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HMACSHA1 Class

Computes a Hash-based Message Authentication Code (HMAC) using the SHA1 hash function.

Namespace:  System.Security.Cryptography
Assembly:  mscorlib (in mscorlib.dll)
'Declaration
<ComVisibleAttribute(True)> _
Public Class HMACSHA1 _
	Inherits HMAC

The HMACSHA1 type exposes the following members.

  NameDescription
Public methodHMACSHA1Initializes a new instance of the HMACSHA1 class with a randomly generated key.
Public methodHMACSHA1(Byte())Initializes a new instance of the HMACSHA1 class with the specified key data.
Public methodHMACSHA1(Byte(), Boolean)Initializes a new instance of the HMACSHA1 class with the specified key data and a value that specifies whether to use the managed version of the SHA1 algorithm.
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  NameDescription
Protected propertyBlockSizeValueGets or sets the block size to use in the hash value. (Inherited from HMAC.)
Public propertyCanReuseTransformGets a value indicating whether the current transform can be reused. (Inherited from HashAlgorithm.)
Public propertyCanTransformMultipleBlocksWhen overridden in a derived class, gets a value indicating whether multiple blocks can be transformed. (Inherited from HashAlgorithm.)
Public propertyHashGets the value of the computed hash code. (Inherited from HashAlgorithm.)
Public propertyHashNameGets or sets the name of the hash algorithm to use for hashing. (Inherited from HMAC.)
Public propertyHashSizeGets the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.)
Public propertyInputBlockSizeWhen overridden in a derived class, gets the input block size. (Inherited from HashAlgorithm.)
Public propertyKeyGets or sets the key to use in the hash algorithm. (Inherited from HMAC.)
Public propertyOutputBlockSizeWhen overridden in a derived class, gets the output block size. (Inherited from HashAlgorithm.)
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  NameDescription
Public methodClearReleases all resources used by the HashAlgorithm class. (Inherited from HashAlgorithm.)
Public methodComputeHash(Byte())Computes the hash value for the specified byte array. (Inherited from HashAlgorithm.)
Public methodComputeHash(Stream)Computes the hash value for the specified Stream object. (Inherited from HashAlgorithm.)
Public methodComputeHash(Byte(), Int32, Int32)Computes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.)
Public methodDisposeReleases all resources used by the current instance of the HashAlgorithm class. (Inherited from HashAlgorithm.)
Protected methodDispose(Boolean)Releases the unmanaged resources used by the HMAC class when a key change is legitimate and optionally releases the managed resources. (Inherited from HMAC.)
Public methodEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodHashCoreWhen 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 methodHashFinalWhen overridden in a derived class, finalizes the hash computation after the last data is processed by the cryptographic stream object. (Inherited from HMAC.)
Public methodInitializeInitializes an instance of the default implementation of HMAC. (Inherited from HMAC.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)
Public methodTransformBlockComputes the hash value for the specified region of the input byte array and copies the specified region of the input byte array to the specified region of the output byte array. (Inherited from HashAlgorithm.)
Public methodTransformFinalBlockComputes the hash value for the specified region of the specified byte array. (Inherited from HashAlgorithm.)
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  NameDescription
Protected fieldHashSizeValueRepresents the size, in bits, of the computed hash code. (Inherited from HashAlgorithm.)
Protected fieldHashValueRepresents the value of the computed hash code. (Inherited from HashAlgorithm.)
Protected fieldKeyValueThe key to use in the hash algorithm. (Inherited from KeyedHashAlgorithm.)
Protected fieldStateRepresents the state of the hash computation. (Inherited from HashAlgorithm.)
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  NameDescription
Explicit interface implemetationPrivate methodIDisposable.DisposeReleases the unmanaged resources used by the HashAlgorithm and optionally releases the managed resources. (Inherited from HashAlgorithm.)
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HMACSHA1 is a type of keyed hash algorithm that is constructed from the SHA1 hash function and used as an HMAC, or hash-based message authentication code. 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 160 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.

The SHA-1 (Secure Hash Algorithm, also called SHS, Secure Hash Standard) is a cryptographic hash algorithm published by the United States Government. It produces a 160-bit hash value from an arbitrary length string.

HMACSHA1 accepts keys of any size, and produces a hash sequence that is 160 bits in length.

The following code example shows how to sign a file by using the HMACSHA1 object and then how to verify the file.

Imports System
Imports System.IO
Imports System.Security.Cryptography

Public Class HMACSHA1example

    Public Shared Sub Main(ByVal Fileargs() As String)
        Dim dataFile As String 
        Dim signedFile As String 
        'If no file names are specified, create them. 
        If Fileargs.Length < 2 Then
            dataFile = "text.txt"
            signedFile = "signedFile.enc" 

            If Not File.Exists(dataFile) Then 
                ' Create a file to write to. 
                Using sw As StreamWriter = File.CreateText(dataFile)
                    sw.WriteLine("Here is a message to sign")
                End Using 
            End If 

        Else
            dataFile = Fileargs(0)
            signedFile = Fileargs(1)
        End If 
        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) {}
            'RNGCryptoServiceProvider is an implementation of a random number generator. 
            Using rng As New RNGCryptoServiceProvider()
                ' The array is now filled with cryptographically strong random bytes.
                rng.GetBytes(secretkey)

                ' Use the secret key to encode the message file.
                SignFile(secretkey, dataFile, signedFile)

                ' Take the encoded file and decode
                VerifyFile(secretkey, signedFile)
            End Using 
        Catch e As IOException
            Console.WriteLine("Error: File not found", e)
        End Try 

    End Sub 'Main

    ' Computes a keyed hash for a source file and creates a target file with the keyed hash 
    ' prepended to the contents of the source file.  
    Public Shared Sub SignFile(ByVal key() As Byte, ByVal sourceFile As String, ByVal destFile As String)
        ' Initialize the keyed hash object. 
        Using myhmac As New HMACSHA1(key)
            Using inStream As New FileStream(sourceFile, FileMode.Open)
                Using outStream As New FileStream(destFile, FileMode.Create)
                    ' Compute the hash of the input file. 
                    Dim hashValue As Byte() = myhmac.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
                End Using 
            End Using 
        End Using 
        Return 

    End Sub 'SignFile
    ' end SignFile 

    ' Compares the key in the source file with a new key created for the data portion of the file. If the keys  
    ' compare the data has not been tampered with. 
    Public Shared Function VerifyFile(ByVal key() As Byte, ByVal sourceFile As String) As Boolean 
        Dim err As Boolean = False 
        ' Initialize the keyed hash object.  
        Using hmac As New HMACSHA1(key)
            ' Create an array to hold the keyed hash value read from the file. 
            Dim storedHash(hmac.HashSize / 8) As Byte 
            ' Create a FileStream for the source file. 
            Using inStream As New FileStream(sourceFile, FileMode.Open)
                ' Read in the storedHash.
                inStream.Read(storedHash, 0, storedHash.Length - 1)
                ' 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() = hmac.ComputeHash(inStream)
                ' compare the computed hash with the stored value 
                Dim i As Integer 
                For i = 0 To storedHash.Length - 2
                    If computedHash(i) <> storedHash(i) Then
                        err = True 
                    End If 
                Next i
            End Using 
        End Using 
        If err Then
            Console.WriteLine("Hash values differ! Signed file has been tampered with!")
            Return False 
        Else
            Console.WriteLine("Hash values agree -- no tampering occurred.")
            Return True 
        End If 

    End Function 'VerifyFile 
End Class 'HMACSHA1example 'end VerifyFile
'end class

.NET Framework

Supported in: 4.5.1, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone 8, Silverlight 8.1

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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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