(VBScript) 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.

Chilkat ActiveX Downloads

ActiveX for 32-bit and 64-bit Windows

Dim fso, outFile
Set fso = CreateObject("Scripting.FileSystemObject")
'Create a Unicode (utf-16) output text file.
Set outFile = fso.CreateTextFile("output.txt", True, True)

success = 0

' 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

set crypt = CreateObject("Chilkat.Crypt2")

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

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

fileToDecrypt = "c:/temp/qa_output/hamlet.enc"
set facIn = CreateObject("Chilkat.FileAccess")
success = facIn.OpenForRead(fileToDecrypt)
If (success <> 1) Then
    outFile.WriteLine("Failed to open file to be decrytped.")
    WScript.Quit
End If

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

crypt.FirstChunk = 1
crypt.LastChunk = 0

set bd = CreateObject("Chilkat.BinData")
set bdDecrypted = CreateObject("Chilkat.BinData")

i = 0
Do While i < numChunks
    i = i + 1
    If (i = numChunks) Then
        crypt.LastChunk = 1
    End If

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

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

    crypt.FirstChunk = 0
Loop

' Make sure both FirstChunk and LastChunk are restored to 1 after
' encrypting or decrypting in chunks.  Otherwise subsequent encryptions/decryptions
' will produce unexpected results.
crypt.FirstChunk = 1
crypt.LastChunk = 1

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.
success = bdDecrypted.WriteFile("c:/temp/qa_output/hamlet_decrypted.xml")

outFile.Close