Create random 128 bit AES Encryption key in iOS

Question

I want to create random AES Encryption key (128 bit) in ios. I have searched in SO but I cannot find a good answer. Please give me some advice. thanks in advance.

UPDATE:

I have used BBAES lib. I used the below code to generate the encryption key but when I convert from NSData to NSString, it shows NULL

  -(NSData*)randomDataWithLength{
    NSData* salt = [BBAES randomDataWithLength:BBAESSaltDefaultLength];
    NSData *key = [BBAES keyBySaltingPassword:@"password" salt:salt keySize:BBAESKeySize128 numberOfIterations:BBAESPBKDF2DefaultIterationsCount];
    NSLog(@"Data ASE Key %@",key);
    NSString *aString  = [[NSString alloc] initWithData:key encoding:NSUTF8StringEncoding];
}

See http://stackoverflow.com/questions/9234686/generating-random-numbers-securely-in-objective-c — rmaddy, May 08 '14 at 02:12
why you want to convert it to NSString? and it is `NULL` because not any random data are valid UTF8 string — Bryan Chen, May 08 '14 at 07:47

score 12 · Answer 1 · edited Apr 01 '15 at 01:35

12

Woah, that's complicated code for a simple task!

- (NSData *)random128BitAESKey {
    unsigned char buf[16];
    arc4random_buf(buf, sizeof(buf));
    return [NSData dataWithBytes:buf length:sizeof(buf)];
}

You probably heard somewhere that you should use salt and hash your passwords. It looks like you took this advice a little too far: there are no passwords here and yet your code still salts and hashes the data! This is completely useless when the input comes from a secure random number generator like arc4random.

Of course it won't convert to an NSString because random data is unlikely to be valid UTF-8 string.

edited Apr 01 '15 at 01:35

Michael Voznesensky

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answered May 08 '14 at 06:38

Dietrich Epp

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Thanks so much. But how can I write this key to file? Thanks – user3214941 May 08 '14 at 06:52
4

You're mostly right, but I believe that arc4random is not always cryptographically secure. `SecRandomCopyBytes` is the RNG function to use for AES keys. – Chris Devereux May 09 '14 at 19:34
1

@ChrisDevereux: Do you have a citation for this? As far as I know, arc4random is designed to be used for cryptography, so it should be fine as long as it was properly seeded. – Dietrich Epp May 09 '14 at 19:43
Hmmm, I might have been thinking about something else. Sorry. Looks like there are some issues with ARC4 but I couldn't say whether its relevant here... http://en.wikipedia.org/wiki/RC4 – Chris Devereux May 09 '14 at 21:27

score 12 · Answer 2 · answered Apr 03 '16 at 00:29

You might want to use Apple's random byte generator for this which is considered more secure than arc4random.

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

https://developer.apple.com/library/ios/documentation/Security/Reference/RandomizationReference/index.html#//apple_ref/c/func/SecRandomCopyBytes

A good explanation for this can be found on a blog post by James Carroll:

http://jamescarroll.xyz/2015/09/09/safely-generating-cryptographically-secure-random-numbers-with-swift/

Open Whisper Systems use this for the iOS version of their popular secure chat app Signal

score -4 · Answer 3 · answered May 08 '14 at 04:56

-4

This might help

- (NSString *)getRandomKey{
    NSString *alphabet  = @"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXZY0123456789/=+";
    NSMutableString *s = [NSMutableString stringWithCapacity:20];
    for (NSUInteger i = 0; i < 20; i++) {
        u_int32_t r = arc4random() % [alphabet length];
        unichar c = [alphabet characterAtIndex:r];
        [s appendFormat:@"%C", c];
    }
    NSLog(@"%@", s);
    NSString *key = s;
    return key;
}

answered May 08 '14 at 04:56

I won't use this algorithm for anything related to security – Bryan Chen May 08 '14 at 05:44
@BryanChen: Please tell me why wont use this algorithm ? – user3214941 May 08 '14 at 06:28
1

Often those random functions do depend on time which makes them predictable for an attacker. Also using arc4random is not recommended because of modulo bias (http://stackoverflow.com/questions/648739/objective-c-modulo-bias) – Quxflux May 08 '14 at 06:39
1

@Lukas: arc4random is not one of those RNGs that gets seeded with `time()`—actually, it's a secure RNG and quite slow compared to other RNGs, and it's the right choice for key generation. However, you are right about the modulo bias. – Dietrich Epp May 08 '14 at 06:41
Do not use `arc4random() % range` there will be some modulo bias, use `arc4random_uniform(range)`. – zaph Apr 03 '16 at 11:55

Create random 128 bit AES Encryption key in iOS

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