PureBasic

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. PureBasic

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

Procedure ChilkatExample()

    success.i = 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

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

    CkCrypt2::setCkCryptAlgorithm(crypt, "aes")
    CkCrypt2::setCkCipherMode(crypt, "cbc")
    CkCrypt2::setCkKeyLength(crypt, 256)

    CkCrypt2::ckSetEncodedKey(crypt,"000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F","hex")
    CkCrypt2::ckSetEncodedIV(crypt,"000102030405060708090A0B0C0D0E0F","hex")

    fileToDecrypt.s = "c:/temp/qa_output/hamlet.enc"
    facIn.i = CkFileAccess::ckCreate()
    If facIn.i = 0
        Debug "Failed to create object."
        ProcedureReturn
    EndIf

    success = CkFileAccess::ckOpenForRead(facIn,fileToDecrypt)
    If success <> 1
        Debug "Failed to open file to be decrytped."
        CkCrypt2::ckDispose(crypt)
        CkFileAccess::ckDispose(facIn)
        ProcedureReturn
    EndIf

    ; Let's decrypt in 32000 byte chunks.
    chunkSize.i = 32000
    numChunks.i = CkFileAccess::ckGetNumBlocks(facIn,chunkSize)

    CkCrypt2::setCkFirstChunk(crypt, 1)
    CkCrypt2::setCkLastChunk(crypt, 0)

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

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

    i.i = 0
    While i < numChunks
        i = i + 1
        If i = numChunks
            CkCrypt2::setCkLastChunk(crypt, 1)
        EndIf

        ; Read the next chunk from the file.
        ; The last chunk will be whatever amount remains in the file..
        CkBinData::ckClear(bd)
        CkFileAccess::ckFileReadBd(facIn,chunkSize,bd)

        ; Decrypt this chunk.
        CkCrypt2::ckDecryptBd(crypt,bd)
        ; Accumulate the decrypted chunks.
        CkBinData::ckAppendBd(bdDecrypted,bd)

        CkCrypt2::setCkFirstChunk(crypt, 0)
    Wend

    ; 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::setCkFirstChunk(crypt, 1)
    CkCrypt2::setCkLastChunk(crypt, 1)

    CkFileAccess::ckFileClose(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::ckWriteFile(bdDecrypted,"c:/temp/qa_output/hamlet_decrypted.xml")


    CkCrypt2::ckDispose(crypt)
    CkFileAccess::ckDispose(facIn)
    CkBinData::ckDispose(bd)
    CkBinData::ckDispose(bdDecrypted)


    ProcedureReturn
EndProcedure