To encrypt/decrypt a key, the top-level functions `openpgp.encryptKey` and
`openpgp.decryptKey` should be used instead: these don't mutate the key;
instead, they either return a new encrypted/decrypted key object or throw an
error.
With `Key.prototype.encrypt` and `decrypt`, which mutated the key, it was
possible to end up in an inconsistent state if some (sub)keys could be
decrypted but others couldn't, they would both mutate the key and throw an
error, which is unexpected.
Note that the `keyID` parameter is not supported by `encryptKey`/`decryptKey`,
since partial key decryption is not recommended. If you still need to decrypt
a single subkey or primary key `k`, you can call `k.keyPacket.decrypt(...)`,
followed by `k.keyPacket.validate(...)`. Similarly, for encryption, call
`k.keyPacket.encrypt(...)`.
Additionally, `openpgp.generateKey` now requires `options.userIDs` again,
since otherwise the key is basically unusable. This was a regression from v4,
since we now allow parsing keys without user IDs (but still not using them).
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