Dissector preferences with alternative representations

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I've got a custom dissector written in Lua. The protocol has encryption, which needs a 16-byte encryption key to decode, so I've made that a protocol preference, with the key entered as 32 hex digits. That's working fine.

I'd like to have the possibility of entering a protocol encryption key as a keyphrase instead, where the encryption key is a hash output from the keyphrase (and non-reversible).

How could dissector preferences be set up so that a user could enter either an encryption key as 32 hex digits, or enter a passphrase that is converted to 32 hex digits? Could it be:

1. two separate text fields (and the key as 32 hex digits is automatically updated when the user types a keyphrase)
2. one text field plus an enum box to choose whether it's a hex key vs keyphrase
3. some other strategy?