PowerShell

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.

PowerShell

Add-Type -Path "C:\chilkat\ChilkatDotNet47-x64\ChilkatDotNet47.dll"

$success = $false

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

$crypt = New-Object Chilkat.Crypt2

# 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

$fac = New-Object Chilkat.FileAccess
$bd = New-Object Chilkat.BinData
$bdHashes = New-Object Chilkat.BinData

$success = $fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg")
if ($success -eq $false) {
    $($fac.LastErrorText)
    exit
}

$blockSize = 4194304
$numBlocks = $fac.GetNumBlocks($blockSize)

$i = 0
while (($i -lt $numBlocks)) {
    # Read the next 4MB block into bd.
    $bd.Clear()
    $success = $fac.ReadBlockBd($i,$blockSize,$bd)

    $hashHex = $crypt.HashBdENC($bd)
    $([string]$i + ": " + $hashHex)
    $bdHashes.AppendEncoded($hashHex,"hex_lower")

    $i = $i + 1
}

$fac.FileClose()

# Hash the concatenated SHA-256 hashes.
$content_hash = $crypt.HashBdENC($bdHashes)
$("content_hash = " + $content_hash)