PureBasic

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.

PureBasic

IncludeFile "CkBinData.pb"
IncludeFile "CkFileAccess.pb"
IncludeFile "CkCrypt2.pb"

Procedure ChilkatExample()

    success.i = 0

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

    crypt.i = CkCrypt2::ckCreate()
    If crypt.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    ; 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.

    CkCrypt2::setCkHashAlgorithm(crypt, "sha256")
    CkCrypt2::setCkEncodingMode(crypt, "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

    fac.i = CkFileAccess::ckCreate()
    If fac.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    bd.i = CkBinData::ckCreate()
    If bd.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    bdHashes.i = CkBinData::ckCreate()
    If bdHashes.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success = CkFileAccess::ckOpenForRead(fac,"qa_data/jpg/milky-way-nasa.jpg")
    If success = 0
        Debug CkFileAccess::ckLastErrorText(fac)
        CkCrypt2::ckDispose(crypt)
        CkFileAccess::ckDispose(fac)
        CkBinData::ckDispose(bd)
        CkBinData::ckDispose(bdHashes)
        ProcedureReturn
    EndIf

    blockSize.i = 4194304
    numBlocks.i = CkFileAccess::ckGetNumBlocks(fac,blockSize)

    i.i = 0
    While (i < numBlocks)
        ; Read the next 4MB block into bd.
        CkBinData::ckClear(bd)
        success = CkFileAccess::ckReadBlockBd(fac,i,blockSize,bd)

        hashHex.s = CkCrypt2::ckHashBdENC(crypt,bd)
        Debug Str(i) + ": " + hashHex
        CkBinData::ckAppendEncoded(bdHashes,hashHex,"hex_lower")

        i = i + 1
    Wend

    CkFileAccess::ckFileClose(fac)

    ; Hash the concatenated SHA-256 hashes.
    content_hash.s = CkCrypt2::ckHashBdENC(crypt,bdHashes)
    Debug "content_hash = " + content_hash


    CkCrypt2::ckDispose(crypt)
    CkFileAccess::ckDispose(fac)
    CkBinData::ckDispose(bd)
    CkBinData::ckDispose(bdHashes)


    ProcedureReturn
EndProcedure