[vnuge]

Hard to say how relevant that is. DMs are simply a collection of events sitting on a relay. It's not really a mutual tunnel, most clients implement nip44 via nip19 (giftwrap DMs) so your ni04 message wouldn't likely make it to them. It's not considered backward compatible such that you could send a user a DM, then cause their client to downgrade to the DM scheme that uses nip04.

It's also worth noting, the user _must_ be made aware of the encryption method that was used, their "signer" application, which is also responsible for encryption and decryption, would require their permission to do an operation in either direction. Users may often choose to grant a trusted client application the permission to decrypt all nip04 or nip44 messages alike, automatically, or generally manually with a popup. That's up the signer application how granular the permissions get.

To be clear this is a client implementation detail, im not a client developer, so I can't say in practice how many have handled the UX on this, but know that the signer, and the user had the final say on which algorithm was granted permission.

Clients and signers alike could choose to block obsolete encryption methods if they choose.

[some_furry]

> It's also worth noting, the user _must_ be made aware of the encryption method that was used, their "signer" application, which is also responsible for encryption and decryption, would require their permission to do an operation in either direction.

Let me ask a more pointed question about downgrade attack resistance then:

Is the algorithm being used determined by the encrypted message contents? Or is it determined by the key controlled by the signer app?

[vnuge]

Regardless of the signer interface the procedure call remains the same. The client application determines what method it wants to use, then the plaintext is passed to the signer (web extension, nip46 remote signing, android etc) with the nip44.encrypt or nip04.encrypt procedure calls.

The user is then requested to confirm the encryption operation. So a "downgrade" could happen in two ways. The client selects nip04 without the user's instructions, and the signer does not properly guard or notify the user that the message to be encrypted is using nip04. Still not really an attack I don't think, since no "sessions" exist in DMs there shouldn't be any way a remote user gets to cause a client to change algorithms.

To answer directly, the client app chooses, makes a remote procedure call with the desired algorithm, user confirms, message is encrypted, returned, signed (another rpc round-trip), then written to relays.

The signer application is ALWAYS authoritative, if it chooses to.