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Hidden channels (covert channels) are communication channels that transmit information without the authorization or knowledge of the channel's designer, owner, or operator.
- Bypassing network filters;
- Bypassing network sniffers.
Information can be hidden within:
- IP ID;
- TCP ISN;
- TCP options;
- DNS ID;
- HTTP cookie;
Generally, it is impossible to detect well-designed hidden channels by means of traffic analysis. For example, information hidden within TLS Client/Server Hello random bytes in encrypted form cannot be distinguished from bytes produced by secure random number generator.
However, it is possible to detect hidden channels by detecting attendant events, such as successful intrusion attempts. Some hidden channels produce network anomalies, for example, hidden channels using DNS ID to hide information may produce large number of DNS queries without further communication between hosts.