how to generate and print barcode in c# windows application Estimated lesson time: 90 minutes in Visual C#.NET

Paint Denso QR Bar Code in Visual C#.NET Estimated lesson time: 90 minutes

Estimated lesson time: 90 minutes
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Encrypting and Decrypting Data with Symmetric Keys
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Many people are introduced to encryption at an early age. Children protect even the most mundane communications from imaginary spies with a secret decoder ring a toy with two rings that translates encrypted characters to unencrypted characters. The rings on a decoder ring rotate, and a message can be decrypted only when the two rings are lined up correctly. To exchange an encrypted message, the children must first agree on how the rings will line up. After they have exchanged this secret piece of information, they can freely pass encrypted messages without worrying that someone will be able to decrypt them. Even if an imaginary spy had a decoder ring, the spy would need to know how to position the rings to decrypt the message. Because both the sender and the recipient of the message must know the same secret to encrypt and decrypt a message, secret decoder rings are an example of symmetric key encryption. Symmetric key encryption is a game for children, but it is also the foundation for most encrypted communications today. As children know, encryption
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User and Data Security
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is a fun topic. You should enjoy building it into your application, and you ll greatly reduce the chance of private data being compromised.
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What Is Symmetric Key Encryption
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Symmetric key encryption, also known as secret-key encryption, is a cryptography technique that uses a single secret key to both encrypt and decrypt data. Symmetric encryption algorithms (also called ciphers) process plain text with the secret encryption key to create encrypted data called cipher text. The cipher text cannot easily be decrypted into the plain text without possession of the secret key. Figure 12-2 shows symmetric key encryption and decryption.
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Encryption Plain text H E L L O
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Secret key Symmetric encryption algorithm S E C R E T
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Cipher text
T R F Y L
Decryption Cypher text T R F Y L
Secret key Symmetric decryption algorithm S E C R E T
Plain text
Figure 12-2
Symmetric encryption uses the same key for encryption and decryption
Symmetric algorithms are extremely fast and are well suited for encrypting large quantities of data. Even though symmetric encryption is very secure, an attacker can identify the plain text, given the cipher text and enough time. To identify the plain text, the attacker needs to use only a brute force attack to sequentially generate symmetric keys until the attacker has tried every single possibility. Typically, the time
Lesson 3: Encrypting and Decrypting Data
required to try all keys is hundreds of years, if not longer, because the attacker would need to try at least 256 key possibilities. More secure symmetric algorithms use longer keys that would take exponentially longer to crack. The disadvantage of secret-key encryption is that it presumes that two parties have already agreed on a key. Agreeing on a symmetric key is a challenge because the key itself cannot be encrypted. If you ve decided to use encryption, it must be because you don t trust your system to prevent an attacker from gaining access to your data. Therefore, users must find a secure way to exchange secret keys. After the secret keys are exchanged, encrypted data can be freely exchanged between the parties. However, keys should be changed on a regular basis for the same reasons that passwords should be changed regularly. Each time the key must be changed, users must resort to the secure communication mechanism. Figure 12-3 shows how users must transfer both the encrypted message and the key using different communication mechanisms to enable the recipient to decrypt the message, while preventing an attacker who can capture your communications across only a single network from decrypting the message. Keys are often transferred by voice across the phone network, sent physically through the mail system, or carried to the recipient. After the shared secret has been established, the two peers can use it to encrypt and decrypt any number of messages.
Transfer the key across one network
Encrypted Document
Transfer encrypted documents across another network
Figure 12-3 Symmetric key encryption requires separately exchanging both the key and the encrypted document
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