B4X

Dropbox Content Hash

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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.

B4X

Dim success As Boolean = False

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

Dim crypt As ChilkatCrypt2
crypt.Initialize("crypt")

'  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 ChilkatFileAccess
fac.Initialize
Dim bd As ChilkatBinData
bd.Initialize
Dim bdHashes As ChilkatBinData
bdHashes.Initialize

success = fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg")
If success = False Then
    Log(fac.LastErrorText)
    Return
End If


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

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

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

    i = i + 1
Loop

fac.FileClose

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