HMACSHA256 Class
Computes a Hash-based Message Authentication Code (HMAC) using the SHA256 hash function.
System.Security.Cryptography.HashAlgorithm
System.Security.Cryptography.KeyedHashAlgorithm
System.Security.Cryptography.HMAC
System.Security.Cryptography.HMACSHA256
Namespace: System.Security.Cryptography
Assembly: mscorlib (in mscorlib.dll)
The HMACSHA256 type exposes the following members.
| Name | Description | |
|---|---|---|
![]() ![]() | HMACSHA256 | Initializes a new instance of the HMACSHA256 class with a randomly generated key. |
![]() ![]() | HMACSHA256(Byte()) | Initializes a new instance of the HMACSHA256 class with the specified key data. |
| Name | Description | |
|---|---|---|
![]() ![]() | BlockSizeValue | Gets or sets the block size to use in the hash value. (Inherited from HMAC.) |
![]() ![]() | 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. (Inherited from HMAC.) |
![]() ![]() | 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. (Inherited from HMAC.) |
![]() ![]() | 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(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(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. (Inherited from HMAC.) |
![]() ![]() | 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. (Inherited from HMAC.) |
![]() ![]() | HashFinal | When overridden in a derived class, finalizes the hash computation after the last data is processed by the cryptographic stream object. (Inherited from HMAC.) |
![]() ![]() | Initialize | Initializes an instance of the default implementation of HMAC. (Inherited from HMAC.) |
![]() ![]() | 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.) |
HMACSHA256 is a type of keyed hash algorithm that is constructed from the SHA-256 hash function and used as a Hash-based Message Authentication Code (HMAC). The HMAC process mixes a secret key with the message data, hashes the result with the hash function, mixes that hash value with the secret key again, and then applies the hash function a second time. The output hash is 256 bits in length.
An 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 hash value as a single message. The receiver recalculates the hash value on the received message and checks that the computed HMAC matches the transmitted HMAC.
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.
HMACSHA256 accepts keys of any size, and produces a hash sequence 256 bits in length.
The following example shows how to encode and decode a file by using the HMACSHA256 class. The example has a very basic user interface, because its purpose is to demonstrate the HMACSHA256 class, not Silverlight controls. To build and run this example, create a Silverlight-based application in Visual Studio named HMACSHA256Example and replace the MainPage.xaml file and the MainPage.xaml.cs (or MainPage.xaml.vb) file with the following code.
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If the XAML code is not displayed, click the Language Filter arrow at the top of this page, and select the XAML check box. |
Imports System Imports System.Windows.Controls Imports System.Windows.Input Imports System.IO Imports System.IO.IsolatedStorage Imports System.Security.Cryptography Imports System.Security.Permissions Imports System.Text Namespace HMACSHA256Example Class MainPage Inherits UserControl Private Shared encryptedFiles As String = "" Private Shared store As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication() Private Const PasswordSalt As String = "PasswordSalt" Private Shared sourceFilePath As String = "" Private Shared signedFilePath As String = "" Public Sub New() InitializeComponent() AddHandler Me.inputBox.KeyDown, AddressOf inputBox_KeyDown AddHandler Me.passwordBox.KeyDown, AddressOf passwordBox_KeyDown AddHandler Me.decodePassWordBox.KeyDown, AddressOf decodePassWordBox_KeyDown AddHandler Me.deleteBox.KeyDown, AddressOf deleteBox_KeyDown AddHandler Me.decodeBox.KeyDown, AddressOf decodeBox_KeyDown store.CreateDirectory("MyFiles") ' Create subdirectory under MyFiles. encryptedFiles = System.IO.Path.Combine("MyFiles", "EncryptedFiles") store.CreateDirectory(encryptedFiles) ListFiles() inputBox.Focus() End Sub 'New Private Sub inputBox_KeyDown(ByVal sender As Object, ByVal e As EventArgs) If CType(e, System.Windows.Input.KeyEventArgs).Key = Key.Enter Then CreateFile() Me.passwordBox.Focus() End If End Sub 'inputBox_KeyDown Private Sub decodeBox_KeyDown(ByVal sender As Object, ByVal e As EventArgs) If CType(e, System.Windows.Input.KeyEventArgs).Key = Key.Enter Then Dim store As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication() signedFilePath = System.IO.Path.Combine(encryptedFiles, "signedFile.txt") Me.decodePassWordBox.Focus() End If End Sub 'decodeBox_KeyDown Private Sub ListFiles() Dim searchpath As String = System.IO.Path.Combine(encryptedFiles, "*.