(Perl) 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 Perl Downloads Perl Module for Windows, MacOS, Linux, Alpine Linux, Solaris

use chilkat();

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

$crypt = chilkat::CkCrypt2->new();

# 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->put_HashAlgorithm("sha256");
$crypt->put_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 = chilkat::CkFileAccess->new();
$bd = chilkat::CkBinData->new();
$bdHashes = chilkat::CkBinData->new();

$success = $fac->OpenForRead("qa_data/jpg/milky-way-nasa.jpg");
if ($success == 0) {
    print $fac->lastErrorText() . "\r\n";
    exit;
}

$blockSize = 4194304;
$numBlocks = $fac->GetNumBlocks($blockSize);

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

    $hashHex = $crypt->hashBdENC($bd);
    print $i . ": " . $hashHex . "\r\n";
    $bdHashes->AppendEncoded($hashHex,"hex_lower");

    $i = $i + 1;
}

$fac->FileClose();

# Hash the concatenated SHA-256 hashes.
$content_hash = $crypt->hashBdENC($bdHashes);
print "content_hash = " . $content_hash . "\r\n";