字符串转16进制

-(NSString*)str2HexStr:(NSString*)str{

    NSString* hexStr= @"";

    NSString* hex = @"0123456789ABCDEF";

    NSString* str4Up = [str uppercaseString];

    //   补绝对0 0x00 而非字符‘0’ 所以要先转16进制补0，再转回

    for (int i =0 ; i<[str4Up length]; i++) {

        char dd = [str4Up characterAtIndex:i];

        hexStr = [NSString stringWithFormat:@"%@%c%c",hexStr,[hex characterAtIndex:(dd & 0x0f0)>>4],[hex characterAtIndex:dd&0x0f]];

    }

    return hexStr;

}