(MFC) 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 C/C++ Library Downloads MS Visual C/C++ Libs

See Also: Using MFC CString in Chilkat

#include <CkCrypt2.h>
#include <CkFileAccess.h>
#include <CkBinData.h>

void ChilkatSample(void)
    {
    CkString strOut;

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

    CkCrypt2 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("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

    CkFileAccess fac;
    CkBinData bd;
    CkBinData bdHashes;

    bool success = fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg");
    if (success == false) {
        strOut.append(fac.lastErrorText());
        strOut.append("\r\n");
        SetDlgItemText(IDC_EDIT1,strOut.getUnicode());
        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 char *hashHex = crypt.hashBdENC(bd);
        strOut.appendInt(i);
        strOut.append(": ");
        strOut.append(hashHex);
        strOut.append("\r\n");
        bdHashes.AppendEncoded(hashHex,"hex_lower");

        i = i + 1;
    }

    fac.FileClose();

    // Hash the concatenated SHA-256 hashes.
    const char *content_hash = crypt.hashBdENC(bdHashes);
    strOut.append("content_hash = ");
    strOut.append(content_hash);
    strOut.append("\r\n");


    SetDlgItemText(IDC_EDIT1,strOut.getUnicode());

    }