(Excel) Encrypt File in Chunks using AES CBC

Demonstrates how to use the FirstChunk/LastChunk properties to encrypt a file chunk-by-chunk.

Important This example requires Chilkat v9.5.0.68 or later.

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' This example requires the Chilkat Crypt API to have been previously unlocked.
' See Unlock Chilkat Crypt for sample code.

Dim crypt As Chilkat.Crypt2
Set crypt = Chilkat.NewCrypt2

crypt.CryptAlgorithm = "aes"
crypt.CipherMode = "cbc"
crypt.KeyLength = 128

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


fileToEncrypt = "qa_data/hamlet.xml"
Dim facIn As Chilkat.FileAccess
Set facIn = Chilkat.NewFileAccess

success = facIn.OpenForRead(fileToEncrypt)
If (success <> True) Then
    Debug.Print "Failed to open file that is to be encrytped."
    Exit Sub
End If


outputEncryptedFile = "qa_output/hamlet.enc"
Dim facOutEnc As Chilkat.FileAccess
Set facOutEnc = Chilkat.NewFileAccess
success = facOutEnc.OpenForWrite(outputEncryptedFile)
If (success <> True) Then
    Debug.Print "Failed to encrypted output file."
    Exit Sub
End If

' Let's encrypt in 10000 byte chunks.

chunkSize = 10000

numChunks = facIn.GetNumBlocks(chunkSize)

crypt.FirstChunk = True
crypt.LastChunk = False

Dim bd As Chilkat.BinData
Set bd = Chilkat.NewBinData


i = 0
Do 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..
    success = bd.Clear()
    success = facIn.FileReadBd(chunkSize,bd)

    ' Encrypt.
    success = crypt.EncryptBd(bd)

    ' Write the encrypted chunk to the output file.
    success = facOutEnc.FileWriteBd(bd,0,0)

    crypt.FirstChunk = False
Loop

' 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 
facOutEnc.FileClose 

' Decrypt the encrypted output file in a single call using CBC mode:

decryptedFile = "qa_output/hamlet_dec.xml"
success = crypt.CkDecryptFile(outputEncryptedFile,decryptedFile)
' Assume success for the example..

' Compare the contents of the decrypted file with the original file:

bSame = facIn.FileContentsEqual(fileToEncrypt,decryptedFile)
Debug.Print "bSame = "; bSame