Tcl

Decrypt File in Chunks using 256-bit AES

See more Encryption Examples

Shows how to decrypt a file chunk-by-chunk using FirstChunk/LastChunk properties and accumulate the results in memory with a Chilkat BinData object. Tcl

load ./chilkat.dll

set success 0

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

# This example decrypts the file previously encrypted in this example:
# Encrypt File in Chunks using AES CBC

set crypt [new_CkCrypt2]

CkCrypt2_put_CryptAlgorithm $crypt "aes"
CkCrypt2_put_CipherMode $crypt "cbc"
CkCrypt2_put_KeyLength $crypt 256

CkCrypt2_SetEncodedKey $crypt "000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F" "hex"
CkCrypt2_SetEncodedIV $crypt "000102030405060708090A0B0C0D0E0F" "hex"

set fileToDecrypt "c:/temp/qa_output/hamlet.enc"
set facIn [new_CkFileAccess]

set success [CkFileAccess_OpenForRead $facIn $fileToDecrypt]
if {$success != 1} then {
    puts "Failed to open file to be decrytped."
    delete_CkCrypt2 $crypt
    delete_CkFileAccess $facIn
    exit
}

# Let's decrypt in 32000 byte chunks.
set chunkSize 32000
set numChunks [CkFileAccess_GetNumBlocks $facIn $chunkSize]

CkCrypt2_put_FirstChunk $crypt 1
CkCrypt2_put_LastChunk $crypt 0

set bd [new_CkBinData]

set bdDecrypted [new_CkBinData]

set i 0
while {$i < $numChunks} {
    set i [expr $i + 1]
    if {$i == $numChunks} then {
        CkCrypt2_put_LastChunk $crypt 1
    }

    # Read the next chunk from the file.
    # The last chunk will be whatever amount remains in the file..
    CkBinData_Clear $bd
    CkFileAccess_FileReadBd $facIn $chunkSize $bd

    # Decrypt this chunk.
    CkCrypt2_DecryptBd $crypt $bd
    # Accumulate the decrypted chunks.
    CkBinData_AppendBd $bdDecrypted $bd

    CkCrypt2_put_FirstChunk $crypt 0
}

# Make sure both FirstChunk and LastChunk are restored to 1 after
# encrypting or decrypting in chunks.  Otherwise subsequent encryptions/decryptions
# will produce unexpected results.
CkCrypt2_put_FirstChunk $crypt 1
CkCrypt2_put_LastChunk $crypt 1

CkFileAccess_FileClose $facIn

# The fully decrypted file is contained in bdDecrypted.
# You can save to a file if desired, or use the decrypted data in your application directly from bdDecrypted.
CkBinData_WriteFile $bdDecrypted "c:/temp/qa_output/hamlet_decrypted.xml"

delete_CkCrypt2 $crypt
delete_CkFileAccess $facIn
delete_CkBinData $bd
delete_CkBinData $bdDecrypted