I need to edit submissions on a form that is encrypted.
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Submission encryption is for contexts where you don't trust your server or you absolutely need to guarantee that the data is only accessible to a set number of individuals. Generally when those are true, ODK Central is only used for initial data capture and then the data is stored somewhere else trusted for further work. That's where you would do your edits rather than within Central.
If you feel like you'd like both submission encryption and Central-based edits, please let us know a little bit about your context and needs.