(Visual FoxPro) 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

Chilkat ActiveX Downloads ActiveX for 32-bit and 64-bit Windows

LOCAL loCrypt
LOCAL loFac
LOCAL loBd
LOCAL loBdHashes
LOCAL lnSuccess
LOCAL lnBlockSize
LOCAL lnNumBlocks
LOCAL i
LOCAL lcHashHex
LOCAL lcContent_hash

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

* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.Crypt2')
loCrypt = CreateObject('Chilkat.Crypt2')

* 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

* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.FileAccess')
loFac = CreateObject('Chilkat.FileAccess')
* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.BinData')
loBd = CreateObject('Chilkat.BinData')
* For versions of Chilkat < 10.0.0, use CreateObject('Chilkat_9_5_0.BinData')
loBdHashes = CreateObject('Chilkat.BinData')

lnSuccess = loFac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg")
IF (lnSuccess = 0) THEN
    ? loFac.LastErrorText
    RELEASE loCrypt
    RELEASE loFac
    RELEASE loBd
    RELEASE loBdHashes
    CANCEL
ENDIF

lnBlockSize = 4194304
lnNumBlocks = loFac.GetNumBlocks(lnBlockSize)

i = 0
DO WHILE (i < lnNumBlocks)
    * Read the next 4MB block into bd.
    loBd.Clear()
    lnSuccess = 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