**2.1 Covert channel characteristics and properties**

Although a covert channel transfers information in a hidden way, it has the same characteristics as other communication channels. These characteristics are:


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*Secure Communication Using Cryptography and Covert Channel*

**2.2 The covert channel is more private and undetectable if it satisfies** 

1.Plausibility: the TCP is usually used for Internet traffic, and it always employs using time stamp option. As a result, TCP using time stamp is a plausible covert channel because the majority of users using TCP will not use it for sending covert data. So, the adversary will believe that TCP time stamps will not be

2.Undetectability: in order for a channel to be more undetectable, the channel must satisfy that the distribution of bits with covert data must be similar to the distribution of the normal channel. If an adversary notices that there are differences (using statistical tests) in bit distribution, then he will detect that the channel is a covert channel. Also, to achieve undetectability, the channel's

bits must be random; otherwise, it will be noticed by the adversary.

3.Indispensability: Lampson [3] reports that a communication channel is a covert channel if it is neither designed nor intended to transfer information at all. The channel should introduce several benefits to the users besides sending data covertly; thus, the adversary cannot or will not close off that channel.

Covert channels can be classified as storage or timing channels, noisy or noise-

The covert storage channel depends on a shared variable or a storage location, whereby one process (sender) can be allowed to write directly or indirectly to the storage location and the other process (receiver) reads from that storage location. On the other hand, the covert timing channel enables senders to send information to the receiver through signals, whereby the sender manages the time that is needed to perform some operation in such a way that when the receiver observes the time, it will understand a special event or a special piece of information. The main disadvantage of the timing channel is that it is considered very noisy because of the several external factors that affect the execution time of a process. Covert storage channels and timing channels need a synchronization process, which enables the sender and receiver to synchronize with each other to send and receive information. The storage covert channel uses a data variable to enable the sender and receiver to communicate. Therefore, a synchronization variable, called sender-receiver, is needed by the sender to notify the receiver that he has completed reading or writing a data variable. The covert channel uses another synchronization variable, called receiver-sender. To distinguish between storage and timing channels, if a channel uses a storage variable to transfer data between the sender and receiver, it is considered a storage channel. On the other hand, a covert timing channel uses time reference (e.g., a clock) to transfer data between the sender and receiver, whereby

I discussed previously that the characteristics of the covert channel are similar to any communication channel. One of these characteristics is that the channel may be

*DOI: http://dx.doi.org/10.5772/intechopen.82580*

used for sending data covertly.

**2.3 Covert channel classification**

less channels, and program-flow channels.

the sender and receiver use a common time reference.

*2.3.2 Noisy and noiseless channels*

*2.3.1 Storage channels and timing channels*

**the following**

*Computer and Network Security*

original key has fake keys. I used encryption algorithm to guarantee the confidentiality. HMAC is used to check integrity. Finally, the time that is needed for the client

In this chapter, secure communication channel for transferring data is implemented. The channel between the client and server is considered a covert channel

Lampson was the first to introduce the idea of a covert channel [3]. Transferring data between two entities indirectly through a channel is called a covert channel. Before the client and server use the channel to transfer data, they must agree on a pre-agreement knowledge (e.g., shared memory, table). For example, a word containing "mm" means bit "0" other than this means bit "1." So, if the client wants to send "10" to the server indirectly, the client will send "secure communication" to the server. The attacker hardly breaks the covert channel and it is considered to be

Although a covert channel transfers information in a hidden way, it has the same

• Capacity: the amount of data that can be transmitted through the channel. From security viewpoint, increasing channel capacity leads to more information leakage. The covert channel capacity is measured in bits/second. To obtain maximum bandwidth through a covert channel, encoding schemes must be

• Noise: transmitted data through a covert channel are exposed to an amount of perturbations that makes the transmitted and received information between

• Transmission mode: the transmission of information in covert channels (as in normal channels) can be synchronous or asynchronous. The sender and receiver in synchronous mode should manage their transmission based on a condition or a specific event. On the other hand, in asynchronous mode, the

characteristics as other communication channels. These characteristics are:

and server to agree on secret information (e.g., secret keys) is measured.

that depends on authentication and encryption.

*Secure communication using covert channel, encryption, and authentication.*

more secure if it is undetectable [3, 4].

**2.1 Covert channel characteristics and properties**

chosen between the sender and receiver.

transmission occurs without a prior condition.

two entities not the same.

**2. Background**

**Figure 1.**

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