What is RSA encryption output size when using 2048 bit key (using pkcs1padding). Is it always 256 bytes independent of input size? how can i calculate it for other key sizes?

**Yes**, it is.

The *output-size* **should** always equals the size of the `Modulus`

(part of the key), so:

```
2048 bit Modulus -> 2048 bit output
1024 bit Modulus -> 1024 bit output
...
```

If it is not, there exist numerous attacks on *RSA*, see here for basic information about that.

So to guarantee that the *output* is `2048 bit`

even when the *input* to encrypt is, let's say `7`

,

**a padding must always be applied!**