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Comment by vnuge | Hacker News Reader

nostr cryptographic developer here (author of libnoscrypt C library)

Nip04 has been deprecated, and to be clear, in practice the nip04 payload is in a signed nip01 event wrapper.

nip44 replaced nip04, which has been reviewed/audited. Does use authenticated encryption in the message payload with forward secrecy, again in practice wrapped in a nip01 event, singed by the author, usually by the same cryptographic software used to encrypt the message.

nip44 is becoming more widely used for direct messages and other "private" metadata stored on relays. It's chacha20 + hkdf.

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Hey, I should say this, and wish I had earlier:

I don't really so much care whether Nostr is good or bad. I'm a connoisseur of cryptographic vulnerabilities, and the ones in that paper are fun. We host a podcast (me, Deirdre Connolly, and David Adrian) that is mostly about good crypto vulns. If there's someone affiliated with Nostr that would want to chat for an hour or so about how applicable the vulns in this paper are or aren't, and how they're addressed in NIP44 --- we'd love to talk. My email address is in my profile. Whoever showed up, they'd be in good company!

https://securitycryptographywhatever.com/

How robust is nostr against downgrade attacks - can an attacker induce users to use nip04, or are there safeguards against this?

(I have no significant knowledge of the protocol; if this is a meaningless question just say so.)

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.

> 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?

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.

That doesn't answer my question at all.

Is the decision (regardless of who fucking decides) based on metadata attached to the key the client controls or from a breadcrumb included in the message itself?

I think that the (unsatisfying) answer is that there's no established standard for how protocol selection works. nip04 and nip44 are completely different types of messages, and it's up to the client how it'll use and/or respond to them.

Kind of. They are standards for encryption, not the direct message itself. You'd be speaking about nip17. Which i've been mis-speaking as nip19.

https://github.com/nostr-protocol/nips/blob/master/17.md

Notice that nip44 IS the standard. again, nip04 is deprecated, and should be treated by clients as such.

I guess it's worth also saying, there is no algorithm selection. Nip04 is dead for DMs. It doesn't need to be backward compatible. A user can't know which client another user is on, nor what their capabilities are, nostr is not smart in that way. Most, if not all, operations on nostr are completely stateless.

When a user decides to send a DM to another user, the client must choose the standard for encryption, and message wrapping. Then hope the other user is using a client that implements the same standard, in order to decrypt the message.

Again, remember, DMs don't have a session. Every message derives a new symmetric key. The only metadata that makes a "chat" session is the timestamp, and the public keys of the users.

We are obviously speaking from different understandings. I would say neither. I would need you to define your terms differently maybe.

In all cases the client application chooses the algorithm used when the user writes a DM. What do you mean by breadcrumb in the message. Message in what context? Message sent to the signer?

Edit: Maybe I should say the client developer? Is that the answer you're looking for? The developer _could_ give the user the option of choosing which to use, but clients generally are hard-coded to use one or the other.

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