如何使用objective c从iOS中的64位字符串中分离出16个字节？

NSString *hexString = @"0123456789ABCDEF"; // 64位十六进制字符串
NSData *data = [self dataWithHexString:hexString];

- (NSData *)dataWithHexString:(NSString *)hexString {
    NSMutableData *data = [[NSMutableData alloc] init];
    unsigned char whole_byte;
    char byte_chars[3] = {'\0','\0','\0'};
    int i;
    for (i=0; i < [hexString length] / 2; i++) {
        byte_chars[0] = [hexString characterAtIndex:i*2];
        byte_chars[1] = [hexString characterAtIndex:i*2+1];
        whole_byte = strtol(byte_chars, NULL, 16);
        [data appendBytes:&whole_byte length:1];
    }
    return data;
}

NSUInteger length = 16; // 要提取的字节数
NSData *subdata = [data subdataWithRange:NSMakeRange(0, length)];