Function open 

pub fn open(
    ciphertext: &[u8],
    key: &MessageKey,
    ctx: &MessageContext,
    sender_verifying_key: &VerifyingKey,
) -> Result<Vec<u8>, EnvelopeError>

Decrypt a ciphertext blob with K and verify the embedded signature against the sender’s Ed25519 key and the trusted context. Returns the plaintext.

Callers on both ends use this: the recipient after unwrap_key, and the server at reveal with the reporter-supplied K and context fields taken from the stored row.