(Mono C#) 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

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

Chilkat.Crypt2 crypt = new 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

Chilkat.FileAccess fac = new Chilkat.FileAccess();
Chilkat.BinData bd = new Chilkat.BinData();
Chilkat.BinData bdHashes = new Chilkat.BinData();

bool success = fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg");
if (success == false) {
    Debug.WriteLine(fac.LastErrorText);
    return;
}

int blockSize = 4194304;
int numBlocks = fac.GetNumBlocks(blockSize);

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

    string hashHex = crypt.HashBdENC(bd);
    Debug.WriteLine(Convert.ToString(i) + ": " + hashHex);
    bdHashes.AppendEncoded(hashHex,"hex_lower");

    i = i + 1;
}

fac.FileClose();

// Hash the concatenated SHA-256 hashes.
string content_hash = crypt.HashBdENC(bdHashes);
Debug.WriteLine("content_hash = " + content_hash);