Objective-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. Objective-C

#import <CkoCrypt2.h>
#import <NSString.h>
#import <CkoFileAccess.h>
#import <CkoBinData.h>

BOOL success = NO;

//  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

CkoCrypt2 *crypt = [[CkoCrypt2 alloc] init];

crypt.CryptAlgorithm = @"aes";
crypt.CipherMode = @"cbc";
crypt.KeyLength = [NSNumber numberWithInt:256];

[crypt SetEncodedKey: @"000102030405060708090A0B0C0D0E0F000102030405060708090A0B0C0D0E0F" encoding: @"hex"];
[crypt SetEncodedIV: @"000102030405060708090A0B0C0D0E0F" encoding: @"hex"];

NSString *fileToDecrypt = @"c:/temp/qa_output/hamlet.enc";
CkoFileAccess *facIn = [[CkoFileAccess alloc] init];
success = [facIn OpenForRead: fileToDecrypt];
if (success != YES) {
    NSLog(@"%@",@"Failed to open file to be decrytped.");
    return;
}

//  Let's decrypt in 32000 byte chunks.
int chunkSize = 32000;
int numChunks = [[facIn GetNumBlocks: [NSNumber numberWithInt: chunkSize]] intValue];

crypt.FirstChunk = YES;
crypt.LastChunk = NO;

CkoBinData *bd = [[CkoBinData alloc] init];
CkoBinData *bdDecrypted = [[CkoBinData alloc] init];

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

    //  Read the next chunk from the file.
    //  The last chunk will be whatever amount remains in the file..
    [bd Clear];
    [facIn FileReadBd: [NSNumber numberWithInt: chunkSize] binData: bd];

    //  Decrypt this chunk.
    [crypt DecryptBd: bd];
    //  Accumulate the decrypted chunks.
    [bdDecrypted AppendBd: bd];

    crypt.FirstChunk = NO;
}

//  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.FirstChunk = YES;
crypt.LastChunk = YES;

[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"];