Software Engineering Institute

To weary network users, encryption provides privacy for data in transit. To network operators and security analysts, encryption hinders visibility. Breaking encryption and inspecting content can be costly and error prone.

By analyzing the lengths and ordering of encrypted data exchanged throughout a connection (i.e., signals that don't require breaking encryption) network monitoring systems can infer protocol state without parsing the content of the connection. By modeling a protocol's state transitions and overlaying that model on a connection's sequence of lengths (SOL), inferences can be made about how the protocol is being use. This provides a sort of compromise between privacy and visibility.