HMACMD5 Class
Updated: May 2011
Computes a Hash-based Message Authentication Code (HMAC) using the MD5 hash function.
Assembly: mscorlib (in mscorlib.dll)
HMACMD5 is a type of keyed hash algorithm that is constructed from the MD5 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, then applies the hash function a second time. The output hash will be 128 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.
MD5 (Message Digest Algorithm 5) is a cryptographic hash algorithm developed at RSA Laboratories. HMACMD5 accepts keys of any size, and produces a hash sequence 128 bits in length.
The following example shows how to sign a file by using the HMACMD5 object and then how to verify the file.
Imports System Imports System.IO Imports System.Security.Cryptography Public Class HMACMD5example 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 HMACMD5(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 HMACMD5(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 'HMACMD5example 'end VerifyFile 'end class
System.Security.Cryptography.HashAlgorithm
System.Security.Cryptography.KeyedHashAlgorithm
System.Security.Cryptography.HMAC
System.Security.Cryptography.HMACMD5
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.