This documentation is archived and is not being maintained.


Updated: May 2011

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

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

<ComVisibleAttribute(True)> _
Public Class HMACSHA1 _
	Inherits HMAC
Dim instance As HMACSHA1

HMACSHA1 is a type of keyed hash algorithm that is constructed from the SHA-1 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, 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 will result 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 of length 160 bits.

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

    ' Computes a keyed hash for a source file, creates a target file with the keyed hash 
    ' prepended to the contents of the source file, then decrypts the file and compares 
    ' the source and the decrypted files. 
    Public Shared Sub EncodeFile(ByVal key() As Byte, ByVal sourceFile As String, ByVal destFile As String)
        ' Initialize the keyed hash object. 
        Dim myhmacsha1 As New HMACSHA1(key)
        Dim inStream As New FileStream(sourceFile, FileMode.Open)
        Dim outStream As New FileStream(destFile, FileMode.Create)
        ' Compute the hash of the input file. 
        Dim hashValue As Byte() = myhmacsha1.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
            ' Read from the wrapping CryptoStream.
            bytesRead = inStream.Read(buffer, 0, 1024)
            outStream.Write(buffer, 0, bytesRead)
        Loop While bytesRead > 0
        ' Close the streams

    End Sub 

    ' Decrypt the encoded file and compare to original file. 
    Public Shared Function DecodeFile(ByVal key() As Byte, ByVal sourceFile As String) As Boolean 
        ' Initialize the keyed hash object.  
        Dim hmacsha1 As New HMACSHA1(key)
        ' Create an array to hold the keyed hash value read from the file. 
        Dim storedHash(hmacsha1.HashSize / 8) As Byte
        ' Create a FileStream for the source file. 
        Dim inStream As New FileStream(sourceFile, FileMode.Open)
        ' 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() = hmacsha1.ComputeHash(inStream)
        ' compare the computed hash with the stored value 
        Dim err As Boolean = False 
        Dim i As Integer 
        For i = 0 To storedHash.Length
            If computedHash(i) <> storedHash(i) Then
                Console.WriteLine("Hash values differ! Encoded file has been tampered with!")
                Return False 
            End If 
        Next i
        If (err) Then
            Console.WriteLine("Hash values differ! Encoded file has been tampered with!")
            Return False 
            Console.WriteLine("Hash values agree -- no tampering occurred.")
            Return True 
        End If 
    End Function 
    Private Const usageText As String = "Usage: HMACSHA1 inputfile.txt encryptedfile.hsh" + vbLf + "You must specify the two file names. Only the first file must exist." + vbLf

    Public Shared Sub Main(ByVal Fileargs() As String)
        'If no file names are specified, write usage text. 
        If Fileargs.Length < 2 Then
                ' 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. 
                Dim rng As New RNGCryptoServiceProvider()
                ' The array is now filled with cryptographically strong random bytes.

                ' Use the secret key to encode the message file.
                EncodeFile(secretkey, Fileargs(0), Fileargs(1))

                ' Take the encoded file and decode
                DecodeFile(secretkey, Fileargs(1))
            Catch e As IOException
                Console.WriteLine("Error: File not found", e)
            End Try 
        End If 

    End Sub 
End Class

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

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98

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

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0




May 2011

Updated the code example.

Information enhancement.