Unicode C++

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.

Unicode C++

#include <CkCrypt2W.h>
#include <CkFileAccessW.h>
#include <CkBinDataW.h>

void ChilkatSample(void)
    {
    bool success = false;

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

    CkCrypt2W 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.put_HashAlgorithm(L"sha256");
    crypt.put_EncodingMode(L"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

    CkFileAccessW fac;
    CkBinDataW bd;
    CkBinDataW bdHashes;

    success = fac.OpenForRead(L"qa_data/jpg/milky-way-nasa.jpg");
    if (success == false) {
        wprintf(L"%s\n",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);

        const wchar_t *hashHex = crypt.hashBdENC(bd);
        wprintf(L"%d: %s\n",i,hashHex);
        bdHashes.AppendEncoded(hashHex,L"hex_lower");

        i = i + 1;
    }

    fac.FileClose();

    // Hash the concatenated SHA-256 hashes.
    const wchar_t *content_hash = crypt.hashBdENC(bdHashes);
    wprintf(L"content_hash = %s\n",content_hash);
    }