C++

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. C++

#include <CkCrypt2.h>
#include <CkFileAccess.h>
#include <CkBinData.h>

void ChilkatSample(void)
    {
    bool success = false;

    //  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

    CkCrypt2 crypt;

    crypt.put_CryptAlgorithm("aes");
    crypt.put_CipherMode("cbc");
    crypt.put_KeyLength(256);

    crypt.SetEncodedKey("000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F","hex");
    crypt.SetEncodedIV("000102030405060708090A0B0C0D0E0F","hex");

    const char *fileToDecrypt = "c:/temp/qa_output/hamlet.enc";
    CkFileAccess facIn;
    success = facIn.OpenForRead(fileToDecrypt);
    if (success != true) {
        std::cout << "Failed to open file to be decrytped." << "\r\n";
        return;
    }

    //  Let's decrypt in 32000 byte chunks.
    int chunkSize = 32000;
    int numChunks = facIn.GetNumBlocks(chunkSize);

    crypt.put_FirstChunk(true);
    crypt.put_LastChunk(false);

    CkBinData bd;
    CkBinData bdDecrypted;

    int i = 0;
    while (i < numChunks) {
        i = i + 1;
        if (i == numChunks) {
            crypt.put_LastChunk(true);
        }

        //  Read the next chunk from the file.
        //  The last chunk will be whatever amount remains in the file..
        bd.Clear();
        facIn.FileReadBd(chunkSize,bd);

        //  Decrypt this chunk.
        crypt.DecryptBd(bd);
        //  Accumulate the decrypted chunks.
        bdDecrypted.AppendBd(bd);

        crypt.put_FirstChunk(false);
    }

    //  Make sure both FirstChunk and LastChunk are restored to true after
    //  encrypting or decrypting in chunks.  Otherwise subsequent encryptions/decryptions
    //  will produce unexpected results.
    crypt.put_FirstChunk(true);
    crypt.put_LastChunk(true);

    facIn.FileClose();

    //  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.
    bdDecrypted.WriteFile("c:/temp/qa_output/hamlet_decrypted.xml");
    }