websocket_handshake.nx

websocket_handshake.nx -- compute Sec-WebSocket-Accept. RFC 6455 §4.2.2: the server's response to a WebSocket upgrade request includes a Sec-WebSocket-Accept header whose value is base64( sha1( client_key || GUID ) ) where GUID = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" is a literal defined in the RFC (solely to make accidental handshake-value forgery harder -- it doesn't add security). client_key is the value of the Sec-WebSocket-Key request header (a 16-byte random value, base64-encoded by the client, so 24 ASCII chars including "=" padding). Composes sha1.nx + base64.nx. The concatenation buffer is at most 24 + 36 = 60 bytes. Invariants: WH1 Caller passes the raw client key bytes (typically 24 ASCII characters); we don't decode them -- the spec says concatenate the client-sent base64 string AS IS with the GUID string and hash that. WH2 Output is 28 ASCII characters (base64 of 20 bytes).