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

loCrypt = createobject("CkCrypt2")

// 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.

loCrypt.HashAlgorithm = "sha256"
loCrypt.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

loFac = createobject("CkFileAccess")
loBd = createobject("CkBinData")
loBdHashes = createobject("CkBinData")

llSuccess = loFac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg")
if (llSuccess = .F.) then
    ? loFac.LastErrorText
    release loCrypt
    release loFac
    release loBd
    release loBdHashes
    return
endif

lnBlockSize = 4194304
lnNumBlocks = loFac.GetNumBlocks(lnBlockSize)

i = 0
do while (i < lnNumBlocks)
    // Read the next 4MB block into bd.
    loBd.Clear()
    llSuccess = loFac.ReadBlockBd(i,lnBlockSize,loBd)

    lcHashHex = loCrypt.HashBdENC(loBd)
    ? str(i) + ": " + lcHashHex
    loBdHashes.AppendEncoded(lcHashHex,"hex_lower")

    i = i + 1
enddo

loFac.FileClose()

// Hash the concatenated SHA-256 hashes.
lcContent_hash = loCrypt.HashBdENC(loBdHashes)
? "content_hash = " + lcContent_hash


release loCrypt
release loFac
release loBd
release loBdHashes