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随机加密

- (void)key {
    _changeArray =  @[@"0",@"1",@"2",@"3",@"4",@"5",@"6",@"7",@"8",@"9",@"A",@"B",@"C",@"D",@"E",@"F",@"G",@"H",@"I",@"J",@"K",@"L",@"M",@"N",@"O",@"P",@"Q",@"R",@"S",@"T",@"U",@"V",@"W",@"X",@"Y",@"Z"];
    NSMutableString *string = [[NSMutableString alloc]initWithCapacity:5];
    _changeStr = [[NSMutableString alloc] initWithCapacity:5];
    for (NSInteger i = 0; i < 4; i ++) {
        NSInteger index = arc4random() % [_changeArray count] - 1;
        string = [_changeArray objectAtIndex:index];
        _changeStr = (NSMutableString *)[_changeStr stringByAppendingString:string];
        _pictures = [NSString stringWithFormat:@"%@",_changeStr];
        _pictures = _changeStr;
        
        NSLog(@"%@",_pictures);
        
    }
}