Classic ASP

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.

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Classic ASP

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<%
success = 0

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

set crypt = Server.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.

crypt.HashAlgorithm = "sha256"
crypt.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

set fac = Server.CreateObject("Chilkat.FileAccess")
set bd = Server.CreateObject("Chilkat.BinData")
set bdHashes = Server.CreateObject("Chilkat.BinData")

success = fac.OpenForRead("qa_data/jpg/milky-way-nasa.jpg")
If (success = 0) Then
    Response.Write "<pre>" & Server.HTMLEncode( fac.LastErrorText) & "</pre>"
    Response.End
End If

blockSize = 4194304
numBlocks = fac.GetNumBlocks(blockSize)

i = 0
Do While (i < numBlocks)
    ' Read the next 4MB block into bd.
    success = bd.Clear()
    success = fac.ReadBlockBd(i,blockSize,bd)

    hashHex = crypt.HashBdENC(bd)
    Response.Write "<pre>" & Server.HTMLEncode( i & ": " & hashHex) & "</pre>"
    success = bdHashes.AppendEncoded(hashHex,"hex_lower")

    i = i + 1
Loop

fac.FileClose 

' Hash the concatenated SHA-256 hashes.
content_hash = crypt.HashBdENC(bdHashes)
Response.Write "<pre>" & Server.HTMLEncode( "content_hash = " & content_hash) & "</pre>"

%>
</body>
</html>