the thing is, you should not be sending anything on a public network that you aren't willing to risk being compromised

this is the most elementary understanding you need to have before you start talking about security

they can't crack the cipher used on data that they don't have

any other data you need to really think about it, are you ok to have that leak, if not, don't send it

if you have a backup plan, and that's a whole nother level of complexity of cryptography, then ok

the encryption keys used for group chats with matrix, simplex, and other decently secure group chat protocols have the property that if you break the key for one message, you don't get any other message, those are separately encrypted

the base principle is the same, it's the same encryption, essentially XOR using a stream of random bytes derived from your secret and the "nonce" you publish at the front of the message

our DMs on nostr use this same model, and the strength of them is based on the 16 byte nonces, mainly, each message has a different one, this is 128 bits of security, or, a little less if you read the details of the theory about it, maybe 121, maybe 112, it partly depends on the elliptic curve, supposedly secp256k1 is a little weaker than edwards (ed25519) but i severely doubt this is actually true because those other shitcoin curves are also far more complex to derive and the number of roots to them means they could actually be backdoored but the koblitz curve of bitcoin, definitely can't be, it's literally just 7 and 10, if i remember correctly, maybe also 2