feat: initial webssh tooling

agent/internal/sshutil/encoder.go

@@ -0,0 +1,41 @@
+package sshutil
+
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+func encodeFrame(id string, data []byte) ([]byte, error) {
+	sid := []byte(id)
+	if len(sid) > 255 {
+		return nil, fmt.Errorf("session ID too long")
+	}
+
+	payloadLen := len(data)
+	buf := make([]byte, 1+len(sid)+4+payloadLen)
+
+	buf[0] = byte(len(sid))                                          // SID length
+	copy(buf[1:], sid)                                               // SID
+	binary.BigEndian.PutUint32(buf[1+len(sid):], uint32(payloadLen)) // Payload length
+	copy(buf[1+len(sid)+4:], data)                                   // Payload
+
+	return buf, nil
+}
+
+func decodeFrame(buf []byte) (id string, payload []byte, ok bool) {
+	if len(buf) < 5 {
+		return "", nil, false
+	}
+	sidLen := int(buf[0])
+	if len(buf) < 1+sidLen+4 {
+		return "", nil, false
+	}
+	id = string(buf[1 : 1+sidLen])
+	payloadLen := int(binary.BigEndian.Uint32(buf[1+sidLen:]))
+
+	if len(buf) < 1+sidLen+4+payloadLen {
+		return "", nil, false
+	}
+	payload = buf[1+sidLen+4 : 1+sidLen+4+payloadLen]
+	return id, payload, true
+}