qr code generator asp net c# Encrypting and Decrypting Data with Asymmetric Keys in C#

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Encrypting and Decrypting Data with Asymmetric Keys
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Asymmetric encryption, also known as public-key encryption, overcomes symmetric encryption s most significant disability: requiring both the encryptor and decryptor to know a shared secret. Asymmetric encryption relies on key pairs. In a key pair, there is one public key and one private key. The public key can be freely shared because it cannot be easily abused, even by an attacker. Messages encrypted with the public key can be decrypted only with the private key, allowing anyone to send encrypted messages that can be decrypted only by a single individual. The asymmetric encryption process begins with a public key being exchanged. Generally, both the client and server exchange public keys. However, if only one side of the communication needs to be encrypted, only the peer receiving encrypted communications must provide a public key. After the public keys are exchanged, communications are encrypted using the recipient s public key. Such communications can be decrypted only by the recipient because only the recipient holds the private key that
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User and Data Security
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matches the public key. Figure 12-4 shows a simple asymmetric encryption arrangement in which only one side of the communications provides a public key.
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Transfer the public key
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Private key kept secret
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Encrypted Document Encrypted documents with public key can be decrypted only with the private key
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Figure 12-4
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Asymmetric cryptography uses separate keys for encryption and decryption
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Asymmetric algorithms are not as fast as symmetric algorithms, but are much more difficult to break. Asymmetric algorithms are not well suited to encrypting large amounts of data because of the performance overhead. One common use of asymmetric algorithms is to encrypt and transfer a symmetric key and IV. The symmetric encryption algorithm is then used for all messages being sent back and forth. This is the technique used by HTTPS and Secure Sockets Layer (SSL) to encrypt Web communications asymmetric encryption is used only during session establishment. This common combination of asymmetric and symmetric encryption is shown in Figure 12-5. The other significant challenge of asymmetric encryption is key management. To manage keys, organizations typically implement a public key infrastructure (PKI), such as Certificate Services included with Windows Server 2003. A PKI is an infrastructure for distributing, managing, and revoking certificates in an organization. As a developer, you will generally not be responsible for configuring a PKI.
Lesson 3: Encrypting and Decrypting Data
1 Transfer the asymmetric public key
2 Transfer the secret symmetric key, asymmetrically encrypted
3 Communicate using symmetric encryption
Private key kept secret
Figure 12-5 Combine asymmetric and symmetric algorithms to optimize security and performance
Asymmetric Algorithm Classes in the .NET Framework
The .NET Framework provides two classes for working with asymmetric encryption, and they are both based on the System.Security.Cryptography.AsymmetricAlgorithm class. This base class has the following properties, several of which are identical to the SymmetricAlgorithm counterparts:
KeyExchangeAlgorithm Gets the name of the key exchange algorithm. Generally,
you do not need to directly access this property.
KeySize Gets or sets the size of the secret key used by the symmetric algorithm in
bits. Asymmetric keys are much larger than symmetric keys. For example, although a typical symmetric key is 182 bits, the .NET Framework implementation of the RSA algorithm supports key lengths from 384 through 16384 bits.
LegalKeySizes A KeySizes array that gets the key sizes that are supported by the symmetric algorithm. Each array member contains MinSize and MaxSize properties that define the valid key ranges in bits, and a SkipSize property that specifies the interval between valid key sizes in bits.
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SignatureAlgorithm Gets the URL of an XML document describing the signature
algorithm. Generally, you do not need to directly access this property. Unlike the SymmetricAlgorithm base class, the AsymmetricAlgorithm base class has no useful methods. Instead, the encryption functionality is built into the objects that implement the AsymmetricAlgorithm class. The .NET Framework provides two implementations of this class:
RSACryptoServiceProvider Used for all asymmetric encryption and decryption.
RSACryptoServiceProvider is the .NET Framework implementation of the RSA algorithm. RSA is named for the last initial of its three creators Ronald Rivest, Adi Shamir, and Leonard Adleman who developed the algorithm in 1977. The RSACryptoServiceProvider class is a managed wrapper around the unmanaged RSA implementation provided by the Cryptography API.
DSACryptoServiceProvider
Used for digitally signing messages, it is also a managed wrapper around unmanaged code.
In addition to the properties provided by AsymmetricAlgorithm, RSACryptoServiceProvider provides the following properties:
PersistKeyInCsp Gets or sets a value indicating whether the key should be persisted
in the CSP. Set this to true when you want to reuse the key without exporting it.
UseMachineKeyStore Gets or sets a value indicating whether the key should be
persisted in the computer's key store instead of the user profile store. The default constructors always populate the algorithm parameters with the strongest defaults available to the run-time environment, giving you the strongest algorithm possible without changing any settings. The RSACryptoServiceProvider class also includes methods for encrypting and decrypting, as well as for importing and exporting keys. The following list describes each of these methods:
Decrypt Decrypts data with the RSA algorithm. Encrypt Encrypts data with the RSA algorithm. ExportParameters Exports an RSAParameters structure, which defines the algo-
rithm s key pair. Pass true to this method to export both the private and public key, or pass false to export only the public key.
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