C

Dropbox Content Hash

See more Dropbox Examples

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.

C

#include <C_CkCrypt2.h>
#include <C_CkFileAccess.h>
#include <C_CkBinData.h>

void ChilkatSample(void)
    {
    BOOL success;
    HCkCrypt2 crypt;
    HCkFileAccess fac;
    HCkBinData bd;
    HCkBinData bdHashes;
    int blockSize;
    int numBlocks;
    int i;
    const char *hashHex;
    const char *content_hash;

    success = FALSE;

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

    crypt = CkCrypt2_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.

    CkCrypt2_putHashAlgorithm(crypt,"sha256");
    CkCrypt2_putEncodingMode(crypt,"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 = CkFileAccess_Create();
    bd = CkBinData_Create();
    bdHashes = CkBinData_Create();

    success = CkFileAccess_OpenForRead(fac,"qa_data/jpg/milky-way-nasa.jpg");
    if (success == FALSE) {
        printf("%s\n",CkFileAccess_lastErrorText(fac));
        CkCrypt2_Dispose(crypt);
        CkFileAccess_Dispose(fac);
        CkBinData_Dispose(bd);
        CkBinData_Dispose(bdHashes);
        return;
    }

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

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

        hashHex = CkCrypt2_hashBdENC(crypt,bd);
        printf("%d: %s\n",i,hashHex);
        CkBinData_AppendEncoded(bdHashes,hashHex,"hex_lower");

        i = i + 1;
    }

    CkFileAccess_FileClose(fac);

    // Hash the concatenated SHA-256 hashes.
    content_hash = CkCrypt2_hashBdENC(crypt,bdHashes);
    printf("content_hash = %s\n",content_hash);


    CkCrypt2_Dispose(crypt);
    CkFileAccess_Dispose(fac);
    CkBinData_Dispose(bd);
    CkBinData_Dispose(bdHashes);

    }