(Tcl) 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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load ./chilkat.dll

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

set crypt [new_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.

CkCrypt2_put_HashAlgorithm $crypt "sha256"
CkCrypt2_put_EncodingMode $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

set fac [new_CkFileAccess]

set bd [new_CkBinData]

set bdHashes [new_CkBinData]

set success [CkFileAccess_OpenForRead $fac "qa_data/jpg/milky-way-nasa.jpg"]
if {$success == 0} then {
    puts [CkFileAccess_lastErrorText $fac]
    delete_CkCrypt2 $crypt
    delete_CkFileAccess $fac
    delete_CkBinData $bd
    delete_CkBinData $bdHashes
    exit
}

set blockSize 4194304
set numBlocks [CkFileAccess_GetNumBlocks $fac $blockSize]

set i 0
while {$i < $numBlocks} {
    # Read the next 4MB block into bd.
    CkBinData_Clear $bd
    set success [CkFileAccess_ReadBlockBd $fac $i $blockSize $bd]

    set hashHex [CkCrypt2_hashBdENC $crypt $bd]
    puts "$i: $hashHex"
    CkBinData_AppendEncoded $bdHashes $hashHex "hex_lower"

    set i [expr $i + 1]
}

CkFileAccess_FileClose $fac

# Hash the concatenated SHA-256 hashes.
set content_hash [CkCrypt2_hashBdENC $crypt $bdHashes]
puts "content_hash = $content_hash"

delete_CkCrypt2 $crypt
delete_CkFileAccess $fac
delete_CkBinData $bd
delete_CkBinData $bdHashes