(Visual Basic 6.0) 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

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

Dim crypt As New ChilkatCrypt2

' 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.

crypt.HashAlgorithm = "sha256"
crypt.EncodingMode = "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

Dim fac As New CkFileAccess
Dim bd As New ChilkatBinData
Dim bdHashes As New ChilkatBinData

Dim success As Long
success = fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg")
If (success = 0) Then
    Debug.Print fac.LastErrorText
    Exit Sub
End If

Dim blockSize As Long
blockSize = 4194304
Dim numBlocks As Long
numBlocks = fac.GetNumBlocks(blockSize)

Dim i As Long
i = 0
Do While (i < numBlocks)
    ' Read the next 4MB block into bd.
    success = bd.Clear()
    success = fac.ReadBlockBd(i,blockSize,bd)

    Dim hashHex As String
    hashHex = crypt.HashBdENC(bd)
    Debug.Print i & ": " & hashHex
    success = bdHashes.AppendEncoded(hashHex,"hex_lower")

    i = i + 1
Loop

fac.FileClose 

' Hash the concatenated SHA-256 hashes.
Dim content_hash As String
content_hash = crypt.HashBdENC(bdHashes)
Debug.Print "content_hash = " & content_hash