Objective-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.

Objective-C

#import <CkoCrypt2.h>
#import <CkoFileAccess.h>
#import <CkoBinData.h>
#import <NSString.h>

BOOL success = NO;

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

CkoCrypt2 *crypt = [[CkoCrypt2 alloc] init];

// 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

CkoFileAccess *fac = [[CkoFileAccess alloc] init];
CkoBinData *bd = [[CkoBinData alloc] init];
CkoBinData *bdHashes = [[CkoBinData alloc] init];

success = [fac OpenForRead: @"qa_data/jpg/milky-way-nasa.jpg"];
if (success == NO) {
    NSLog(@"%@",fac.LastErrorText);
    return;
}

int blockSize = 4194304;
int numBlocks = [[fac GetNumBlocks: [NSNumber numberWithInt: blockSize]] intValue];

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

    NSString *hashHex = [crypt HashBdENC: bd];
    NSLog(@"%d%@%@",i,@": ",hashHex);
    [bdHashes AppendEncoded: hashHex encoding: @"hex_lower"];

    i = i + 1;
}

[fac FileClose];

// Hash the concatenated SHA-256 hashes.
NSString *content_hash = [crypt HashBdENC: bdHashes];
NSLog(@"%@%@",@"content_hash = ",content_hash);