Xbase++

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.

Xbase++

LOCAL nSuccess
LOCAL oCrypt
LOCAL oFac
LOCAL oBd
LOCAL oBdHashes
LOCAL nBlockSize
LOCAL nNumBlocks
LOCAL i
LOCAL cHashHex
LOCAL cContent_hash

nSuccess := 0

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

oCrypt := 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.

oCrypt:HashAlgorithm := "sha256"
oCrypt: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

oFac := CreateObject("Chilkat.FileAccess")
oBd := CreateObject("Chilkat.BinData")
oBdHashes := CreateObject("Chilkat.BinData")

nSuccess := oFac:OpenForRead("qa_data/jpg/milky-way-nasa.jpg")
IF (nSuccess == 0)
    ? oFac:LastErrorText
    oCrypt:destroy()
    oFac:destroy()
    oBd:destroy()
    oBdHashes:destroy()
    RETURN
ENDIF

nBlockSize := 4194304
nNumBlocks := oFac:GetNumBlocks(nBlockSize)

i := 0
DO WHILE (i < nNumBlocks)
    //  Read the next 4MB block into bd.
    oBd:Clear()
    nSuccess := oFac:ReadBlockBd(i, nBlockSize, oBd)

    cHashHex := oCrypt:HashBdENC(oBd)
    ? Str(i) + ": " + cHashHex
    oBdHashes:AppendEncoded(cHashHex, "hex_lower")

    i := i + 1
ENDDO

oFac:FileClose()

//  Hash the concatenated SHA-256 hashes.
cContent_hash := oCrypt:HashBdENC(oBdHashes)
? "content_hash = " + cContent_hash

oCrypt:destroy()
oFac:destroy()
oBd:destroy()
oBdHashes:destroy()