VB.NET

Decrypt File in Chunks using 256-bit AES

See more Encryption Examples

Shows how to decrypt a file chunk-by-chunk using FirstChunk/LastChunk properties and accumulate the results in memory with a Chilkat BinData object. VB.NET

Dim success As Boolean = False

' This example assumes the Chilkat API to have been previously unlocked.
' See Global Unlock Sample for sample code.

' This example decrypts the file previously encrypted in this example:
' Encrypt File in Chunks using AES CBC

Dim crypt As New Chilkat.Crypt2

crypt.CryptAlgorithm = "aes"
crypt.CipherMode = "cbc"
crypt.KeyLength = 256

crypt.SetEncodedKey("000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F","hex")
crypt.SetEncodedIV("000102030405060708090A0B0C0D0E0F","hex")

Dim fileToDecrypt As String = "c:/temp/qa_output/hamlet.enc"
Dim facIn As New Chilkat.FileAccess
success = facIn.OpenForRead(fileToDecrypt)
If (success <> True) Then
    Debug.WriteLine("Failed to open file to be decrytped.")
    Exit Sub
End If


' Let's decrypt in 32000 byte chunks.
Dim chunkSize As Integer = 32000
Dim numChunks As Integer = facIn.GetNumBlocks(chunkSize)

crypt.FirstChunk = True
crypt.LastChunk = False

Dim bd As New Chilkat.BinData
Dim bdDecrypted As New Chilkat.BinData

Dim i As Integer = 0
While i < numChunks
    i = i + 1
    If (i = numChunks) Then
        crypt.LastChunk = True
    End If


    ' Read the next chunk from the file.
    ' The last chunk will be whatever amount remains in the file..
    bd.Clear()
    facIn.FileReadBd(chunkSize,bd)

    ' Decrypt this chunk.
    crypt.DecryptBd(bd)
    ' Accumulate the decrypted chunks.
    bdDecrypted.AppendBd(bd)

    crypt.FirstChunk = False
End While

' Make sure both FirstChunk and LastChunk are restored to True after
' encrypting or decrypting in chunks.  Otherwise subsequent encryptions/decryptions
' will produce unexpected results.
crypt.FirstChunk = True
crypt.LastChunk = True

facIn.FileClose()

' The fully decrypted file is contained in bdDecrypted.
' You can save to a file if desired, or use the decrypted data in your application directly from bdDecrypted.
bdDecrypted.WriteFile("c:/temp/qa_output/hamlet_decrypted.xml")