This is a test of a rudimentary encryption/decryption 
algorithm.

Note: It is possible for the encrypted string to include Chr$(0). 
Visual Basic handles this just fine but Windows (and 
therefore the TextBox control) uses Chr$(0) to signify the end 
of the string. Therefore, programs such as this one that store 
the encrypted data in a text box may end up truncating the data.

First, add a textbox, called txtText, with its MultiLine property set to true. Then, add another textbox called txtPassword, and two command buttons named cmdEncrypt, and cmdDecrypt. Finally, add the code below.

'This program may be distributed on the condition that it is
'distributed in full and unchanged, and that no fee is charged for
'such distribution with the exception of reasonable shipping and media
'charged. In addition, the code in this program may be incorporated
'into your own programs and the resulting programs may be distributed
'without payment of royalties.
'
'This example program was provided by:
' SoftCircuits Programming
' http://www.softcircuits.com
' P.O. Box 16262
' Irvine, CA 92623

Option Explicit

'Set to True to make the password case-sensitive
#Const CASE_SENSITIVE_PASSWORD = False

Private Sub cmdEncrypt_Click()
    ' You can encrypt twice for extra security
    txtText = EncryptText((txtText), txtPassword)
    txtText = EncryptText((txtText), txtPassword)
End Sub

Private Sub cmdDecrypt_Click()
    txtText = DecryptText((txtText), txtPassword)
    txtText = DecryptText((txtText), txtPassword)
End Sub

'Encrypt text
Private Function EncryptText(strText As String, ByVal strPwd As String)
    Dim i As Integer, c As Integer
    Dim strBuff As String

#If Not CASE_SENSITIVE_PASSWORD Then

    'Convert password to upper case
    'if not case-sensitive
    strPwd = UCase$(strPwd)

#End If

    'Encrypt string
    If Len(strPwd) Then
        For i = 1 To Len(strText)
            c = Asc(Mid$(strText, i, 1))
            c = c + Asc(Mid$(strPwd, (i Mod Len(strPwd)) + 1, 1))
            strBuff = strBuff & Chr$(c And &HFF)
        Next i
    Else
        strBuff = strText
    End If
    EncryptText = strBuff
End Function

'Decrypt text encrypted with EncryptText
Private Function DecryptText(strText As String, ByVal strPwd As String)
    Dim i As Integer, c As Integer
    Dim strBuff As String

#If Not CASE_SENSITIVE_PASSWORD Then

    'Convert password to upper case
    'if not case-sensitive
    strPwd = UCase$(strPwd)

#End If

    'Decrypt string
    If Len(strPwd) Then
        For i = 1 To Len(strText)
            c = Asc(Mid$(strText, i, 1))
            c = c - Asc(Mid$(strPwd, (i Mod Len(strPwd)) + 1, 1))
            strBuff = strBuff & Chr$(c And &HFF)
        Next i
    Else
        strBuff = strText
    End If
    DecryptText = strBuff
End Function