Generate random bytes Cocoa?

Question

I need to 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

Marvin

How will you know what bytes are the random addition and which are part of the original plaintext when you decrypt? There are already a number of different cryptographic padding schemes for use in different circumstances, most of which are easier to chop off afterwards than a bunch of random bytes. – Antemortem

Answer 1

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.

Androecium

Yes; you can use [NSData initWithBytes: length:] to get to the data into an NSData object. – Androecium

Could you be a bit more specific? – Androecium

You could save some work by creating an NSMutableData object of the desired length and passing its mutableBytes pointer as the buffer for SecRandomCopyBytes to write into. – Schappe

@PeterHosey Agreed. See robnapier.net/blog/aes-commoncrypto-564 for an example of this. Search for randomDataOfLength:. This is a very convenient way to manage buffers. – Despondent

I believe you need to link Security.framework to get SecRandomCopyBytes(). That may be what's causing the trouble for the OP. – Despondent

@RobNapier: One avenue of improvement there: You can use the strerror function to get a description of the errno value. – Schappe

Answer 2

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.

Cyruscyst

Note this is very easy but it fails if your app is in the App Sandbox, where it can't get at /dev – Hostler

/dev/random is accessible from an unplugged iOS device. Is it sandboxed on macOS? – Cyruscyst