Question

i am trying to use CCCrypt to encrypt my data , but something wrong with it, the data length changed after i encrypted, but didn’t happen on the decrypt, here is the following code.

- (NSData *)AES256EncryptWithKeyGenerateIV:(NSString *)key {
    char keyPtr[kCCKeySizeAES128+1];
    bzero(keyPtr, sizeof(keyPtr));

    [key getCString:keyPtr maxLength:sizeof(keyPtr) encoding:NSUTF8StringEncoding];

    char ivPtr[kCCKeySizeAES128+1];

    unsigned char result[CC_MD5_DIGEST_LENGTH];
    CC_MD5([key UTF8String], strlen([key UTF8String]), result);

    NSString* resultString = [NSString stringWithFormat:
                              @"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
                          result[0], result[1], result[2], result[3],
                          result[4], result[5], result[6], result[7],
                          result[8], result[9], result[10], result[11],
                          result[12], result[13], result[14], result[15]
                          ];
    resultString = [resultString substringToIndex:16];

    bzero(ivPtr, sizeof(ivPtr));
    [resultString getCString:ivPtr maxLength:sizeof(ivPtr)     encoding:NSUTF8StringEncoding];
    NSUInteger dataLength = [self length];

    NSLog(@"Before encrypt dataLength is %d",dataLength);

    size_t bufferSize = dataLength + kCCBlockSizeAES128;
    void *buffer = malloc(bufferSize);

    size_t numBytesDecrypted = 0;
    CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, kCCAlgorithmAES128,
                                      0,
                                      keyPtr, kCCBlockSizeAES128,
                                      ivPtr,
                                      [self bytes], dataLength,
                                      buffer, bufferSize,
                                      &numBytesDecrypted);

    NSLog(@"After file encrypted dataLength is %d",numBytesDecrypted);
    if (cryptStatus == kCCSuccess) {
        return [NSData dataWithBytesNoCopy:buffer length:numBytesDecrypted];
    }

    free(buffer);
    return nil;
}

and here is the log i print the data length out:

2012-07-09 18:29:20.787 Before encrypt dataLength is 248
2012-07-09 18:29:20.788 After file encrypted dataLength is 240
2012-07-09 18:29:20.789 Before encrypt dataLength is 20
2012-07-09 18:29:20.790 After file encrypted dataLength is 16
2012-07-09 18:29:20.792 Before encrypt dataLength is 583
2012-07-09 18:29:20.792 After file encrypted dataLength is 576
2012-07-09 18:29:20.795 Before encrypt dataLength is 31120
2012-07-09 18:29:20.796 After file encrypted dataLength is 31120
2012-07-09 18:29:20.797 Before encrypt dataLength is 27551
2012-07-09 18:29:20.798 After file encrypted dataLength is 27536
2012-07-09 18:29:20.800 Before encrypt dataLength is 1266
2012-07-09 18:29:20.800 After file encrypted dataLength is 1264
2012-07-09 18:29:20.802 Before encrypt dataLength is 431315
2012-07-09 18:29:20.809 After file encrypted dataLength is 431312
2012-07-09 18:29:20.815 Before encrypt dataLength is 204
2012-07-09 18:29:20.815 After file encrypted dataLength is 192
2012-07-09 18:29:20.817 Before encrypt dataLength is 1083
2012-07-09 18:29:20.817 After file encrypted dataLength is 1072
2012-07-09 18:29:20.818 Before encrypt dataLength is 440
2012-07-09 18:29:20.819 After file encrypted dataLength is 432

can someone tell me why please?

Answer 1

Padding has to be applied when you are using a block cipher, to make the plain text fit into the cipher blocks. AES is a block cipher, and uses a block size of 128 bits, 16 bytes. Try it with explicit padding and CBC mode in the CCOptions, see the Apple documentation of CCCrypt how to do that (although, again, I’ve seen better descriptions of cryptographic methods in my life).

[edit]
Note: try kCCOptionPKCS7Padding, CBC is probably the default.