Lua Requires Chilkat v9.5.0.79+

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. Lua

local chilkat = require("chilkat")

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

local crypt = chilkat.Crypt2.new()

-- 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.hash_algorithm = "sha256"
crypt.encoding_mode = "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

local fac = chilkat.FileAccess.new()
local bd = chilkat.BinData.new()
local bdHashes = chilkat.BinData.new()

local success = fac:open_for_read("qa_data/jpg/milky-way-nasa.jpg")
if not success then
    print(fac.last_error_text)
    return
end

local blockSize = 4194304
local numBlocks = fac:get_num_blocks(blockSize)

local i = 0
while i < numBlocks do
    -- Read the next 4MB block into bd.
    bd:clear()
    success = fac:read_block_bd(i, blockSize, bd)

    local hashHex = crypt:hash_bd_enc(bd)
    print(i .. ": " .. hashHex)
    bdHashes:append_encoded(hashHex, "hex_lower")

    i = i + 1
end

fac:file_close()

-- Hash the concatenated SHA-256 hashes.
local content_hash = crypt:hash_bd_enc(bdHashes)
print("content_hash = " .. content_hash)