MS Access VBA Substitution Cipher Encrypt/Decrypt - ms-access

Question

Could anyone suggest please how I can achieve a substitution cipher style; encrypt and decrypt function in VBA. I appreciate hashing is considered the better way but I need reversible encryption. Many Thanks.

Answer 1

You can use Blowfish. There's a Visual Basic 6 version that will work in Access, available here:
http://www.di-mgt.com.au/cryptoBlowfishVer6.html
You can also try TwoFish.

Answer 2

There is a simple example here or you can use the even simpler ROT13 cipher.
These are useful for obscuring a little text, but I'd not use them for anything that actually needs to be kept secure.

Answer 3

Many thanks for all the answers provided in reference to my question, it's good to see there are different approaches, this is one I coded yesterday morning. It allows a different cipher keyword/phrase to be used for both Upper & Lowercase letters, I have used 'Zebras' in this example, it also runs a second pass with the ROT13 cipher. Encryption method:
Public Function Encrypt(strvalue As String) As String
Const LowerAlpha As String = "abcdefghijklmnopqrstuvwxyz"
Const LowerSub As String = "zebrascdfghijklmnopqtuvwxy" 'zebras
Const UpperAlpha As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
Const UpperSub As String = "ZEBRASCDFGHIJKLMNOPQTUVWXY" 'ZEBRAS
Dim lngi As Long
Dim lngE As Long
Dim strEncrypt As String
Dim strLetter As String
If strvalue & "" = "" Then Exit Function
For lngi = 1 To Len(strvalue)
strLetter = Mid(strvalue, lngi, 1)
Select Case Asc(strLetter)
Case 65 To 90 'Uppercase
'Find position in alpha string
For lngE = 1 To Len(UpperAlpha)
If Mid(UpperAlpha, lngE, 1) = strLetter Then GoTo USub
Next
USub:
strEncrypt = strEncrypt & Mid(UpperSub, lngE, 1)
Case 97 To 122 'Lowercase
'Find position in alpha string
For lngE = 1 To Len(LowerAlpha)
If Mid(LowerAlpha, lngE, 1) = strLetter Then GoTo LSub
Next
LSub:
strEncrypt = strEncrypt & Mid(LowerSub, lngE, 1)
Case Else 'Do not substitute
strEncrypt = strEncrypt & strLetter
End Select
Next
'Now pass this string through ROT13 for another tier of security
For lngi = 1 To Len(strEncrypt)
Encrypt = Encrypt & Chr(Asc(Mid(strEncrypt, lngi, 1)) + 13)
Next
End Function
And this is the Decryption that goes with it:
Public Function Decrypt(strvalue As String) As String
Const LowerAlpha As String = "abcdefghijklmnopqrstuvwxyz"
Const LowerSub As String = "zebrascdfghijklmnopqtuvwxy" 'zebras
Const UpperAlpha As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
Const UpperSub As String = "ZEBRASCDFGHIJKLMNOPQTUVWXY" 'ZEBRAS
Dim lngi As Long
Dim lngE As Long
Dim strDecrypt As String
Dim strLetter As String
If strvalue & "" = "" Then Exit Function
'Reverse the ROT13 cipher
For lngi = 1 To Len(strvalue)
strDecrypt = strDecrypt & Chr(Asc(Mid(strvalue, lngi, 1)) - 13)
Next
'Now reverse the encryption
For lngi = 1 To Len(strDecrypt)
strLetter = Mid(strDecrypt, lngi, 1)
Select Case Asc(strLetter)
Case 65 To 90 'Uppercase
'Find position in sub string
For lngE = 1 To Len(UpperSub)
If Mid(UpperSub, lngE, 1) = strLetter Then GoTo USub
Next
USub:
Decrypt = Decrypt & Mid(UpperAlpha, lngE, 1)
Case 97 To 122 'Lowercase
'Find position in sub string
For lngE = 1 To Len(LowerSub)
If Mid(LowerSub, lngE, 1) = strLetter Then GoTo LSub
Next
LSub:
Decrypt = Decrypt & Mid(LowerAlpha, lngE, 1)
Case Else 'Do not substitute
Decrypt = Decrypt & strLetter
End Select
Next
End Function
I hope the coding is very simple to follow for those who do not have vast experience with VBA coding and it can be lifted straight from the page; but again thanks for all the other answers.