Best way to generate NSData object with random bytes of a specific length?

Question

If I create a new NSData object of a specific size using dataWithBytes:length:, what is the most efficient way to create the input bytes (20 Mb worth) of random characters, preferably without reading the data in from a file? I need a unique buffer of a specific size each time.

Thanks

Answer 1

You can create a 20*2^20b NSData object, then append a random 4 byte integer to it 20*2^20/4 times with arc4random(). I believe you need to include stdlib.h (via Generating random numbers in Objective-C).

#include <stdlib.h>

-(NSData*)create20mbRandomNSData
{
  int twentyMb           = 20971520;
  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];
  }
  return theData;
}

Answer 2

void * bytes = malloc(numberOfBytes);
NSData * data = [NSData dataWithBytes:bytes length:numberOfBytes];
free(bytes);

The bytes are not 'random', but will contain garbage values (whatever was on the heap before this was run). The advantage being its fast and the code is concise.

Answer 3

Here's a 3-liner swift version:

Swift 2

let length = 2048
let bytes = [UInt32](count: length, repeatedValue: 0).map { _ in arc4random() }
let data = NSData(bytes: bytes, length: bytes.count * sizeof(UInt32))

Swift 3

let bytes = [UInt32](repeating: 0, count: length).map { _ in arc4random() }
let data = Data(bytes: bytes, count: length)

Answer 4

You might consider using CCRandomGenerateBytes function from CommonCrypto to generate random data. Like:

func generateBytes(length : Int) throws -> NSData? {
    var bytes = [UInt8](count: length, repeatedValue: UInt8(0))
    let statusCode = CCRandomGenerateBytes(&bytes, bytes.count)
    if statusCode != CCRNGStatus(kCCSuccess) {
        return nil
    }
    return NSData(bytes: bytes, length: bytes.count)
}

Answer 5

Swift 3:

import Security

func randomBytes(length: Int) -> Data {
    var data = Data(capacity: length)
    data.withUnsafeMutableBytes { (bytes: UnsafeMutablePointer<UInt8>) -> Void in
        let _ = SecRandomCopyBytes(kSecRandomDefault, length, bytes)
    }
    return data
}

Swift 5 Update:

func randomBytes(length: Int) -> Data? {
    var data = Data(count: length)
    let result = data.withUnsafeMutableBytes {
        SecRandomCopyBytes(kSecRandomDefault, length, $0.baseAddress!)
    }
    guard result == errSecSuccess else {
        return nil
    }
    return data
}

Thanks to this post.

Answer 6

Use arc4random_buf to fill the buffer with random bytes

Obj-C

+ (nullable NSData *)radomDataOfSize:(size_t)sizeInBytes
{
    void *buff = malloc(sizeInBytes);
    if (buff == NULL) {
        return nil;
    }
    arc4random_buf(buff, sizeInBytes);

    return [NSData dataWithBytesNoCopy:buff length:sizeInBytes freeWhenDone:YES];
}

Answer 7

The original version has a bug but mine takes care of that and hopefully doesn't introduce any new one. Hope it helps.

- (NSData *)randomDataWithBytes: (NSUInteger)length {
    NSMutableData *mutableData = [NSMutableData dataWithCapacity: length];
    for (unsigned int i = 0; i < size; i++) { 
        NSInteger randomBits = arc4random();
        [mutableData appendBytes: (void *) &randomBits length: 1];
    } return mutableData;
}

Here is its unit test:

NSInteger givenLength = INT16_MAX;
NSData *randomData = [self randomDataWithBytes: givenLength];
STAssertTrue([randomData length] == givenLength,
             @"RandomDataWithBytes Failed Expected size %d and got %d", 
             givenLength, [randomData length]);

Answer 8

I've open sourced my JFRandom class over at github which can do exactly this. Here's a blog post demonstrating how to obtain/use it to achieve your goal...

http://jayfuerstenberg.com/devblog/generating-random-numbers-strings-and-data-in-objective-c

Answer 9

urandom is more efficient.

Here is a category to generate random buffers:

@interface NSMutableData(Random)
+(id)randomDataWithLength:(NSUInteger)length;
@end

@implementation NSMutableData(Random)
+(id)randomDataWithLength:(NSUInteger)length
{
    NSMutableData* data=[NSMutableData dataWithLength:length];
    [[NSInputStream inputStreamWithFileAtPath:@"/dev/urandom"] read:(uint8_t*)[data mutableBytes] maxLength:length];
    return data;
}
@end