*") Dim filesInSubDirs As String() = store.GetFileNames(searchpath) Dim sb As New StringBuilder() ' List files in MyFiles\EncryptedFiles. sb.AppendLine("Files in MyFiles\EncryptedFiles:") Dim fileName As String For Each fileName In filesInSubDirs sb.AppendLine(" - " + fileName) Next fileName sb.AppendLine() outputBlock.Text = sb.ToString() End Sub 'ListFiles Private Sub deleteBox_KeyDown(ByVal sender As Object, ByVal e As EventArgs) If CType(e, System.Windows.Input.KeyEventArgs).Key = Key.Enter Then store.DeleteFile(encryptedFiles + "\" + deleteBox.Text) ListFiles() Me.passwordBox.Focus() End If End Sub 'deleteBox_KeyDown Private Sub passwordBox_KeyDown(ByVal sender As Object, ByVal e As EventArgs) If CType(e, System.Windows.Input.KeyEventArgs).Key = Key.Enter Then Try ' Create a random key using a random number generator. This would be the ' secret key shared by sender and receiver. Dim secretkey() As Byte = New [Byte](63) {} Dim deriveBytes As New Rfc2898DeriveBytes(passwordBox.Password, Encoding.UTF8.GetBytes(PasswordSalt)) secretkey = deriveBytes.GetBytes(64) ' Use the secret key to encode the message file. EncodeFile(secretkey, sourceFilePath, signedFilePath) Catch ex As Exception outputBlock.Text = ex.Message End Try Me.decodePassWordBox.Focus() End If End Sub 'passwordBox_KeyDown Private Sub decodePassWordBox_KeyDown(ByVal sender As Object, ByVal e As EventArgs) If CType(e, System.Windows.Input.KeyEventArgs).Key = Key.Enter Then Try ' Create a random key using a random number generator. This would be the ' secret key shared by sender and receiver. Dim secretkey() As Byte = New [Byte](63) {} Dim deriveBytes As New Rfc2898DeriveBytes(passwordBox.Password, Encoding.UTF8.GetBytes(PasswordSalt)) secretkey = deriveBytes.GetBytes(64) ' Take the encoded file and decode DecodeFile(secretkey, signedFilePath) Catch ex As Exception outputBlock.Text = ex.Message End Try Me.deleteBox.Focus() End If End Sub 'decodePassWordBox_KeyDown ' Computes a keyed hash for a source file, creates a target file with the keyed hash ' prepended to the contents of the source file Public Sub EncodeFile(ByVal key() As Byte, ByVal sourceFile As String, ByVal destFile As String) Dim isoStore As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication() Try Dim outStream As IsolatedStorageFileStream = isoStore.OpenFile(signedFilePath, FileMode.Open) Try Try ' Initialize the keyed hash object. Dim myhmacsha256 As New HMACSHA256(key) Dim inStream As IsolatedStorageFileStream = isoStore.OpenFile(sourceFilePath, FileMode.Open) inStream.Position = 0 ' Compute the hash of the input file. Dim hashValue As Byte() = myhmacsha256.ComputeHash(inStream) ' Reset inStream to the beginning of the file. inStream.Position = 0 ' Write the computed hash value to the output file. outStream.Write(hashValue, 0, hashValue.Length) ' Copy the contents of the sourceFile to the destFile. Dim bytesRead As Integer ' read 1K at a time Dim buffer(1023) As Byte Do ' Read from the wrapping CryptoStream. bytesRead = inStream.Read(buffer, 0, 1024) outStream.Write(buffer, 0, bytesRead) Loop While bytesRead > 0 myhmacsha256.Clear() ' Close the streams inStream.Close() outStream.Close() decodeBox.Text = destFile.Replace(encryptedFiles + "\", "") decodePassWordBox.Focus() Return Catch e As Exception outputBlock.Text = e.Message End Try Finally outStream.Dispose() End Try Finally isoStore.Dispose() 'end if-else End Try End Sub 'EncodeFile ' end EncodeFile ' Decode the encoded file and compare to original file. Public Function DecodeFile(ByVal key() As Byte, ByVal sourceFile As String) As Boolean Dim isoStore As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication() Try Dim inStream As IsolatedStorageFileStream = isoStore.OpenFile(signedFilePath, FileMode.Open) Try Try ' Initialize the keyed hash object. Dim hmacsha256 As New HMACSHA256(key) ' Create an array to hold the keyed hash value read from the file. Dim storedHash(hmacsha256.HashSize / 8 - 1) As Byte ' Create a FileStream for the source file ' Read in the storedHash. inStream.Read(storedHash, 0, storedHash.Length) ' Compute the hash of the remaining contents of the file. ' The stream is properly positioned at the beginning of the content, ' immediately after the stored hash value. Dim computedHash As Byte() = hmacsha256.ComputeHash(inStream) ' compare the computed hash with the stored value Dim i As Integer For i = 1 To storedHash.Length - 1 If computedHash(i) <> storedHash(i) Then outputBlock.Text = "Hash values differ! Either wrong password or file has changed." Return False End If Next i outputBlock.Text = "Hash values agree -- no tampering occurred." Return True Catch e As Exception outputBlock.Text = e.Message