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DataFlex Examples

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(DataFlex) Dropbox Content Hash

Demonstrates how to compute the Dropbox content_hash of a file. (This is the "content_hash" found in the Dropbox FileMetadata object.

Note: This example requires Chilkat v9.5.0.79 or greater because it uses the fac.ReadBlockBd method.

For more information, see https://www.dropbox.com/developers/reference/content-hash

Chilkat ActiveX Downloads

ActiveX for 32-bit and 64-bit Windows

Use ChilkatAx-9.5.0-win32.pkg

Procedure Test
    Handle hoCrypt
    Handle hoFac
    Variant vBd
    Handle hoBd
    Variant vBd
Hashes    Handle hoBdHashes
    Boolean iSuccess
    Integer iBlockSize
    Integer iNumBlocks
    Integer i
    String sHashHex
    String sContent_hash
    String sTemp1

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

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

    // To calculate the content_hash of a file:
    // Split the file into blocks of 4 MB (4,194,304 or 4 * 1024 * 1024 bytes). The last block (if any) may be smaller than 4 MB.
    // Compute the hash of each block using SHA-256.
    // Concatenate the hash of all blocks in the binary format to form a single binary string.
    // Compute the hash of the concatenated string using SHA-256. Output the resulting hash in hexadecimal format.

    Set ComHashAlgorithm Of hoCrypt To "sha256"
    Set ComEncodingMode Of hoCrypt To "hex_lower"

    // We're going to calculate the content_hash for the Milky Way JPG image linked here:  https://www.dropbox.com/developers/reference/content-hash

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

    Get ComOpenForRead Of hoFac "qa_data/jpg/milky-way-nasa.jpg" To iSuccess
    If (iSuccess = False) Begin
        Get ComLastErrorText Of hoFac To sTemp1
        Showln sTemp1
        Procedure_Return
    End

    Move 4194304 To iBlockSize
    Get ComGetNumBlocks Of hoFac iBlockSize To iNumBlocks

    Move 0 To i
    While (i < iNumBlocks)
        // Read the next 4MB block into bd.
        Get ComClear Of hoBd To iSuccess
        Get pvComObject of hoBd to vBd
        Get ComReadBlockBd Of hoFac i iBlockSize vBd To iSuccess

        Get pvComObject of hoBd to vBd
        Get ComHashBdENC Of hoCrypt vBd To sHashHex
        Showln i ": " sHashHex
        Get ComAppendEncoded Of hoBdHashes sHashHex "hex_lower" To iSuccess

        Move i + 1 To i
    Loop

    Send ComFileClose To hoFac

    // Hash the concatenated SHA-256 hashes.
    Get pvComObject of hoBdHashes to vBdHashes
    Get ComHashBdENC Of hoCrypt vBdHashes To sContent_hash
    Showln "content_hash = " sContent_hash


End_Procedure

 

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