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HMACSHA256 Class

Computes a Hash-based Message Authentication Code (HMAC) using the SHA256 hash function.

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

'Declaration
<ComVisibleAttribute(True)> _
Public Class HMACSHA256 _
	Inherits HMAC

The HMACSHA256 type exposes the following members.

  NameDescription
Public methodSupported by Silverlight for Windows PhoneHMACSHA256Initializes a new instance of the HMACSHA256 class with a randomly generated key.
Public methodSupported by Silverlight for Windows PhoneHMACSHA256(Byte())Initializes a new instance of the HMACSHA256 class with the specified key data.
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  NameDescription
Protected propertySupported by Silverlight for Windows PhoneBlockSizeValueGets or sets the block size to use in the hash value. (Inherited from HMAC.)
Public propertySupported by Silverlight for Windows PhoneCanReuseTransformGets a value indicating whether the current transform can be reused. (Inherited from HashAlgorithm.)
Public propertySupported by Silverlight for Windows PhoneCanTransformMultipleBlocksWhen overridden in a derived class, gets a value indicating whether multiple blocks can be transformed. (Inherited from HashAlgorithm.)
Public propertySupported by Silverlight for Windows PhoneHashGets the value of the computed hash code. (Inherited from HashAlgorithm.)
Public propertySupported by Silverlight for Windows PhoneHashNameGets or sets the name of the hash algorithm to use for hashing. (Inherited from HMAC.)
Public propertySupported by Silverlight for Windows PhoneHashSizeGets the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.)
Public propertySupported by Silverlight for Windows PhoneInputBlockSizeWhen overridden in a derived class, gets the input block size. (Inherited from HashAlgorithm.)
Public propertySupported by Silverlight for Windows PhoneKeyGets or sets the key to use in the hash algorithm. (Inherited from HMAC.)
Public propertySupported by Silverlight for Windows PhoneOutputBlockSizeWhen overridden in a derived class, gets the output block size. (Inherited from HashAlgorithm.)
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  NameDescription
Public methodSupported by Silverlight for Windows PhoneClearReleases all resources used by the HashAlgorithm class. (Inherited from HashAlgorithm.)
Public methodSupported by Silverlight for Windows PhoneComputeHash(Byte())Computes the hash value for the specified byte array. (Inherited from HashAlgorithm.)
Public methodSupported by Silverlight for Windows PhoneComputeHash(Stream)Computes the hash value for the specified Stream object. (Inherited from HashAlgorithm.)
Public methodSupported by Silverlight for Windows PhoneComputeHash(Byte(), Int32, Int32)Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.)
Protected methodSupported by Silverlight for Windows PhoneDisposeReleases the unmanaged resources used by the HMAC class when a key change is legitimate and optionally releases the managed resources. (Inherited from HMAC.)
Public methodSupported by Silverlight for Windows PhoneEquals(Object)Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected methodSupported by Silverlight for Windows PhoneFinalizeAllows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
Public methodSupported by Silverlight for Windows PhoneGetHashCodeServes as a hash function for a particular type. (Inherited from Object.)
Public methodSupported by Silverlight for Windows PhoneGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodSupported by Silverlight for Windows PhoneHashCoreWhen 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.)
Protected methodSupported by Silverlight for Windows PhoneHashFinalWhen overridden in a derived class, finalizes the hash computation after the last data is processed by the cryptographic stream object. (Inherited from HMAC.)
Public methodSupported by Silverlight for Windows PhoneInitializeInitializes an instance of the default implementation of HMAC. (Inherited from HMAC.)
Protected methodSupported by Silverlight for Windows PhoneMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodSupported by Silverlight for Windows PhoneToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodSupported by Silverlight for Windows PhoneTransformBlockComputes 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.)
Public methodSupported by Silverlight for Windows PhoneTransformFinalBlockComputes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.)
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  NameDescription
Protected fieldSupported by Silverlight for Windows PhoneHashSizeValueRepresents the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.)
Protected fieldSupported by Silverlight for Windows PhoneHashValueRepresents the value of the computed hash code. (Inherited from HashAlgorithm.)
Protected fieldSupported by Silverlight for Windows PhoneKeyValueThe key to use in the hash algorithm. (Inherited from KeyedHashAlgorithm.)
Protected fieldSupported by Silverlight for Windows PhoneStateRepresents the state of the hash computation. (Inherited from HashAlgorithm.)
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  NameDescription
Explicit interface implemetationPrivate methodSupported by Silverlight for Windows PhoneIDisposable.DisposeReleases the unmanaged resources used by the HashAlgorithm and optionally releases the managed resources. (Inherited from HashAlgorithm.)
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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.

NoteNote:

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>


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

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

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

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