Thank you for this excellent question! You’ve identified an important architectural consideration that
deserves clarification.
Short answer: Yes, the encryption key can serve as a general-purpose public encryption key.
You’re absolutely right that since the encryption key is:
- Stable (part of the pv-address)
- Publicly accessible
- Based on standard ECDH cryptography (Baby Jubjub)
It could technically support all the use cases you mentioned:
- Private messaging between users
- Cross-protocol encrypted notifications
- Encrypted metadata/memos attached to transactions
- Offline stealth-style encrypted messages
- Wallet-to-wallet encrypted communication
Design Philosophy
This proposal is an address format standard - it defines how to encode and decode three public keys, but
intentionally does not restrict how implementations use them. The decision to extend the encryption key
usage beyond note encryption is left to individual implementations. This flexibility enables innovation
while maintaining the core standard.
If a developer wants to build a private messaging system using the encryption key from pv-addresses, that’s
perfectly valid. If another wants to use it only for note encryption, that’s also fine.