how can we identify ASCII or Unicode Encoded thru Encrypted String?

Question

we are using ASCIIEncoding for saving data we have the new requirement for support an Unicode value (regional language like (Hindi, Gujarati) so we modify the code as below we have not done a big change just change over ASCIIEncoding to UnicodeEncoding . the issue is we are not able to decrypt old records using UnicodeEncoding

is there any way to identify the encryption method from an encrypted string which is stored in DB

we are using Below Code

Public Function DESEncrypt(ByVal PlainText As String) As String
    Dim KeyBArray As Byte()
    Dim IVBArray As Byte()
    Dim Encryptor As ICryptoTransform
    Dim PlainTextBArray As Byte()
    'Dim Ascii As New ASCIIEncoding 
    Dim Ascii As New UnicodeEncoding 
    Dim I As Integer
    Dim Result As String

    PlainTextBArray = Ascii.GetBytes(PlainText)
    KeyBArray = Convert.FromBase64String(desKey)
    IVBArray = Convert.FromBase64String(desIV)

    Dim TDES As New TripleDESCryptoServiceProvider
    Encryptor = TDES.CreateEncryptor(KeyBArray, IVBArray)      

    Dim CypherTextBArray(Encryptor.OutputBlockSize - 1) As Byte
    CypherTextBArray = Encryptor.TransformFinalBlock(PlainTextBArray, I, PlainTextBArray.Length)
    Dim TempBArray(CypherTextBArray.Length - 1) As Byte
    For I = 0 To CypherTextBArray.Length - 1
        TempBArray(I) = CypherTextBArray(I)
    Next
    Result = Convert.ToBase64String(TempBArray)
    KeyBArray = Nothing
    IVBArray = Nothing
    Encryptor = Nothing
    PlainTextBArray = Nothing
    Ascii = Nothing
    Return Result
End Function

Public Function DESDecrypt(ByVal CypherText As String) As String
    Dim KeyBArray As Byte()
    Dim IVBArray As Byte()       
    Dim CypherTextBArray As Byte()
    Dim Decryptor As ICryptoTransform
    Dim PlainText As New StringBuilder("")
    Dim I As Integer
    Dim Result As String = String.Empty

    If CypherText <> "" Then
        CypherTextBArray = Convert.FromBase64String(CypherText)
        KeyBArray = Convert.FromBase64String(desKey)
        IVBArray = Convert.FromBase64String(desIV)

        If cryptoPrivider = "TripleDES" Or cryptoPrivider = "CITripleDES" Then
            Dim TDES As New TripleDESCryptoServiceProvider
            Decryptor = TDES.CreateDecryptor(KeyBArray, IVBArray)
        Else
            Dim DES As New DESCryptoServiceProvider
            Decryptor = DES.CreateDecryptor(KeyBArray, IVBArray)
        End If

        Dim PlainTextBArray(Decryptor.OutputBlockSize - 1) As Byte

        PlainTextBArray = Decryptor.TransformFinalBlock(CypherTextBArray, I, CypherTextBArray.Length)
        'Result = System.Text.Encoding.ASCII.GetString(PlainTextBArray) 
        Result = System.Text.Encoding.Unicode.GetString(PlainTextBArray) 

    End If        
    KeyBArray = Nothing
    IVBArray = Nothing
    CypherTextBArray = Nothing
    Decryptor = Nothing
    PlainText = Nothing
    Return Result
End Function

Answer 1

You should use UTF8Encoding instead of UnicodeEncoding . The latter is encoded with UTF16, so not bytes compatible with ASCII. Instead UTF8 is byte compatible with ASCII. Both support all Unicode codes, so there is no loss of information.

But often one is not so lucky to have a superset encoder. So in general you should keep track of the encoding (eg in a new field), or just use one encoding (so you convert all your databases).