Mira te dejo dos funciones que sirven para encryptar cadenas en VB 6.0; en .net nose, pero teniendo el codigo de vb lo puedes adaptar...
incluso yo lo traduci a JAVA; asi que es mas sencillo traducirlo a .net ....
no recuerdo quien es el autor, lo tengo varios años usando y es muy efectivo
Function cifra(ByVal Pass As String) As String
Dim clave As String, i As Integer, Pass2 As String
Dim CAR As String, Codigo As String
clave = "%ü&/@#$A"
Pass2 = ""
For i = 1 To Len(Pass)
CAR = Mid(Pass, i, 1)
Codigo = Mid(clave, ((i - 1) Mod Len(clave)) + 1, 1)
Pass2 = Pass2 & Right("0" & Hex(Asc(Codigo) Xor Asc(CAR)), 2)
Next i
cifra = Pass2
End Function
Function descifra(ByVal Pass As String) As String
Dim clave As String, i As Integer, Pass2 As String
Dim CAR As String, Codigo As String
Dim j As Integer
clave = "%ü&/@#$A"
Pass2 = ""
j = 1
For i = 1 To Len(Pass) Step 2
CAR = Mid(Pass, i, 2)
Codigo = Mid(clave, ((j - 1) Mod Len(clave)) + 1, 1)
Pass2 = Pass2 & Chr(Asc(Codigo) Xor Val("&h" + CAR))
j = j + 1
Next i
descifra = Pass2
End Function
Saludos