes, though it's not yet certain what the exact charges will be, nor even whether the indictment will be made at all. An outgoing president wants to ensure this individual remains protected, even after he leaves office, and so considers issuing a blanket pardon to this individual for any and all crimes they may have committed (similar to how Ford pardoned Nixon). However, the president knows that such a move would be wildly unpopular and so wants to mitigate the risk of tarnishing his own reputation and legacy.
In such a scenario, could the president issue a pardon in the presence of witnesses who agree to hold it confidential, and then not publish it? Copies of the pardon would be given to the individual in question, and maybe also to the soon-to-be-ex-president and the witnesses. The individual does not disclose the pardon unless and until they are actually charged with or indicted for a crime, thus sparing the president any embarrassment in the event that the charge or indictment never happens. The authenticity of the pardon can be confirmed, if necessary, by checking with the ex-president and/or the witnesses.
Is there any law which prevents this scenario from playing out? If so, would it invalidate the pardon itself, or would it just constitute a separate crime for those party to its clandestine issuance?