Question

I am trying to create a 16 byte and later 32 byte initialization vector in objective-c (Mac OS). I took some code on how to create random bytes and modified it to 16 bytes, but I have some difficulty with this. The NSData dumps the hex, but an NSString dump gives nil, and a cstring NSLog gives the wrong number of characters (not reproduced the same in the dump here).

Here is my terminal output:

2012-01-07 14:29:07.705 Test3Test[4633:80f] iv hex <48ea262d efd8f5f5 f8021126 fd74c9fd>
2012-01-07 14:29:07.710 Test3Test[4633:80f] IV string: (null)
2012-01-07 14:29:07.711 Test3Test[4633:80f] IV char string t^Q¶�^��^A

Here is the main program:

int main (int argc, const char * argv[])
{
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
    //NSString *iv_string = [NSString stringWithCString:iv encoding:NSUTF8StringEncoding];
    testclass *obj = [testclass alloc];
    NSData *iv_data = [obj createRandomNSData];
    //[iv_string dataUsingEncoding:NSUTF8StringEncoding];
    NSLog(@"iv hex %@",iv_data);

    //NSString *iv_string = [[NSString alloc] initWithBytes:[iv_data bytes] length:16 encoding:NSUTF8StringE$
    NSString *iv_string = [[NSString alloc] initWithData:iv_data encoding:NSUTF8StringEncoding];
    NSLog(@"IV string: %@",iv_string);
    NSLog(@"IV char string %.*s",[iv_data bytes]);

    return 0;
]

(I left in the above some commented code that I tried and did not work also).

Below is my random number generater, taken from a stack overflow example:

@implementation testclass
-(NSData*)createRandomNSData
{
    int twentyMb           = 16;
    NSMutableData* theData = [NSMutableData dataWithCapacity:twentyMb];
    for( unsigned int i = 0 ; i < twentyMb/4 ; ++i )
    {
            u_int32_t randomBits = arc4random();
            [theData appendBytes:(void*)&randomBits length:4];
    }
    NSData *data = [NSData dataWithData:theData];
    [theData dealloc];
    return data;
}
@end

I am really quite clueless as to what could be the problem here. If I have data as bytes, it should convert to a string or not necessarily? I have looked over the relevant examples here on stackoverflow, but none of them have worked in this situation.

Thanks,
Elijah

Answer 1

An arbitrary byte sequence may not be legal UTF8 encoding. As @Joachim Isaksson notes, there is seldom reason to convert to strings this way. If you need to store random data as a string, you should use an encoding scheme like Base64, serialize the NSData to a plist, or similar approach. You cannot simply use a cstring either, since NULL is legal inside of a random byte sequence, but is not legal inside of a cstring.

You do not need to build your own random byte creator on Mac or iOS. There’s one built-in called SecRandomCopyBytes(). For example (from Properly encrypting with AES with CommonCrypto):

+ (NSData *)randomDataOfLength:(size_t)length {
  NSMutableData *data = [NSMutableData dataWithLength:length];

  int result = SecRandomCopyBytes(kSecRandomDefault, 
                                  length,
                                  data.mutableBytes);
  NSAssert(result == 0, @"Unable to generate random bytes: %d",
           errno);

  return data;
}