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calculating legth of triple DES  encrypted string

Posted on 2009-04-07
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Last Modified: 2012-05-06
I am encrypting a varchar(30) column and storing in a different column in the database.

I need to calculate what the maximum length of the encrypted text could be so that I can make sure the new column is the right size. How do I calculate the size?

I am using vb.net to do the encryption and SQL server 2005 to store the data.
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Question by:darrenwright
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Expert Comment

by:udaya kumar laligondla
ID: 24087691
for a 30 character string it will be approximately 36 chars. you can use the following caluculation to calculate the length n + 8 - (n % 8)
where n is the length of the source string
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Author Comment

by:darrenwright
ID: 24088329
I'm get back encryption strings of 88 chars so something not right
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Expert Comment

by:udaya kumar laligondla
ID: 24088695
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Author Comment

by:darrenwright
ID: 24088866
I use encrypttostring function
        Public Overloads Function EncrypttoString(ByVal plainText As String) As String
        Return Convert.ToBase64String(Encrypt(plainText))
    End Function
Public Function Encrypt(ByVal plainText As String) As Byte()
 
        Dim tdes As New TripleDESCryptoServiceProvider()
        Dim encryptor As ICryptoTransform = tdes.CreateEncryptor(mvarTripleDesKey, mvarTripleIV)
        Dim data As Byte() = Encoding.Unicode.GetBytes(plainText)
 
        Return encryptor.TransformFinalBlock(data, 0, data.Length)
 
    End Function

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Accepted Solution

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udaya kumar laligondla earned 500 total points
ID: 24093483
you are using three conversions (for 30 chars)
1.  unicode ( 30 *2 = 60)
2.encryption ( 60 +8 (60 % 8) = 64 (63.2))
3. Base64 Encoding (88)  calculated as Base64 = ((64 + 3 - (64 MOD 3)) /3) x 4

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