Article
11/18/2011

HMAC Class

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Represents the abstract class from which all implementations of Hash-based Message Authentication Code (HMAC) must derive.

Inheritance Hierarchy

System.Object
  System.Security.Cryptography.HashAlgorithm
    System.Security.Cryptography.KeyedHashAlgorithm
      System.Security.Cryptography.HMAC
        System.Security.Cryptography.HMACSHA1
        System.Security.Cryptography.HMACSHA256

Namespace: System.Security.Cryptography
Assembly: mscorlib (in mscorlib.dll)

Syntax

'Declaration
<ComVisibleAttribute(True)> _
Public MustInherit Class HMAC _
    Inherits KeyedHashAlgorithm

[ComVisibleAttribute(true)]
public abstract class HMAC : KeyedHashAlgorithm

The HMAC type exposes the following members.

Constructors

	Name	Description
	HMAC	Initializes a new instance of the HMAC class.

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Properties

	Name	Description
	BlockSizeValue	Gets or sets the block size to use in the hash value.
	CanReuseTransform	Gets a value indicating whether the current transform can be reused. (Inherited from HashAlgorithm.)
	CanTransformMultipleBlocks	When overridden in a derived class, gets a value indicating whether multiple blocks can be transformed. (Inherited from HashAlgorithm.)
	Hash	Gets the value of the computed hash code. (Inherited from HashAlgorithm.)
	HashName	Gets or sets the name of the hash algorithm to use for hashing.
	HashSize	Gets the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.)
	InputBlockSize	When overridden in a derived class, gets the input block size. (Inherited from HashAlgorithm.)
	Key	Gets or sets the key to use in the hash algorithm. (Overrides KeyedHashAlgorithm.Key.)
	OutputBlockSize	When overridden in a derived class, gets the output block size. (Inherited from HashAlgorithm.)

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Methods

	Name	Description
	Clear	Releases all resources used by the HashAlgorithm class. (Inherited from HashAlgorithm.)
	ComputeHash(array<Byte[])	Computes the hash value for the specified byte array. (Inherited from HashAlgorithm.)
	ComputeHash(Stream)	Computes the hash value for the specified Stream object. (Inherited from HashAlgorithm.)
	ComputeHash(array<Byte[], Int32, Int32)	Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.)
	Dispose	Releases the unmanaged resources used by the HMAC class when a key change is legitimate and optionally releases the managed resources. (Overrides KeyedHashAlgorithm.Dispose(Boolean).)
	Equals(Object)	Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
	Finalize	Allows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
	GetHashCode	Serves as a hash function for a particular type. (Inherited from Object.)
	GetType	Gets the Type of the current instance. (Inherited from Object.)
	HashCore	When overridden in a derived class, routes data written to the object into the default HMAC hash algorithm for computing the hash value. (Overrides HashAlgorithm.HashCore(array<Byte[], Int32, Int32).)
	HashFinal	When overridden in a derived class, finalizes the hash computation after the last data is processed by the cryptographic stream object. (Overrides HashAlgorithm.HashFinal().)
	Initialize	Initializes an instance of the default implementation of HMAC. (Overrides HashAlgorithm.Initialize().)
	MemberwiseClone	Creates a shallow copy of the current Object. (Inherited from Object.)
	ToString	Returns a string that represents the current object. (Inherited from Object.)
	TransformBlock	Computes the hash value for the specified region of the input byte array and copies the resulting hash value to the specified region of the output byte array. (Inherited from HashAlgorithm.)
	TransformFinalBlock	Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.)

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Fields

	Name	Description
	HashSizeValue	Represents the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.)
	HashValue	Represents the value of the computed hash code. (Inherited from HashAlgorithm.)
	KeyValue	The key to use in the hash algorithm. (Inherited from KeyedHashAlgorithm.)
	State	Represents the state of the hash computation. (Inherited from HashAlgorithm.)

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Explicit Interface Implementations

	Name	Description
	IDisposable.Dispose	Releases the unmanaged resources used by the HashAlgorithm and optionally releases the managed resources. (Inherited from HashAlgorithm.)

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Remarks

A Hash-based Message Authentication Code (HMAC) can be used to determine whether a message sent over an insecure channel has been tampered with, provided that the sender and receiver share a secret key. The sender computes the hash value for the original data and sends both the original data and the HMAC as a single message. The receiver recomputes the hash value on the received message and checks that the computed hash value matches the transmitted hash value.

HMAC can be used with any iterative cryptographic hash function, such as MD5 or SHA-1, in combination with a secret shared key. The cryptographic strength of HMAC depends on the properties of the underlying hash function.

Any change to the data or the hash value results in a mismatch, because knowledge of the secret key is required to change the message and reproduce the correct hash value. Therefore, if the original and computed hash values match, the message is authenticated.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Thread Safety

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.