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



Public Class HMACSHA256example

    ' 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 myhmacsha256 As New HMACSHA256(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() = 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()
        Return

    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 hmacsha256 As New HMACSHA256(key)
        ' Create an array to hold the keyed hash value read from the file.
        Dim storedHash(hmacsha256.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() = hmacsha256.ComputeHash(inStream)
        ' compare the computed hash with the stored value
        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
        Console.WriteLine("Hash values agree -- no tampering occurred.")
        Return True

    End Function
    Private Const usageText As String = "Usage: HMACSHA256 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
            Console.WriteLine(usageText)
        Else
            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.
                Dim 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.
                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