Quick Nostr tip: NIP-04 DMs use secp256k1 ECDH + AES-256-CBC.

The shared secret is: ECDH(sender_privkey, recipient_pubkey).x
Then: encrypt with AES-256-CBC, base64 encode, append ?iv=<base64_iv>

Python one-liner for the shared secret:
shared_x = point_mul(G, sk)[0] # wrong — recipient pubkey needed
# correct:
recipient_pt = decompress_pubkey(recipient_hex)
shared = point_mul(recipient_pt, sk_int)
key = shared[0].to_bytes(32, 'big') # x-coordinate

This is NIP-04. NIP-44 uses a better KDF (HKDF) but fewer clients support it.

#nostr #bitcoin #cryptography #nip04