Generate random bytes Cocoa?

I need t开发者_C百科o generate some random data to append to my file for encryption. How would I go about doing this? Would it be sort of the same idea as generating a string of random characters?

Something like:

NSData *randomData = @"what should i put here?";

And then use the rand() function to randomize the data?

Your help is greatly appreciated

---

int SecRandomCopyBytes ( SecRandomRef rnd, size_t count, uint8_t *bytes );

For example:

uint8_t data[100];
int err = 0;

// Don't ask for too many bytes in one go, that can lock up your system
err = SecRandomCopyBytes(kSecRandomDefault, 100, data);
if(err != noErr)
    @throw [NSException exceptionWithName:@"..." reason:@"..." userInfo:nil];

NSData* randomData = [[NSData alloc] initWithBytes:data length:100];

As noted by Peter in the comments, you can also do this:

NSMutableData* data = [NSMutableData dataWithLength:100];
err = SecRandomCopyBytes(kSecRandomDefault, 100, [data mutableBytes]);

And as noted by Rob in the comments, you need to link Security.framework for SecRandomCopyBytes to be available. You also need to include SecRandom.h.

---

On UNIX you can rely on /dev/random or /dev/urandom to obtain randomness of cryptographic quality.

NSData *bytes = [[NSFileHandle fileHandleForReadingAtPath: @"/dev/random"] readDataOfLength: 100];

As it seems, it reads 100 bytes from /dev/random. Refer to man urandom for details.