From an upcoming app that needs to encrypt your data using a passphrase, using industry standard methods:

#import <CommonCrypto/CommonKeyDerivation.h>

...

// Makes a random 256-bit salt
- (NSData*)generateSalt256 {
    unsigned char salt[32];
    for (int i=0; i<32; i++) {
        salt[i] = (unsigned char)arc4random();
    }
    return [NSData dataWithBytes:salt length:32];
}

...

// Make keys!
NSString* myPass = @"MyPassword1234";
NSData* myPassData = [myPass dataUsingEncoding:NSUTF8StringEncoding];
NSData* salt = [self generateSalt256];

// How many rounds to use so that it takes 0.1s ?
int rounds = CCCalibratePBKDF(kCCPBKDF2, myPassData.length,
    salt.length, kCCPRFHmacAlgSHA256, 32, 100);

// Open CommonKeyDerivation.h for help
unsigned char key[32];
CCKeyDerivationPBKDF(kCCPBKDF2, myPassData.bytes, myPassData.length,
    salt.bytes, salt.length, kCCPRFHmacAlgSHA256, rounds, key, 32);
NSData* keyData = [NSData dataWithBytes:key length:32];

Thanks for reading! And if you want to get in touch, I'd love to hear from you: chris.hulbert at gmail.