(Unicode 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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#include <C_CkCrypt2W.h>
#include <C_CkFileAccessW.h>
#include <C_CkBinDataW.h>

void ChilkatSample(void)
    {
    HCkCrypt2W crypt;
    HCkFileAccessW fac;
    HCkBinDataW bd;
    HCkBinDataW bdHashes;
    BOOL success;
    int blockSize;
    int numBlocks;
    int i;
    const wchar_t *hashHex;
    const wchar_t *content_hash;

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

    crypt = CkCrypt2W_Create();

    // 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.

    CkCrypt2W_putHashAlgorithm(crypt,L"sha256");
    CkCrypt2W_putEncodingMode(crypt,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

    fac = CkFileAccessW_Create();
    bd = CkBinDataW_Create();
    bdHashes = CkBinDataW_Create();

    success = CkFileAccessW_OpenForRead(fac,L"qa_data/jpg/milky-way-nasa.jpg");
    if (success == FALSE) {
        wprintf(L"%s\n",CkFileAccessW_lastErrorText(fac));
        CkCrypt2W_Dispose(crypt);
        CkFileAccessW_Dispose(fac);
        CkBinDataW_Dispose(bd);
        CkBinDataW_Dispose(bdHashes);
        return;
    }

    blockSize = 4194304;
    numBlocks = CkFileAccessW_GetNumBlocks(fac,blockSize);

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

        hashHex = CkCrypt2W_hashBdENC(crypt,bd);
        wprintf(L"%d: %s\n",i,hashHex);
        CkBinDataW_AppendEncoded(bdHashes,hashHex,L"hex_lower");

        i = i + 1;
    }

    CkFileAccessW_FileClose(fac);

    // Hash the concatenated SHA-256 hashes.
    content_hash = CkCrypt2W_hashBdENC(crypt,bdHashes);
    wprintf(L"content_hash = %s\n",content_hash);


    CkCrypt2W_Dispose(crypt);
    CkFileAccessW_Dispose(fac);
    CkBinDataW_Dispose(bd);
    CkBinDataW_Dispose(bdHashes);

    }