The snap-specific entropy used to sign messages can also be used to derive an encryption key-pair.
The secret decryption key should not be the entropy, to avoid key reuse, but it has to be deterministically generated from it.
To align with other MetaMask implementations, the encryption system should use ERC1024 messages and keys.
See here an implementation.

• add a getEncryptionPublicKey(): Promise<string> method that returns a hex encoded, 0x prefixed x25519 public key
• add a decryptMessage(message: Eip1024EncryptedData): Promise<string> method that accepts an Eip1024EncryptedData object and returns the decrypted message (string).
• add corresponding logic to onRpcRequest to expose the 2 new methods
• If possible, avoid using extra dependencies