Encryption and Decryption in VB.Net

Public Class clsCrypto
    Private bytIV() As Byte = {121, 241, 10, 1, 132, 74, 11, 39, 255, 91, 45, 78, 14, 211, 22, 62}
    Private Const chrKeyFill As Char = "X"c
    Private Const strTextErrorString As String = "#ERROR - {0}"
    Private Const intMinSalt As Integer = 4
    Private Const intMaxSalt As Integer = 8
    Private Const intHashSize As Integer = 16
    Private Const intKeySize As Integer = 32
    Public Function EncryptString128Bit(ByVal strPlainText As String, ByVal strKey As String) As String
        Try
            Dim bytPlainText() As Byte
            Dim bytKey() As Byte
            Dim bytEncoded() As Byte
            Dim objMemoryStream As New MemoryStream
            Dim objRijndaelManaged As New RijndaelManaged

            strPlainText = strPlainText.Replace(vbNullChar, String.Empty)
            bytPlainText = Encoding.UTF8.GetBytes(strPlainText)
            bytKey = ConvertKeyToBytes(strKey)
            Dim objCryptoStream As New CryptoStream(objMemoryStream, _
                objRijndaelManaged.CreateEncryptor(bytKey, bytIV), _
                CryptoStreamMode.Write)
            objCryptoStream.Write(bytPlainText, 0, bytPlainText.Length)
            objCryptoStream.FlushFinalBlock()
            bytEncoded = objMemoryStream.ToArray
            objMemoryStream.Close()
            objCryptoStream.Close()
            Return Convert.ToBase64String(bytEncoded)
        Catch ex As Exception
            Return String.Format(strTextErrorString, ex.Message)
        End Try
    End Function

    Public Function DecryptString128Bit(ByVal strCryptText As String, ByVal strKey As String) As String
        Try
            Dim bytCryptText() As Byte
            Dim bytKey() As Byte
            Dim objRijndaelManaged As New RijndaelManaged

            bytCryptText = Convert.FromBase64String(strCryptText)
            bytKey = ConvertKeyToBytes(strKey)

            Dim bytTemp(bytCryptText.Length) As Byte
            Dim objMemoryStream As New MemoryStream(bytCryptText)
            Dim objCryptoStream As New CryptoStream(objMemoryStream, _
                objRijndaelManaged.CreateDecryptor(bytKey, bytIV), _
                CryptoStreamMode.Read)
            objCryptoStream.Read(bytTemp, 0, bytTemp.Length)
            objMemoryStream.Close()
            objCryptoStream.Close()
            Return Encoding.UTF8.GetString(bytTemp).Replace(vbNullChar, String.Empty)
        Catch ex As Exception
            Return String.Format(strTextErrorString, ex.Message)
        End Try

    End Function

    Public Function ComputeMD5Hash(ByVal strPlainText As String, Optional ByVal bytSalt() As Byte = Nothing) As String
        Try
            Dim bytPlainText As Byte() = Encoding.UTF8.GetBytes(strPlainText)
            Dim hash As HashAlgorithm = New MD5CryptoServiceProvider()

            If bytSalt Is Nothing Then
                Dim rand As New Random
                Dim intSaltSize As Integer = rand.Next(intMinSalt, intMaxSalt)
                bytSalt = New Byte(intSaltSize - 1) {}
                Dim rng As New RNGCryptoServiceProvider
                rng.GetNonZeroBytes(bytSalt)
            End If
            Dim bytPlainTextWithSalt() As Byte = New Byte(bytPlainText.Length + bytSalt.Length - 1) {}
            bytPlainTextWithSalt = ConcatBytes(bytPlainText, bytSalt)
            Dim bytHash As Byte() = hash.ComputeHash(bytPlainTextWithSalt)
            Dim bytHashWithSalt() As Byte = New Byte(bytHash.Length + bytSalt.Length - 1) {}
            bytHashWithSalt = ConcatBytes(bytHash, bytSalt)
            Return Convert.ToBase64String(bytHashWithSalt)
        Catch ex As Exception
            Return String.Format(strTextErrorString, ex.Message)
        End Try
    End Function

    Public Function VerifyHash(ByVal strPlainText As String, ByVal strHashValue As String) As Boolean
        Try
            Dim bytWithSalt As Byte() = Convert.FromBase64String(strHashValue)
            If bytWithSalt.Length < intHashSize Then Return False
            Dim bytSalt() As Byte = New Byte(bytWithSalt.Length - intHashSize - 1) {}
            Array.Copy(bytWithSalt, intHashSize, bytSalt, 0, bytWithSalt.Length - intHashSize)
            Dim strExpectedHashString As String = ComputeMD5Hash(strPlainText, bytSalt)
            Return strHashValue.Equals(strExpectedHashString)
        Catch ex As Exception
            Return Nothing
        End Try
    End Function

    Private Function ConcatBytes(ByVal bytA() As Byte, ByVal bytB() As Byte) As Byte()
        Try
            Dim bytX() As Byte = New Byte(((bytA.Length + bytB.Length)) - 1) {}
            Array.Copy(bytA, bytX, bytA.Length)
            Array.Copy(bytB, 0, bytX, bytA.Length, bytB.Length)
            Return bytX
        Catch ex As Exception
            Return Nothing
        End Try
    End Function

    Private Function ConvertKeyToBytes(ByVal strKey As String) As Byte()
        Try
            Dim intLength As Integer = strKey.Length
            If intLength < intKeySize Then
                strKey &= Strings.StrDup(intKeySize - intLength, chrKeyFill)
            Else
                strKey = strKey.Substring(0, intKeySize)
            End If
            Return Encoding.UTF8.GetBytes(strKey)
        Catch ex As Exception
            Return Nothing
        End Try
    End Function
End Class