Zig

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.

Zig

const std = @import("std");
const chilkat = @import("chilkat");

pub fn main(init: std.process.Init) !void {
    const alloc = init.arena.allocator();

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

    const crypt = try chilkat.Crypt2.init();
    defer crypt.deinit();

    // 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.setHashAlgorithm("sha256");
    crypt.setEncodingMode("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

    const fac = try chilkat.FileAccess.init();
    defer fac.deinit();
    const bd = try chilkat.BinData.init();
    defer bd.deinit();
    const bd_hashes = try chilkat.BinData.init();
    defer bd_hashes.deinit();

    fac.openForRead("qa_data/jpg/milky-way-nasa.jpg") catch {
        std.debug.print("{s}\n", .{try fac.getLastErrorText(alloc)});
        return;
    };

    const block_size = 4194304;
    const num_blocks = fac.getNumBlocks(block_size);

    var i: i32 = 0;
    while (i < num_blocks) {
        // Read the next 4MB block into bd.
        bd.clear() catch {};
        try fac.readBlockBd(i, block_size, bd);

        const hash_hex = try crypt.hashBdENC(alloc, bd);
        std.debug.print("{d}: {s}\n", .{ i, hash_hex });
        bd_hashes.appendEncoded(hash_hex, "hex_lower") catch {};

        i = i + 1;
    }

    fac.fileClose();

    // Hash the concatenated SHA-256 hashes.
    const content_hash = try crypt.hashBdENC(alloc, bd_hashes);
    std.debug.print("content_hash = {s}\n", .{content_hash});
}