Encryption Basics

Encryption is the process of changing the form of data in such a drastic way that that data can only be understood by authorized parties. The name for the reverse process of encryption, in which data is translated from encrypted form back into readable form, is called decryption. Using encryption and decryption, online merchants can be more confident that sensitive customer data such as credit card information) is not being stolen by unauthorized third parties (such as hackers or unscrupulous competitors) while in transit over the Internet.

Several key concepts underlie the process of encryption — in particular, ciphers, keys, and certificates. 

Ciphers and codes 

The process of encrypting data involves the use of mathematical algorithms to translate readable data into seemingly meaningless nonsense called ciphertext (some call this nonsense a hash). Only authorized parties (those who hold the proper decryption key) can reassemble ciphertext into properly readable text. Some people confuse the concept of a code with the concept of ciphertext, but the two are distinct. 

Codes actually have nothing to do with encryption; a code is merely a way of saying one thing using a different language. For example, computer systems use binary codes of 1s and 0s to represent data; Morse code uses a series of long and short tones to indicate letters. In effect, a code is a communication tool; ciphers, on the other hand, are arrangements of symbols specifically designed to hide the contents of a message, obscuring its meaning to everyone except those by whom the data was meant to be read.

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ARP Process

The following scenario details the ARP process for two clients on a single network segment.
- Client 1 has the following configuration: 

IP address 131.107.40.16 MAC address 00-3B-1E-34-B6-73
- Client 2 has the following configuration: 

IP address 131.107.40.19 MAC address 00-2E-6B-81-1D-4A

Client 1 ARPs (sends out an ARP broadcast message) for the MAC address of Client 2. The ARP request is sent as a network broadcast and every client on the network receives the request; however, only the client that exactly matches the IP address responds.

When Client 1 sent the ARP request, the MAC address of Client 1 was included in the packet. When  Client 2 receives the ARP request, the ARP table or ARP cache is updated with Client 1’s information. 

The ARP table/cache is a listing of IP addresses to the corresponding MAC address.
By default, most Microsoft operating systems use a cache life of 2 minutes. During the 2 minutes, the ARP cache is checked to try to avoid sending out a network broadcast to obtain resolution. When 2 minutes have elapsed, the entry is flushed from the cache. 

Client 2 responds to the request from Client 1 by sending its MAC address back, which results in both clients now having the MAC address of the other.

The ARP process is different when clients are on different network segments. Because the ARP process uses broadcasts, routers will not forward the broadcasts.
Client 1 is located on Segment 1 and Client 2 is located on Segment 2. Router A connects the two segments. The following details the process between Client 1 and Client 2 in this situation.
Client 1 sends an ARP packet to Router A. Router A connects to Client 2 with an ARP request. Client 2 updates its ARP cache with the MAC address of Router A and Router A is updated with the MAC address of Client 2. 

Router A then updates Client 1 with the MAC address of Router A. Thus Client 1 and Client 2 have received the MAC address of Router A — and Router A has received the MAC addresses of Client 1 and Client 2. Maintaining the ARP cache.

Here are the more commonly used ARP commands: 

Run these command on command prompt
>ARP –a:   displays the ARP cache. 

>ARP –s:   creates a static entry in the ARP cache.
>ARP –d:    deletes a static entry from the ARP cache.
>ARP –IP:  address displays the MAC address of the entered IP address.

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ARP and RARP

Address Resolution Protocol (ARP) and the Reverse Address Resolution Protocol (RARP). Ways to resolve an IP address to its equivalent MAC address are ARP and RARP .

ARP is a broadcast-based resolution method  and because it uses broadcasts, ARP traffic cannot pass through routers. RARP requires a server for IP-MAC resolution and is most commonly used in Unix environments. Microsoft networks (on the other hand) use ARP exclusively to resolve IP addresses.

As an example of how the ARP process works, consider the following: When a user wishes to access a resource on the network using its computer name, the name is resolved to its IP address equivalent using WINS or NetBIOS.
When the IP address of the destination device is known, the client then sends out an ARP request to obtain the device’s (or the computer’s) MAC address. When this process (which occurs solely through broadcasts) is complete, the client that initially sent the ARP request updates its ARP cache.

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Broadcasts

Broadcasts are packets sent out on the cable topology with no specific destination address given (the 6-byte source address field is set to all zeros). All devices on the wire will pick up a broadcast packet to determine if it has any reason to respond. A good example of this is a common login sequence.

When a PC first comes on line, it performs a broadcast looking for a server to authenticate to. Basically, the PC is telling every device on the cable that it is looking for a server so that it can log in to the network. All devices will see this data packet, but only file servers will respond.

When a broadcast packet is sent out on the LAN, every device connected to that network segment will stop what it’s doing and look at the broadcast packet. Because a device does not know whom the specific broadcast message is for, it must stop what it’s doing to check the packet even if it does not need to respond.

Note: 

When too many broadcasts are being sent on a LAN, all devices will be too busy checking these messages to do any real work. This is a common Ethernet problem known as a broadcast storm. Broadcast storms can be caused deliberately by a potential intruder, or (most likely), from a malfunctioning NIC.

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Basic Ethernet Transmits

Ethernet Transmits data in frames. The first 12 bytes of every frame contain the 6-byte destination address (the recipient) and a 6-byte source address (the sender). Each Ethernet adapter card has a unique factory installed address (the “Media Access Control”, or MAC address). Use of this hardware address guarantees a unique identity to each card.

Ethernet uses a transmission protocol method known as Carrier Sense Multiple Access/Collision Detection (CSMA/CD). With this concept Ethernet only allows one device at a time to “talk” on the wire. 

When a network device wants to transmit data, the Ethernet protocol states that the cable must be without any other traffic. This is the Carrier Sense and Multiple Access portion of the CSMA technology. It is likely that devices will “listen” on the wire at the exact same time and not hear any other traffic. They will then assume that the cable is free and begin transmitting their data. 

Because only one device is allowed to “talk” on the wire at a time, this situation breaks the rules of the Ethernet protocol. When this happens, the data bits collide with one another and basically make a real mess of things. 

But in the fact collision often ocur when the data was transmit . When a collision is detected, the devices are programmed to “back off” and wait a random amount of time before trying the process over again. This back-off algorithm is built into the Ethernet devices to ensure different timing states between attempts to retransmit.

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Word Ether of Ethernet

The base word ether of ethernet was chosen as a way of describing an essential feature of the system; the physical medium (a cable) carries bits to all stations, in much the same way that astronomers once thought a “luminiferous aether” propagated electromagnetic waves universally through space. That was ethernet story. 

The Institute of Electricians and Electrical Engineers (IEEE) was assigned the task of developing formal international standards for all local-area network technology. It formed the “802” committee to look at Ethernet, Token Ring, Fiber-optic, and other LAN technology. 

The objective of the project was to Standardize each LAN individually ? Establish rules that would be global to all types of LANs so data could easily move from Ethernet to Token Ring or fiber-optic networks In the process of standardizing the Ethernet protocols, some conflicts with the original Xerox system were inevitable. 

The IEEE was careful to separate the new and old rules. It recognized that there would be a period when old DIX messages and new IEEE 802 messages would have to coexist on the same LAN. The result was a published set of standards known as 802.2 and 802.3. 

For Token Ring networks, this committee published the 802.5 standard. In the following section, we’ll look at these IEEE standards in detail, and then we’ll follow-up with a primer on the basic function of the Ethernet and Token Ring protocols.

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