Return False End Try Finally inStream.Dispose() End Try Finally isoStore.Dispose() End Try 'end DecodeFile End Function 'DecodeFile Public Sub CreateFile() Dim store As IsolatedStorageFile = IsolatedStorageFile.GetUserStoreForApplication() sourceFilePath = System.IO.Path.Combine(encryptedFiles, "sourceFile.txt") signedFilePath = System.IO.Path.Combine(encryptedFiles, "signedFile.txt") Dim sourceStream As IsolatedStorageFileStream = store.CreateFile(sourceFilePath) Dim sw As New StreamWriter(sourceStream) sw.WriteLine(inputBox.Text) sw.Close() sourceStream.Close() Dim signedStream As IsolatedStorageFileStream = store.CreateFile(signedFilePath) signedStream.Close() decodeBox.Text = "sourceFile.txt" End Sub 'CreateFile End Class 'Page End Namespace
<UserControl x:Class="HMACSHA256Example.MainPage" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Width="1024" Height="768"> <Grid x:Name="LayoutRoot" Background="White" > <Grid.ColumnDefinitions> <ColumnDefinition Width="550" /> <ColumnDefinition Width="250" /> <!--<ColumnDefinition Width="250" />--> </Grid.ColumnDefinitions> <Grid.RowDefinitions> <RowDefinition Height="Auto" /> <RowDefinition Height="Auto" /> <RowDefinition Height="Auto"/> <RowDefinition Height="Auto"/> <RowDefinition Height="Auto"/> <RowDefinition Height="Auto"/> </Grid.RowDefinitions> <TextBlock Grid.Row="0" Grid.Column="0" FontSize="14" Text="Enter data for source file, press Enter to create the file"/> <TextBlock Grid.Row="1" Grid.Column="0" FontSize="14" Text="Enter password and press Enter to create a signed copy" /> <TextBlock Grid.Row="2" Grid.Column="0" FontSize="14" Text="Enter file to check" /> <TextBlock Grid.Row="3" Grid.Column="0" FontSize="14" Text="Enter password and press Enter to check" /> <TextBlock Grid.Row="4" Grid.Column="0" FontSize="14" Text="Enter file name to delete" /> <TextBox x:Name="inputBox" Grid.Row="0" Grid.Column="1" TabIndex="0" FontSize="12" IsReadOnly="False" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <PasswordBox x:Name="passwordBox" Grid.Row="1" Grid.Column="1" TabIndex ="1" FontSize="12" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <TextBox x:Name="decodeBox" Grid.Row="2" Grid.Column="1" TabIndex="2" FontSize="12" IsReadOnly="False" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <PasswordBox x:Name="decodePassWordBox" Grid.Row="3" TabIndex="3" Grid.Column="1" FontSize="12" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <TextBox x:Name="deleteBox" Grid.Row="4" Grid.Column="1" TabIndex="4" FontSize="12" IsReadOnly="False" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <TextBlock x:Name="outputBlock" Grid.Row="5" Grid.Column="0" FontSize="12" TextWrapping="Wrap"> </TextBlock> </Grid> </UserControl>
<UserControl x:Class="HMACSHA256Example.MainPage" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Width="1024" Height="768"> <Grid x:Name="LayoutRoot" Background="White" > <Grid.ColumnDefinitions> <ColumnDefinition Width="550" /> <ColumnDefinition Width="250" /> <!--<ColumnDefinition Width="250" />--> </Grid.ColumnDefinitions> <Grid.RowDefinitions> <RowDefinition Height="Auto" /> <RowDefinition Height="Auto" /> <RowDefinition Height="Auto"/> <RowDefinition Height="Auto"/> <RowDefinition Height="Auto"/> <RowDefinition Height="Auto"/> </Grid.RowDefinitions> <TextBlock Grid.Row="0" Grid.Column="0" FontSize="14" Text="Enter data for source file, press Enter to create the file"/> <TextBlock Grid.Row="1" Grid.Column="0" FontSize="14" Text="Enter password and press Enter to create a signed copy" /> <TextBlock Grid.Row="2" Grid.Column="0" FontSize="14" Text="Enter file to check" /> <TextBlock Grid.Row="3" Grid.Column="0" FontSize="14" Text="Enter password and press Enter to check" /> <TextBlock Grid.Row="4" Grid.Column="0" FontSize="14" Text="Enter file name to delete" /> <TextBox x:Name="inputBox" Grid.Row="0" Grid.Column="1" TabIndex="0" FontSize="12" IsReadOnly="False" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <PasswordBox x:Name="passwordBox" Grid.Row="1" Grid.Column="1" TabIndex ="1" FontSize="12" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <TextBox x:Name="decodeBox" Grid.Row="2" Grid.Column="1" TabIndex="2" FontSize="12" IsReadOnly="False" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <PasswordBox x:Name="decodePassWordBox" Grid.Row="3" TabIndex="3" Grid.Column="1" FontSize="12" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <TextBox x:Name="deleteBox" Grid.Row="4" Grid.Column="1" TabIndex="4" FontSize="12" IsReadOnly="False" BorderThickness="5" Height="40" Width="160" HorizontalAlignment="Center" /> <TextBlock x:Name="outputBlock" Grid.Row="5" Grid.Column="0" FontSize="12" TextWrapping="Wrap"> </TextBlock> </Grid> </UserControl>
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.
