Xojo Plugin

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. Xojo Plugin

Dim success As Boolean
success = 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
fileToDecrypt = "c:/temp/qa_output/hamlet.enc"
Dim facIn As New Chilkat.FileAccess
success = facIn.OpenForRead(fileToDecrypt)
If (success <> True) Then
    System.DebugLog("Failed to open file to be decrytped.")
    Return
End If

// Let's decrypt in 32000 byte chunks.
Dim chunkSize As Int32
chunkSize = 32000
Dim numChunks As Int32
numChunks = facIn.GetNumBlocks(chunkSize)

crypt.FirstChunk = True
crypt.LastChunk = False

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

Dim i As Int32
i = 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..
    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 = False
Wend

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