Arduino keypad and RF 433MHz transmitter problem
The problem is you send a character of type char, thus exactly one byte.
However, when you send it you have to give the length and you use strlen. However, msg is an array (at least that is what you want), but that array contains only the character to sent. What you want is to send only 1 character, so use 1 instead of strlen.

Also, you can get rid of the special msg pointer and instead of

  const char *msg = key;//here is the problem, when i send the string above, it works okay, when I send this, i get strange string on other end
  driver.send((uint8_t *)msg, strlen(msg


use

  driver.send(&amp;key, 1);


Where &amp;key defines the address (pointer to) key, and since there is always exactly 1 byte in key (the key itself), 1 can be used. To be more safe, you can use sizeof(key) thus

  driver.send(&amp;key, sizeof(key));


What you also can do is create a string that ends with \0, so the first is the character to send, and strlen can be used, but the easiest way is to set the length to exactly 1.

Actually, using strlen on a string that does not end with \0 can result in strange behavior, because maybe there is no \0 in memory.