DataFlex

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. DataFlex

Use ChilkatAx-win32.pkg

Procedure Test
    Boolean iSuccess
    Handle hoCrypt
    String sFileToDecrypt
    Handle hoFacIn
    Integer iChunkSize
    Integer iNumChunks
    Variant vBd
    Handle hoBd
    Handle hoBdDecrypted
    Integer i

    Move False To iSuccess

    // 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

    Get Create (RefClass(cComChilkatCrypt2)) To hoCrypt
    If (Not(IsComObjectCreated(hoCrypt))) Begin
        Send CreateComObject of hoCrypt
    End

    Set ComCryptAlgorithm Of hoCrypt To "aes"
    Set ComCipherMode Of hoCrypt To "cbc"
    Set ComKeyLength Of hoCrypt To 256

    Send ComSetEncodedKey To hoCrypt "000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F" "hex"
    Send ComSetEncodedIV To hoCrypt "000102030405060708090A0B0C0D0E0F" "hex"

    Move "c:/temp/qa_output/hamlet.enc" To sFileToDecrypt
    Get Create (RefClass(cComCkFileAccess)) To hoFacIn
    If (Not(IsComObjectCreated(hoFacIn))) Begin
        Send CreateComObject of hoFacIn
    End
    Get ComOpenForRead Of hoFacIn sFileToDecrypt To iSuccess
    If (iSuccess <> True) Begin
        Showln "Failed to open file to be decrytped."
        Procedure_Return
    End

    // Let's decrypt in 32000 byte chunks.
    Move 32000 To iChunkSize
    Get ComGetNumBlocks Of hoFacIn iChunkSize To iNumChunks

    Set ComFirstChunk Of hoCrypt To True
    Set ComLastChunk Of hoCrypt To False

    Get Create (RefClass(cComChilkatBinData)) To hoBd
    If (Not(IsComObjectCreated(hoBd))) Begin
        Send CreateComObject of hoBd
    End
    Get Create (RefClass(cComChilkatBinData)) To hoBdDecrypted
    If (Not(IsComObjectCreated(hoBdDecrypted))) Begin
        Send CreateComObject of hoBdDecrypted
    End

    Move 0 To i
    While (i < iNumChunks)
        Move (i + 1) To i
        If (i = iNumChunks) Begin
            Set ComLastChunk Of hoCrypt To True
        End

        // Read the next chunk from the file.
        // The last chunk will be whatever amount remains in the file..
        Get ComClear Of hoBd To iSuccess
        Get pvComObject of hoBd to vBd
        Get ComFileReadBd Of hoFacIn iChunkSize vBd To iSuccess

        // Decrypt this chunk.
        Get pvComObject of hoBd to vBd
        Get ComDecryptBd Of hoCrypt vBd To iSuccess
        // Accumulate the decrypted chunks.
        Get pvComObject of hoBd to vBd
        Get ComAppendBd Of hoBdDecrypted vBd To iSuccess

        Set ComFirstChunk Of hoCrypt To 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.
    Set ComFirstChunk Of hoCrypt To True
    Set ComLastChunk Of hoCrypt To True

    Send ComFileClose To hoFacIn

    // 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.
    Get ComWriteFile Of hoBdDecrypted "c:/temp/qa_output/hamlet_decrypted.xml" To iSuccess


End_Procedure