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I created a sample for encrypt / decrypt a string using DES algorithm.

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Full codes:
Code:
Imports System.Security.Cryptography

Imports System.IO

Public Class CryptoMemoryStream

    Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click

        Try

            ' Create a new DES key.

            Using key As New DESCryptoServiceProvider()

                ' Encrypt a string to a byte array.

                Dim buffer As Byte() = Encrypt(TextBox1.Text, key)

                ' Display the unicodesbytestostring value to the textboxes.

                TextBox2.Text = UnicodeBytesToString(buffer)

                TextBox3.Text = UnicodeBytesToString(buffer)

            End Using

        Catch ex As Exception

            MsgBox(ex.Message, MsgBoxStyle.Exclamation, "Message")

        End Try

    End Sub

    Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click

        Try

            Using key As New DESCryptoServiceProvider()

                ' Decrypt the byte array back to a string.

                Dim buffer As Byte() = Encrypt(TextBox1.Text, key)

                Dim plaintext As String = Decrypt(buffer, key)

                ' Display the plaintext value to the textbox4.

                TextBox4.Text = plaintext

            End Using

        Catch ex As Exception

            MsgBox(ex.Message, MsgBoxStyle.Exclamation, "Message")

        End Try

    End Sub

    'Convert the bytes from a byte array into a string

    Private Shared Function UnicodeBytesToString(ByVal bytes() As Byte) As String

        Return System.Text.Encoding.Unicode.GetString(bytes)

    End Function

    ' Encrypt the string.

    Public Shared Function Encrypt(PlainText As String, key As SymmetricAlgorithm) As Byte()

        ' Create a memory stream.

        Dim ms As New MemoryStream()

        ' Create a CryptoStream using the memory stream and the 

        ' CSP DES key.  

        Dim encStream As New CryptoStream(ms, key.CreateEncryptor(), CryptoStreamMode.Write)

        ' Create a StreamWriter to write a string

        ' to the stream.

        Dim sw As New StreamWriter(encStream)

        ' Write the plaintext to the stream.

        sw.WriteLine(PlainText)

        ' Close the StreamWriter and CryptoStream.

        sw.Close()

        encStream.Close()

        ' Get an array of bytes that represents

        ' the memory stream.

        Dim buffer As Byte() = ms.ToArray()

        ' Close the memory stream.

        ms.Close()

        ' Return the encrypted byte array.

        Return buffer

    End Function 'Encrypt

    ' Decrypt the byte array.

    Public Shared Function Decrypt(CypherText() As Byte, key As SymmetricAlgorithm) As String

        ' Create a memory stream to the passed buffer.

        Dim ms As New MemoryStream(CypherText)

        ' Create a CryptoStream using the memory stream and the 

        ' CSP DES key. 

        Dim encStream As New CryptoStream(ms, key.CreateDecryptor(), CryptoStreamMode.Read)

        ' Create a StreamReader for reading the stream.

        Dim sr As New StreamReader(encStream)

        ' Read the stream as a string.

        Dim val As String = sr.ReadLine()

        ' Close the streams.

        sr.Close()

        encStream.Close()

        ms.Close()

        Return val

    End Function 'Decrypt

End Class
euverve
