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 <C_CkCrypt2.h>
#include <C_CkFileAccess.h>
#include <C_CkBinData.h>

void ChilkatSample(void)
    {
    BOOL success;
    HCkCrypt2 crypt;
    const char *fileToDecrypt;
    HCkFileAccess facIn;
    int chunkSize;
    int numChunks;
    HCkBinData bd;
    HCkBinData bdDecrypted;
    int i;

    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

    crypt = CkCrypt2_Create();

    CkCrypt2_putCryptAlgorithm(crypt,"aes");
    CkCrypt2_putCipherMode(crypt,"cbc");
    CkCrypt2_putKeyLength(crypt,256);

    CkCrypt2_SetEncodedKey(crypt,"000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F","hex");
    CkCrypt2_SetEncodedIV(crypt,"000102030405060708090A0B0C0D0E0F","hex");

    fileToDecrypt = "c:/temp/qa_output/hamlet.enc";
    facIn = CkFileAccess_Create();
    success = CkFileAccess_OpenForRead(facIn,fileToDecrypt);
    if (success != TRUE) {
        printf("Failed to open file to be decrytped.\n");
        CkCrypt2_Dispose(crypt);
        CkFileAccess_Dispose(facIn);
        return;
    }

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

    CkCrypt2_putFirstChunk(crypt,TRUE);
    CkCrypt2_putLastChunk(crypt,FALSE);

    bd = CkBinData_Create();
    bdDecrypted = CkBinData_Create();

    i = 0;
    while (i < numChunks) {
        i = i + 1;
        if (i == numChunks) {
            CkCrypt2_putLastChunk(crypt,TRUE);
        }

        //  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_putFirstChunk(crypt,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.
    CkCrypt2_putFirstChunk(crypt,TRUE);
    CkCrypt2_putLastChunk(crypt,TRUE);

    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");


    CkCrypt2_Dispose(crypt);
    CkFileAccess_Dispose(facIn);
    CkBinData_Dispose(bd);
    CkBinData_Dispose(bdDecrypted);

    }