Where do I put my legacy key on iPad OS 15?
it should be the same place as described at Import backup to iOS
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the last imported key overwrites the existing “primary” key and will be used for Autocrypt encrypting/signing. old keys will still be used for decryption, however.
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to import legacy keys that should not become the primary key, you have to add the word
legacysomewhere to the filename. in this case the key is only used for decryption.
And also, if I am chatting with someone who is using my old, legacy key, will that chat continue to use that old key?
old key will be used for decryption only, there is no way to encrypt to different primary keys for different users.
there is only one primary key at any point, that key is used for Autocrypt encrypting/signing on outgoing messages.