I’m assuming that DMs use asymmetric encryption. 

Doesn’t that mean that if I send you a DM I’m encrypting it with your public key so you can decrypt it with your private key and vice versa. 

If usage were to help with guessing the nsec couldn’t someone just constantly use my pubkey to encrypt data until they could “guess the number”. The decryption of the data happens “off network”. 

Maybe I’m misunderstanding how DMs work but that’s what I originally thought.