Best way to generate NSData object with random byt

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

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
}

Answer 2

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 3

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