HMAC Class
[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]
Represents the abstract class from which all implementations of Hash-based Message Authentication Code (HMAC) must derive.
Assembly: mscorlib (in mscorlib.dll)
The HMAC type exposes the following members.
| 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.) |
| 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.) |
| 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.) |
| Name | Description | |
|---|---|---|
  | IDisposable::Dispose | Releases the unmanaged resources used by the HashAlgorithm and optionally releases the managed resources. (Inherited from HashAlgorithm.) |
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.